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of manufacture and presence of leaf tip (for orthodox

manufactured leaf) and/or undesirable stalk and fiber,

feel, and aroma. Attention is then drawn to the in-

fused leaf, which, disconcertingly, is often referred to

as the ’infusion.’ Ideally, this should be of a bright

copper color and virtually free of the green tinge of

chlorophyl. From the color and evenness of the

infused leaf, the taster is able to make an assessment

of the degree and quality of the fermentation.

0006 On removing the lid, the aroma of the vapor pre-

sent in the cup is evaluated. The liquor is then judged

whilst it is still warm, first for appearance, which

should be bright and clear with a distinct reddish

tinge and a faint pink meniscus where the liquor

touches the cup, then for taste. When cool, the

better-quality liquors become opaque due to a process

called ’creaming down,’ which is essentially the pre-

cipitation of very finely divided colloidal material

derived from caffeine, protein, and polyphenols, par-

ticularly the theaflavins.

0007 The taste characteristics of the liquor are of

particular importance. In nonscientific vocabulary,

the terms ’taste’ and ’flavor’ are often considered

synonymous, but in scientific nomenclature, these

words are used to describe two different and distinct

characteristics. The flavor of an aqueous solution

comprises:

0008 The basic taste sensations of sweet, sour, bitter,

and salt, which are detected by the tongue and

produced by nonvolatile substances present in

appreciable amounts.

0009 The odor, or aroma, detected by the nose and

produced by vapors of volatile components, many

of which are present only in minute amounts.

0010 ’Taste’ refers only to those sensations produced

by nonvolatile components. In tea tasting, the term

’taste’ is used in its wider sense and therefore includes

the aroma. The actual tasting (Figure 1) is carried out

by sucking, rather than sipping, so that the liquor is

drawn to the back of the mouth, on an inward breath,

and up to the olfactory nerve in the nose. The liquor is

then swished backwards and forwards and brought

into contact with the tongue, palate, and other areas

of the mouth where sensory receptors are located.

Using this skillful, if somewhat noisy, technique, the

taster is able to feel, taste, and smell the liquor virtu-

ally simultaneously and is thus able to determine its

briskness, strength, body, and flavor. The liquor is

either taken directly from the cup or from a special

spoon. After tasting, the liquor is not swallowed, but

discarded into a mobile spittoon.

0011 The tea tasters’ report concentrates on appearance,

color, strength, pungency, and flavor. For each par-

ameter or character, a large number of specialized

descriptive terms are available for use by the tea

taster. A tea taster can taste over 50 samples per

session. The procedure is strictly standardized, and

tea tasters require considerable individual training

and experience in order to attain the necessary stand-

ards of competence.

0012Although it is clearly a subjective method, prone to

influence by variables such as supply and demand, the

tasters’ state of health, and personal preferences and

prejudices, tea tasting is still considered to be the best

method available for assessing tea quality as far as

the tea trade is concerned. The success of the method

lies in the fact that it is quick and inexpensive, and

requires little equipment. From a scientific point of

view, the method is less acceptable, and several

chemical methods are currently being developed,

and employed, as replacements or supplements to

the tea tasters’ evaluation of tea quality. Despite the

advanced analytical equipment and procedures that

are currently available, it is still not possible to re-

place tea tasting with a single, reliable, and objective

scientific method.

Chemical Analysis of Black Tea

0013The International Organization for Standardization

specification Black Tea – Definition and Basic Re-

quirements (ISO 3720) specifies certain chemical

requirements for black tea, which, when met, are

considered to be indicative of a tea produced by

recognized good production practice. These chemical

requirements can be found in Table 1 together with

the individual ISO specification numbers for each

parameter. ISO 3720 defines black tea as that tea

derived solely from the leaves, buds and tender

stems of varieties of the species Camellia sinensis.

With the exception of ISO 6078 (Black Tea

fig0001Figure 1 (see color plate 133) Tea-tasting session (courtesy of

the Tea Board of Kenya, Nairobi).

5758 TEA/Analysis and Tasting

Vocabulary, a glossary of terms relating to black tea),

the International Organization for Standardization

has not dealt with a methodology for pricing black

teas. More recently, the ISO has embarked on the

development of chemical methods for classifying tea

based on polyphenol content. However, efforts have

not reached the standards stage. Tea prices, and qual-

ity, continue to be based on tea tasters’ evaluations. In

fact, it is generally accepted that expert tea tasters

have the ability to assess whether a tea would be

unlikely to comply with ISO 3720, and in practice,

chemical analysis tends to be carried out only if the

tea taster considers a tea to be ’suspect.’

0014 Several chemical methods have been developed

to determine specific quality parameters of tea. The

volatile components of tea, often referred to as the

’aroma complex,’ have been studied in depth by

gas chromatography and mass spectrometry, and

over 600 compounds have been identified. Routine

methods for determining aroma have been developed.

In one such method, the ratio of the gas chromato-

graphic peak areas of the volatile compounds eluting

before linalool to the sum of peak area of linalool plus

all volatile compounds eluting after linalool was used

as the aroma quantifying index. In another method,

the ratio of the sum of the gas chromatographic peak

areas of terpenoid compounds to nonterpenoid com-

pounds was used as the aroma index. In yet another

method, the ratio of the gas chromatographic peak

area of linalool to that of 2-E-hexenal was used. For

the last aroma index development, the major volatiles

are divided into two distinct groups; substances that

have a pleasant aroma and are highly desirable (group

II) and substances that, although essential constitu-

ents of tea, are considered to have a deleterious effect

on flavor when present in large amounts (group I).

Lists of typical compounds in groups II and I are

shown in Table 2. The ratio of the sum of the group

II to group I volatile compounds gas chromatographic

peak areas called the flavor index (FI) was used to

quantify the black tea aroma quality. Using Kenya

clonal black teas, the FI was recently shown to be a

better measure for aroma quality compared with the

other indices. The volatiles are usually extracted

by simultaneous steam distillation/solvent extraction,

separated by gas chromatography, and identified

by mass spectrometry. The volatile FI is the ratio

of group II to group I volatiles. Flavory teas have

larger amounts of group II compounds in relation

to group I.

0015Phenolic substances dominate the nonvolatile

flavor components. In the case of black tea, most of

the taste is believed to be due to the presence of the

unique phenolic fermentation products theaflavins

and thearubigins plus unoxidized catechins. The sens-

ory qualities of the theaflavins, thearubigins and cate-

chins are usually described in terms of mouth feel and

color characteristics. The theaflavins are responsible

for briskness and brightness, the thearubigins are

considered responsible for body and color, and the

catechins contribute to astringency. In general terms,

teas with a high level of thearubigins relative to

theaflavins and low catechin levels are considered to

impart a soft mouth feel and dull color, and in the

reverse situation, the teas are often described as

lacking body and color.

0016The catechins, theaflavins, and thearubigins are

determined by spectrophotometric methods, which

gives a total value for each group. However, high-

performance liquid chromatography (HPLC) is now

tbl0001 Table 1 Chemical requirements for black tea

Characteristic Requirement (%) Test method

Water extract

Minimum 32 ISO 1574

To t a l a s h

Maximum 8 ISO 1575

Minimum 4

Water-soluble ash of total ash

Minimum 45 ISO 1576

Alkalinity of water-soluble ash (as KOH)

Maximum 1.0

ISO 1578

Minimum 3.0

Acid-insoluble ash

Maximum 1.0 ISO 1577

Maximum 16.5 ISO 5498

When the alkalinity of water-soluble ash is expressed in terms of mm

KOH per 100 g of ground sample, the limits shall be 178 (minimum) and

53 mm (maximum).

Courtesy of the Kenya Bureau of Standards.

tbl0002Table 2 Some volatile flavor compounds used to determine the

flavor index of black teas

Group I volatiles Group II volatiles

Isovaleraldehyde Linalool oxide, (Z)-furanoid

2-Ethylfuran Linalool oxide, (E)-furanoid

Hexanal Benzaldehyde

1-Penten-3-ol Linalool

Heptanal b-Cyclocitral

(Z)-3-Hexenal Ho-trienol

(E)-2-Hexenal 1-Ethylformylpyrrole

Penten-1-ol Phenylacetaldehyde

(Z)-2-Penten-1-ol a-Terpineol

Hexan-1-ol Linalool oxide, (Z)-pyranoid

(Z)-3-Hexen-1-ol Linalool oxide, (E)-pyranoid

Nonanal Methylsalicylate

(E)-2-Hexen-1-ol Nerol

(E)-2,4-Beptadienal Geraniol

(E, E )-2,4-Heptadienal Benzy alcohol

Nerolidol

Bovolide

Dihydroactimdiolide

TEA/Analysis and Tasting 5759

applied to these analyses as it allows the catechins

and theaflavins to be determined individually. Black

teas have four dominant theaflavins which have

different astringencies and have variable contribu-

tion to tea taste. Their contribution to taste has

been normalized to theaflavin digallate equivalent,

which relates better to sensory evaluations. The

chemical structures of thearubigins have not been

determined.

Contaminants: Heavy Metals and

Pesticide Residues

0017 Several producer and consumer countries have passed

legislation stipulating maximum permissible levels of

heavy metal contaminants in tea. For example, in

Kenya, according to the Food, Drugs and Chemical

Substances Act (1978), limits were set for heavy metal

and fluoride contaminants in black tea, based on

the dry weight (Table 3). In the UK, statutory limits

have been set for black tea, on a dry-weight basis,

at 1 mg kg

1

for arsenic and 5 mg kg

1

for lead, and

guideline limits have been set at 50 mg kg

1

for zinc

and 150 mg kg

1

for copper.

0018 High levels of lead have often been associated

with tea, particularly in the past when lead linings

were used in tea chests and lead-coated drier

trays were used in tea factories. Lower concentrations

were achieved by stopping such practices, but even so,

levels of about 2 mg kg

1

are considered to be un-

avoidable. Evidence shows, however, that most of

the lead remains in the tea leaves on brewing and is

not consumed. Furthermore, tea leaves have been

shown to absorb lead from water.

0019 The Codex Alimentarius of the Food and Agri-

culture Organization of the United Nations World

Health Organization has set down maximum permis-

sible levels of various pesticide residues in black tea

on a dry-weight basis (Table 4).

0020 Most of the heavy metal and pesticide residue levels

that have been set are based on the dry weight of

processed tea. However, tea is drunk as an infusion

using water of widely variable quality and at different

temperatures. The solubility of the contaminants

therefore varies. This has been largely ignored in the

setting of the various limits.

Off-flavors and Taints

Causes of Off-flavors and Taints

0021Off-flavor, or taint, may be defined as any taste or

odor that is considered to be abnormal or foreign for

tea. Off-flavors originate from a myriad of sources

and can be attributed to either single or multiple

chemical substances that are not normally present in

tea or occasionally to unusually high concentrations

of compounds that are generally found to be present

only in trace amounts.

0022The chemicals responsible for taints are mostly

volatile organic compounds with very low odor

thresholds. Taint detection is an extremely time-

consuming and difficult process as the compounds

responsible are usually present at very low concen-

trations. Once tea is tainted, the condition is irrevers-

ible, and the subsequent loss in revenue is very often

substantial. It is therefore imperative to take all

possible measures to avoid the formation of such

undesirable compounds in the first place.

0023Good factory hygiene is of paramount importance,

and strict control of each stage of the manufacturing

process is essential in order to insure an evenly pro-

cessed product of the best possible quality. If hygiene

standards or manufacturing controls are allowed to

slip, an inferior product results, which is very often

tainted or off-flavored. Additional common causes of

taints in tea are described below.

0024Moisture Due to its hygroscopic nature, processed

tea is particularly liable to generate or pick up taints;

therefore, strict control of the moisture content is

crucial. Moisture contents in excess of 6.5% are ex-

tremely detrimental as this encourages rapid fungal

and bacterial growth. Volatile compounds produced

tbl0004Table 4 Maximum permissible levels of pesticide residues

in tea

Pesticide Level (mg kg

1

)

Ethion 6.0

Fenitrothion 0.5

Methidathion 0.1

Methylparathion 0.2

Cyhexatin 2.0

Bromopropylate 5.0

Methylechlorpyrifos 0.1

Cartap 20.0

Permethrin 20.0

Deltamethrin 10.0

tbl0003 Table 3 Maximum limits of heavy metals and fluoride

contaminants permitted in black tea in Kenya

Contaminant Limit (mg kg

1

)

Arsenic 1.0

Lead 10.0

Copper 150.0

Zinc 50.0

Fluoride 100.0

5760 TEA/Analysis and Tasting

during the course of proliferation of fungi and bac-

teria are a major source of taints in tea. Taints from

this source are variously described by tea tasters

as ’musty,’’fusty,’’moldy,’’gone-off,’’sweaty,’ and

’sour.’ The presence of high levels of moisture in the

tea at the time the taint is first perceived is indicative

of the cause of such taints, and the identities of the

individual compounds responsible are thus rarely

pursued. However, such fungal and bacterial infec-

tions can also occur during the course of processing,

particularly during the withering and fermentation

stages, if the leaf is excessively damp or the conditions

unclean. In such cases, the moisture content of the

finished product would not necessarily be elevated,

and the source of the contamination would be much

more difficult to locate.

0025 Conversely, teas that come out of the drier with

moisture contents of 2.5%, or less, generally have a

’smoky’ taint, if firing has been carried out using

a conventional dryer. This renders them equally

unacceptable.

0026 Chlorophenols and chloroanisoles Chlorophenols

impart disinfectant-like taints and are responsible

for numerous incidences of tainting throughout the

food industry. A major cause for concern is that many

fungi and bacteria can readily convert chlorophenols

to chloroanisoles by a simple process of methylation.

Chloroanisoles possess intense musty/moldy odors,

often described as ’old cellars’ or ’damp sacks,’ and

have much lower odor thresholds than the parent

chlorophenols. 2,4,6-Trichloroanisole and 2,3,4,6-

tetrachloroanisole, both of which have been found

in tainted teas, have odor thresholds in water of

2 10

5

and 4 10

3

mgkg

1

, respectively.

0027 Due to the widespread usage of chlorophenols and

their derivatives, considerable vigilance is required

in order to minimize the chances of contamination.

Chlorophenols have been detected in multiwalled

paper sacks (and in the adhesives and printing inks

used in their manufacture), wooden pallets, and con-

tainers used for the storage and transportation of tea.

Chlorophenols from such sources have been deemed

responsible for chloroanisole contamination in teas.

0028 The use of certain chlorinated herbicides, fungi-

cides, and pesticides in the field, and algaecides or

disinfectants in the factory, may give rise to chloro-

phenol contamination. Storage of tea in the vicinity of

such chemicals is also liable to cause tainting. In fact,

tea must not be stored in close proximity to any

commodity with a strong smell due to its remarkable

ability to pick up odors. In addition, factory water

supplies should not be chlorinated, as chlorophenols

can also be formed by the reaction of chlorine with

phenols in the tea or in the environment.

Detection and Analysis of Compounds Responsible

for Taints

0029Tea tasters usually detect taints, in the first instance.

This may take place at the factory, and in such cases,

depending on the description of the taint, the source

can be malfunctioning equipment or bad handling

practices. Once the tea has been transported or

stored, it can be extremely difficult to identify the

source of the contamination, and it is only considered

cost-effective to pursue the source of such taints in the

events that they recur.

0030The techniques used for isolating and eradicating

persistent, recurring taints are many and varied, as,

indeed, are the agents responsible for such taints.

Techniques employed are required to be meticulous

and methodical, and involve two distinct processes.

Firstly, the movements of the product from the time

the taint was first perceived are traced back, even as

far as the field, if necessary, in an attempt to pin-

point the association with any agent, or identify any

deviation from normal practices, capable of causing

the described taint. This requires the cooperation of

all concerned and a knowledgable and skillful co-

ordinator. Circumstantial evidence collected in this

way can be most successful when confirmed by

chemical analysis. Secondly, analysis of the tea is

undertaken to identify the compounds responsible

for the taint. The flavor analyst initially discusses

the description of the taint with the tea tasters and

then proceeds to search for specific compounds

known to cause such taints. Analytical methods

used include headspace gas chromatography, con-

tinuous steam distillation, and solvent extraction

of the volatiles, followed by separation by gas

chromatography (including splitting of the gaseous

mixture exiting the gas chromatography column in

order to enable ’sniffing’ of the individual sample

components), preparative gas chromatography to

concentrate relevant fractions, specific detector

gas chromatography, gas chromatography–mass

spectrometry (including multiple ion monitoring)

and HPLC.

See also: Contamination of Food; Pesticides and

Herbicides: Residue Determination; Sensory

Evaluation: Taste; Taints: Types and Causes; Analysis

and Identification

Further Reading

Owuor PO, Othieno CO and Reeves SG (1987) The Kenyan

position on theaflavins in tea. Journal of Food and Agri-

culture 1: 185–186.

Owuor PO (1992) A comparison of gas chromatographic

volatile profiling methods for assessing the flavour

TEA/Analysis and Tasting 5761

quality of Kenyan black teas. Journal of the Science of

Food and Agriculture. 59: 189–197.

Robinson J and Owuor PO (1991) Tea aroma. In: Willson

KC and Clifford MN (eds) Tea: Cultivation to Con-

sumption. London: Chapman & Hall.

Rothe M (1988) Introduction to Aroma Research, 1st

English edn. Berlin: Akademie Verlag.

Werkhoven J (1974) Tea processing. In: Agricultural Ser-

vices Bulletin, No. 26. Geneva: Food and Agricultural

Organization.

Whitfield FB (1983) Some flavours which industry could

well do without. Case studies of industrial problems.

CSIRO Food Research Quarterly 43: 96–106.

Texture See Rheological Properties of Food Materials; Sensory Evaluation: Sensory Characteristics of

Human Foods; Food Acceptability and Sensory Evaluation; Practical Considerations; Sensory Difference Testing;

Sensory Rating and Scoring Methods; Descriptive Analysis; Appearance; Texture; Aroma; Taste

Therapeutic Diets See Elderly: Nutritional Management of Geriatric Patients; Food Intolerance: Types;

Food Allergies; Milk Allergy; Lactose Intolerance; Hypertension: Nutrition in the Diabetic Hypertensive; HIV

Disease and Nutrition; Infection, Fever, and Nutrition; Multiple Sclerosis – Nutritional Management;

Pregnancy: Nutrition in Diabetic Pregnancy

Thermal Processing See Heat Treatment: Ultra-high Temperature (UHT) Treatments; Chemical and

Microbiological Changes; Electrical Process Heating

THERMOGENESIS

P Trayhurn, University of Liverpool, Liverpool, UK

Background

0001 The term ’thermogenesis’ comes from the Greek word

thermos for heat. All metabolic processes in animals

(and plants) produce heat, as a reflection of thermo-

dynamic inefficiency, and are therefore thermogenic.

In general, however, thermogenesis is used to describe

a facultative, or adaptive, process of heat generation,

i.e., a process in which heat is the primary product of

metabolism. It is also used to describe the heat pro-

duced in direct response to a meal.

0002The concept of facultative thermogenesis is well-

established in relation to the generation of heat

to maintain body temperature (thermoregulatory

thermogenesis). It is also widely used in nutritional

science to describe the adaptive production of heat in

response to overfeeding (diet-induced thermogen-

esis), particularly as a component of the regulation

of whole-body energy balance.

Thermogenesis and Thermoregulation

General

0003In homeotherms, heat is generated to maintain body

temperature (at *37



C in mammals) when an

5762 THERMOGENESIS

animal is in a cold environment. In this context, ’cold’

is defined as a temperature below the thermoneutral

zone, i.e., below the range of temperatures at which

metabolic rate is at a minimum. The thermoneutral

zone varies widely between species, being for

example, approximately 32–33



C in mice, 26–28



in adult humans, and of the order of 0–25



C in mature

sheep (depending on factors such as the plane of

nutrition and the length of the coat). At thermoneu-

trality body temperature is maintained above the am-

bient temperature by the heat generated by obligatory

thermogenesis. This is the heat produced as a bypro-

duct of all the basal metabolic processes of an animal,

such as the pumping of the heart, the synthesis of

proteins, and ion pumping across membranes.

0004 The lowest point of the thermoneutral zone is re-

ferred to as the lower critical temperature. Once the

environmental temperature has fallen below this

point, then the rate of heat loss exceeds the rate of

obligatory, or basal, heat production and body tem-

perature will fall. Factors such as age, nutritional

status, and the degree of thermal insulation all affect

the thermoneutral zone and the lower critical tem-

perature of an animal.

0005 Two distinct mechanisms are available for thermo-

regulatory heat production – shivering and nonshiver-

ing thermogenesis. Shivering, which involves small

contraction of the skeletal muscles, is a widely

employed mechanism for the generation of heat in

the cold. It is, however, behaviorally disadvanta-

geous, in contrast to nonshivering thermogenesis

where heat is produced without muscle contraction.

The term ’thermogenesis’ when applied to thermore-

gulation is often used with exclusive reference to

nonshivering thermogenesis.

Adaptation of Adult Animals to Cold Environments

0006 Exposure of mature homeotherms to a temperature

below, or colder, than the lower critical temperature

imposes an immediate requirement for thermoregula-

tory thermogenesis in order to maintain body tem-

perature. The generation of heat by shivering is a

major part of the initial response to cold. However,

during long-term (days to weeks) cold exposure, heat

generated by shivering is gradually replaced by non-

shivering thermogenesis. In laboratory animals full

adaptation to cold is considered to require a period

of up to 3–4 weeks.

Newborn

0007 The newborn of a species has a higher surface area to

body mass ratio than the mature or adult animal, and

insulation (hair, fur) is often absent or underdevel-

oped. These two factors result in a higher rate of

heat loss at a given environmental temperature in

the neonate than in the adult homeotherm. As a con-

sequence, neonates are particularly vulnerable to cold

and a high capacity for nonshivering thermogenesis is

generally evident. The transition at birth from the

intrauterine environment to the external world can

present a major thermal challenge to the mammalian

neonate, particularly in the case of precocial species.

Lambs, for example, can experience a fall in ambient

temperature in excess of 50



C over a few minutes

when being being born in cold northern climates in

late winter. In some species, such as large ruminants

(e.g., sheep, reindeer) the capacity for nonshivering

thermogenesis falls rapidly over the first days or

weeks of postnatal life.

Arousal from Hibernation

0008A limited number of mammalian species undergo

hibernation during winter. Examples include hedge-

hogs, the European hamster (Cricetus cricetus)and

various species of ground squirrel, e.g., Richardson’s

ground squirrel (Spermophilus richardsonii). Arousal

from the deep torpor of hibernation, when body tem-

perature may be as low as 4–6



C, requires intense

thermogenesis. Both shivering and nonshivering

mechanisms are invoked in order rapidly (within a

few hours) to raise the body temperature during

arousal.

Thermogenic Effect of Food –

Diet-Induced Thermogenesis

Nomenclature

0009Discussion of facultative thermogenesis in the

context of nutrition is compounded by some ter-

minological complexity and confusion. Feeding

leads to a rapid increase in metabolic rate and dif-

ferent expressions have been used to describe this

phenomenon. These include specific dynamic action

(originally used in relation to the presumed particu-

lar effects of protein), thermic effect of food, post-

prandial thermogenesis, diet-induced thermogenesis,

and the heat increment of feeding (Table 1). The

term ’heat increment of feeding’ is used primarily by

those who work on farm animals, while the other

expressions are employed in human nutrition and in

connection with energy metabolism in laboratory

animals.

0010In human and small-animal nutrition, diet-induced

thermogenesis has become the customary expression.

Even this, however, includes two distinct processes –

obligatory and adaptive (or facultative) diet-induced

thermogenesis – and involves both the short-term

THERMOGENESIS 5763

response to a meal and the long-term response to

overfeeding. It is emphasized that there are close par-

allels between adaptive diet-induced thermogenesis

and nonshivering thermogenesis, particularly at a

mechanistic level, and high levels of the former will

reduce the need for the latter.

Short-Term Responses to Food

0011 The consumption of food leads to a rapid increase in

energy expenditure. Although this was originally con-

sidered to be a particular property of dietary protein,

as noted above, it is now recognized that it occurs

with the consumption of each of the macronutrients.

The digestion, absorption, and initial metabolism of

nutrients impose an energy cost on an organism, and

include the cost of processes such as the synthesis of

digestive enzymes.

0012 The main component of obligatory diet-induced

thermogenesis is the energy costs associated with

the initial metabolic processing of substrates, once

absorbed. Thus, glucose derived from dietary carbo-

hydrate is initially stored as glycogen (in liver or

muscle) or triacylglycerols (in liver or adipose tissue),

following transport into tissues by either insulin-

dependent or insulin-independent glucose transport.

Obligatory diet-induced thermogenesis occurs in

humans and in all other animals.

0013 The energy costs associated with the storage of

dietary carbohydrate as lipid amounts to approxi-

mately 25% of the energy potential of the substrate.

In contrast, the deposition of dietary fat as triacyl-

glycerol costs only some 5% of the energy associated

with the substrate. Thus, calorie for calorie, the ob-

ligatory energy expenditure consequent upon the ini-

tial handling and storage of substrates as lipid will be

lower with dietary fat than with carbohydrate (or

protein).

0014 The extent to which the acute response to food

includes a facultative, energy-dissipative component

is uncertain; there has, however, been much discus-

sion of this possibility.

Responses to Chronic Overfeeding

0015In the early part of the twentieth century, German

physiologists suggested that overfeeding leads to the

stimulation of energy expenditure over and above

obligatory diet-induced thermogenesis. The expres-

sion Luxoskonsumption was coined to describe the

phenomenon, but adaptive diet-induced thermogen-

esis is now a more commonly used term. The basis of

the concept is that overfeeding, particularly long-

term, leads to the activation of mechanisms for the

dissipation of the excess energy intake as heat. Thus,

diet-induced thermogenesis is considered to represent

a control mechanism in the regulation of energy

balance.

0016Investigation of the metabolic responses to over-

feeding in experimental animals has been greatly

aided by the introduction of the cafeteria or super-

market diet. This diet, which provides a variety of

human food items in addition to the standard la-

boratory animal chow, may induce up to twofold

increases in voluntary food intake; the degree of

hyperphagia depends, however, primarily on the var-

iety and palatability of the foods. Energy balance

measurements in rats and mice, in particular, have

demonstrated that adaptive diet-induced thermogen-

esis is markedly activated in response to chronic

overfeeding with a cafeteria diet. This reduces the

extent to which fat is deposited in response to the

overfeeding.

0017The phenomenon of adaptive diet-induced thermo-

genesis has at times been controversial. The contro-

versy in part reflects the variability of the metabolic

responses to overfeeding. In laboratory animals

adaptive diet-induced thermogenesis appears to be

strain-dependent, and to vary with other factors

such as age, sex, and environmental temperature.

Thus it is generally more substantial in young than

in old animals, and in females than in males, and it is

inhibited at thermoneutral temperatures. The process

does not necessarily depend on the precise macronu-

trient content of the diet – it is the overfeeding rather

than the nature of the macronutrients that is import-

ant. None the less, adaptive diet-induced thermogen-

esis has been widely noted in animals fed low-protein

diets, and it is suggested that high-fat diets rich in

polyunsaturated fatty acids are more thermogenic

than those based on saturated and monounsaturated

fatty acids.

0018The extent to which adaptive diet-induced thermo-

genesis is present, and a component of the regulation

of energy balance, in humans is a matter of continu-

ing debate. While some evidence in support of the

concept is available, none is currently considered to

be convincing.

tbl0001 Table 1 Synonyms for the thermogenic response to food

Name

Short-term

Specific dynamic action

Thermic effect of food

Postprandial thermogenesis

Heat increment of feeding

Diet-induced thermogenesis (obligatory and adaptive)

Chronic (overfeeding)

Luxoskonsumption

Diet-induced thermogenesis (adaptive)

5764 THERMOGENESIS

Obesity

0019 Obesity has been a major focus in studies on the

regulation of energy balance, and some emphasis

has been given to the hypothesis that obesity results,

at least in part, from thermogenic abnormalities.

Some animal models of obesity (e.g., the obese ob/

ob mouse and the diabetic db/db mouse) exhibit a

reduced capacity for nonshivering thermogenesis and

under normal environmental conditions have a de-

creased energy expenditure on this process. Reduced

adaptive diet-induced thermogenesis is a general char-

acteristic of obesity in laboratory animals, and there

is little doubt that this abnormality is significant in

the energetics of the development of the obese condi-

tion, involving the recently discovered hormone

leptin.

0020 The situation is much more problematic in humans,

and there is little convincing evidence to implicate

reduced facultative thermogenesis (dietary or thermo-

regulatory) as an important factor in the development

of human obesity. A reduction in diet-induced thermo-

genesis in response to nutrients is, however, frequently

observed in the obese, and this appears to relate, at

least in part, to insulin resistance.

0021 Despite the absence of substantive evidence for

major thermogenic abnormalities in human obesity,

drugs which stimulate this component of energy ex-

penditure have been the focus of continuing interest

to the pharmaceutical industry as potential anti-

obesity agents. Such drugs are primarily b-adrenocep-

tor agonists, selective for the b

-adrenoceptor;

however, none has yet progressed beyond the research

and development phase.

0022 Recently, the concept of nonexercise activity

thermogenesis (NEAT) has been introduced and

this proposes that small movements such as fidgeting

and the maintenance of posture may contribute sig-

nificantly to energy expenditure in humans. The ab-

sence of NEAT in response to overfeeding has been

implicated as a factor in the etiology of obesity.

Biochemical Basis of Facultative

Thermogenesis

0023 A number of biochemical mechanisms have been

proposed for the generation of heat by faculative

thermogenesis – both nonshivering and adaptive

diet-induced forms. These mechanisms are associated

with a variety of tissues. Shivering is, of course, local-

ized in skeletal muscle. Mechanisms for facultative

thermogenesis can be divided into those in which

there is a direct hydrolysis of adenosine triphosphate

(ATP), and those in which the synthesis and hydroly-

sis of ATP are bypassed.

0024Thermogenic mechanisms where ATP is hydro-

lyzed include protein turnover, the pumping of Na

across the plasma membrane, and various substrate

(or ’futile’) cycles in specific metabolic pathways

(Table 2). Examples of substrate cycles include the

triacylglycerol/free fatty acid reesterification cycle

which can occur both within and between tissues,

and the fructose-6-phosphate/fructose-1,6-bisphos-

phate cycle in the glycolytic and gluconeogenic

pathways (when both occur within the same tissue).

Although there is good evidence that these mechan-

isms are thermogenic, individually their contribution

to overall heat production is small – with the excep-

tion of highly specific situations such as in the

warming of bumble bee flight muscle in cold environ-

ments. The simultaneous activation of a number of

substrate cycles may, however, lead to the production

of considerable amounts of heat.

Brown Adipose Tissue

0025The best documented and most widely recognized

thermogenic mechanism is one in which the synthesis

and hydrolysis of ATP are bypassed, namely the

proton conductance pathway in brown adipose

tissue. In this process the proton gradient that is

normally established across the mitochondrial inner

membrane during the oxidation of substrates is dissi-

pated as heat rather than being coupled to the synthe-

sis of ATP, in contrast to what generally occurs in

mitochondria from other tissues.

0026The proton conductance pathway is regulated by a

tissue-specific mitochondrial protein, termed uncoup-

ling protein or now uncoupling protein-1 (it has also

been called thermogenin), with a molecular weight of

tbl0002Table 2 Putative biochemical mechanisms of facultative

thermogenesis

Mechanism

Pumping of Na

across plasma membranes

Protein turnover

Mitochondrial Ca

cycle

a-Glycerophosphate shuttle

Substrate (or ’futile’ cycles), e.g., fructose-6-phosphate/

fructose-1,6-bisphosphate; glucose/glucose-6-phosphate;

triacylglycerol/free fatty acid reesterification

’Hot pipes’ in the microvasculature

Mitochondrial proton conductance pathway in brown adipose

tissue

Mitochondrial uncoupling through UCP2 and UCP3

The precise biochemical mechanism involved in this process is unclear.

Exclusive to brown adipose tissue; the other mechanisms may occur in a

number of tissues, particularly skeletal muscle.

Whether uncoupling protein-2 (UCP2) and UCP3 ’uncouple’ is a matter of

debate.

THERMOGENESIS 5765

32 kDa. The amount of this protein, and its activity,

varies according to the requirements for thermogen-

esis in an animal. Brown adipose tissue is particularly

well-developed in small animals, in the newborn of a

number of mammalian species (including humans

and ruminants), and in hibernators. It is also promin-

ent in small, cold-adapted animals and in rodents

exhibiting adaptive diet-induced thermogenesis

where a major activation takes place.

0027 The proton conductance pathway through uncoup-

ling protein-1 is unique to brown adipose tissue, and

this mechanism, together with the tissue itself, has

been the major focus of studies on thermogenic

systems in recent years. A recent major development

is the discovery that there are a family of mitochon-

drial uncoupling proteins, not just uncoupling

protein-1. Uncoupling protein-2 exhibits a wide

tissue distribution, being found even in the brain,

but it is particularly evident in white adipose tissue.

Uncoupling protein-3 is located primarily in skeletal

muscle.

0028 The initial view was that these new uncoupling

proteins uncouple mitochondria in a manner gener-

ally similar to uncoupling protein-1. However, it is far

from clear that this is the case and it is argued that

their role is more concerned with fat oxidation than

energy dissipation per se. This would be consistent

with the fact that brown adipose tissue expresses the

genes encoding uncoupling protein 1–3; a thermo-

genic role for 2 and 3 in the presence of uncoupling

protein-1 would seem superfluous.

Neural and Hormonal Control of Thermogenesis

0029 Catecholamines are the main stimulus for facultative

thermogenesis, both the dietary and nonshivering

forms. Heat production can be stimulated by norepin-

ephrine (noradrenaline) or epinephrine (adrenaline).

Brown adipose tissue is highly innervated by sympa-

thetic nerves, and norepinephrine secreted by them

plays a central role in both the acute and chronic

regulation of the thermogenic process.

0030 The thyroid hormones, thyroxine (T

) and triiodo-

thyronine (T

), have long been recognized to play

a permissive role in thermogenesis. They can

themselves also lead to a direct stimulation of heat

production, and have in the past been used for the

treatment of obesity. In brown adipose tissue, T

generated endogenously through the presence of a

type II iodothyronine 5

-deiodinase enzyme, and is

involved in the regulation of the expression of the

gene coding for the mitochondrial uncoupling

protein.

0031 Insulin also plays a significant role in facultative

heat production. Diabetic animals have an impaired

capacity for both nonshivering thermogenesis and

adaptive diet-induced thermogenesis. Insulin is

considered to stimulate thermogenesis through

a central activation of the sympathetic nervous

system, and by a direct effect on peripheral tissues.

The uncoupling protein content of brown adipose

tissue is regulated by insulin, and this in part in-

volves an interaction with the sympathetic nervous

system. Insulin resistance impairs the thermogenic

response to food in both animals and in humans.

The acute activation of brown adipose tissue in re-

sponse to cold is also inhibited by insulin resistance

in the tissue.

Glossary

Brown Adipose Tissue Form of adipose tissue spe-

cialized for the generation of heat, heat being the

primary product of its metabolism. Heat is generated

in brown adipose tissue through the presence of the

tissue-specific uncoupling protein-1.

Diet-Induced Thermogenesis Currently preferred

term to describe the increase in heat production

associated with feeding. Relates to both short-

term responses to a meal, and to chronic overfeed-

ing; divided into obligatory and adaptive com-

ponents.

Nonshivering Thermogenesis Heat generated spe-

cifically for thermoregulatory purposes, by mechan-

isms that do not involve muscle contraction.

Thermogenesis Generic term generally used to de-

scribe facultative heat production.

Uncoupling Proteins Family of mitochondrial pro-

teins of molecular weight 32 kDa; the first such pro-

tein, uncoupling protein-1, is responsible for the

generation of heat in brown adipose tissue. Uncoup-

ling protein-2 is expressed particularly in white adi-

pose tissue while uncoupling protein-3 is found

particularly in skeletal muscle.

See also: Adipose Tissue: Structure and Function of

Brown Adipose Tissue; Diabetes Mellitus: Etiology;

Chemical Pathology; Energy: Energy Expenditure and

Energy Balance; Hormones: Adrenal Hormones; Thyroid

Hormones; Obesity: Etiology and Diagnosis