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10. Effects of Heat on Appetite
Pages 187-214

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From page 187...
... This literature has been supplemented by a survey designed specifically for this chapter (consumer survey, University of Toronto, unpublished data, 19911. Because of the anticipation that the scientific literature, especially on humans, might be skimpy, a questionnaire was sent to a number of restaurant and grocery chains in the metropolitan Toronto area asking about shifts in customer purchasing behavior as a function of environmental heat.
From page 188...
... A winter trip to the tropics probably represents a greater short-term shift in temperature than might be encountered if one stayed put; how does it affect appetite? To further complicate matters, how hot one feels is not simply a matter of the environment; one's own activity may generate heat, so that being active may be functionally equivalent to raising the environmental temperature.
From page 189...
... ENVIRONMENTAL TEMPERATURE, EATING, AND THERMOREGULATION Any discussion of the effects of heat on eating must begin with a recognition that eating represents the basic means of securing energy for humans. Most analyses of heat and eating go one step further and point out that a major physiological concern of humans is thermoregulation—the maintenance of an appropriate body temperature and that eating provides a major contribution to maintaining body heat (Brobeck, 19481.
From page 190...
... body temperature independent of environmental temperatureas in fever and determine whether this form of hyperthermia suppresses appetite. Alternatively, one might manipulate the need to acquire or dissipate energy more indirectly, through exercise; exercise, by providing a shortterm boost in internal heat, ought to reduce the need for further energy in short, appetite should be suppressed.
From page 191...
... The more complicated interpretation is that heat lowers BW set-point and that appetite subsides because the animal's current weight is now excessive relative to set-point. Hyperthermia following excessive eating may contribute to the decline in intake, but increased heat dissipation and decreased intake may both be understood as mechanisms in the service of attaining a lowered level of BW, which in turn may be a mechanism in the service of more efficient thermoregulation in the heat.
From page 192...
... Hypothalamic disturbances that produce substantial weight gain may do so at least partially by suppressing the heat dissipation by brown adipose tissue (BAT) that normally follows a meal (Hogan et al., 1986)
From page 193...
... The prudent recommendation for heat exposure would seem to be to allow for reduced intake but to avoid, as much as possible, strenuous activity, which not only requires more energy but also generates more undesirable heat, and which also puts fluid balance in jeopardy. Reduced activity is a natural response to heat exposure.
From page 194...
... One might be tempted to suggest underfeeding troops in hot climates in order to minimize their problems with heat. Although a somewhat reduced intake is probably desirable and inevitable, given the various regulatory pressures that are activated automatically, deliberate food restriction below what the troops naturally desire would probably not be desirable, owing to all the negative effects of maintaining a suboptimal body weight.
From page 195...
... The main threat facing an individual in the hot climate would seem to be a failure of thermoregulation: heat dissipation is the main concern. Yet if the nighttime temperature drops precipitously, as may happen in the desert, then heat adaptation during the day may be more than cancelled out by cold adaptation at night.
From page 196...
... Stress Under acute stress, body temperature may rise. For example, boxers before a bout have a higher body temperature than they do before a routine practice.
From page 197...
... Conversely, cold food ought to minimize the thermic effect of eating. Indeed, one gets the impression that if the food is served at a temperature significantly below body temperature it will have a cooling effect; this cooling effect should be more reinforcing for people who are hyperthermic.
From page 198...
... Yet there appears to be only a small variation in the thermic effect of food depending on the type of food ingested. "Eating proteins, which are somewhat more complicated to break down inside the body than carbohydrates or fats are, tends to raise body temperature very slightly more than these other two basic food components do" (Belier, 1977~.
From page 199...
... People do not have the appetite for a large heavy meal when it is hot." Belter (1977) neatly summarized the effect of heat on appetite, explaining it on the basis of the thermic effect of food: The ability to raise body temperature through feeding is one that is shared by all warm-blooded animals.
From page 200...
... The finding suggests that if one spends most of one's time in the heat but eats in an artificially cooled environment, one might end up eating more than needed, with potential problems for heat dissipation when one returns to the hot environment. Nonhumans Research on the effects of variations in environmental temperature on feeding was stimulated by Brobeck's (1948)
From page 201...
... and frequently lose weight in hotter ambient temperatures. Kraly and Blass (1976)
From page 202...
... To extrapolate to the presumably reduced gastric emptying rate in animals exposed to the heat, one might speculate that the slowing of the digestive process is a means of muting the thermic effect of food. Seasonal Elects Casual inquiry yields a broad consensus that BW declines in the summer and rises in the winter.
From page 203...
... The possibility remains, of course, that fever might not represent true hyperthermia but rather a resetting of the thermoregulatory set-point at a higher level (Mrosovsky, 19901; in this case, the body might "want" to maintain a higher temperature, and a decline in feeding would not be expected. Intraperitoneal injection of interleukin-l, normally released in the presence of pathogens, raises body temperature and ordinarily is associated with appetite suppression; but when injected intracerebroventricularly, interleukin1 raises body temperature in rats without affecting intake (McCarthy et al., 19861.
From page 204...
... Holding intake constant, one might expect hot food to suppress appetite by suppressing gastric emptying rate, just as exposure to cold environments speeds gastric emptying, as shown above. Or hot food might suppress appetite by raising body temperature and inducing satiety.
From page 205...
... It is tantalizing to imagine that bright sunlight might contribute to the appetite suppression observed in hot environments; however, there is essentially no evidence that normal control subjects' appetites are affected by light exposure. Rats show a transient decline in appetite when exposed to constant light (Dark et al., 19801; but rats are nocturnal feeders, so the extended presence of light would be expected to disrupt feeding briefly, independent of profound physiological changes.
From page 206...
... In fact, one ice cream chain reported that while ice cream sales tended to rise with increases in environmental temperature from 72° to 82°F, above 82°F customers switch to more thirst-quenching products (for example, ices and "light" ice creams)
From page 207...
... · Studies of the thermic effects of food at different temperatures. · Studies of the palatability of foods at different temperatures while manipulating environmental temperature and, if possible, body temperature.
From page 208...
... Practical Recommendations for Working in Hot Environments In general, work in a hot environment demands close attention to factors that might threaten either thermoregulation or BW regulation. In the absence of knowledge about whether these regulatory mechanisms act in conflict or in concert in hot environments, it is probably safest to focus on thermoregulation, which poses the most immediate physiological challenge.
From page 209...
... Vonotzky 1947 The effect of environmental temperature on food selection.
From page 210...
... Rosenthal 1990 Effects of bright light on resting metabolic rate in patients with seasonal affective disorder and control subjects.
From page 211...
... Albu, and F.X. Pi-Sunyer 1987 Thermic effect of food and exercise in lean and obese men of similar lean body mass.
From page 212...
... PARTICIPANT: Mike Sawka mentioned yesterday that discomfort is closely related to skin temperature. I wonder if you think that a feeling of discomfort in the heat would also affect appetite, and if that might be part of the appetite suppression mechanism as opposed to solely internal body temperature?
From page 213...
... EFFECTS OF HEAT ON APPETITE 213 We know that heating deep core organs like the hypothalamus in animals will have the same effect. So my guess would be that almost anywhere along the chain that you apply heat, it is likely to have a suppressive effect on appetite.


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