Food Intake, Appetite, and Work in Hot Environments
Allison A. Yates1
Research to determine the specific internal metabolic mechanisms by which environmental heat affects appetite has focused on measurement of changes in intake induced in animal models in artificial hot environments. External ambient temperature and body temperature, and the regulation thereof, have been looked at in detail in animal models because food intake has been shown to markedly decrease in hot environments in all species of animals studied (Young, 1987), and usually in humans (Mitchell and Edman, 1951).
The prime theory ascribed to the mechanism by which heat decreases food intake (and therefore appetite) involves thermoregulation and the thermic effect of food. If continued consumption of normal intakes occurs under heat stress conditions, the additional heat required to be dissipated by the normal amounts ingested may result in an inability to dissipate heat adequately. In a series of experiments by Hamilton (1963a), rats, upon exposure to a temperature of 35°C, ate only 2 grams of food during the first 24 hours, compared with a previous intake of more than 20 grams at 24°C; mild (32°C) and severe (35°C) heat stress over 21 days resulted in a contin-
ued lower level of food intake. The marked decrease initially was thought to be due to the initial dehydration; the continued lower level of intake was due to the adaptation to the increased ambient temperature. Body weights in the growing rats dropped initially by as much as 30 grams and then remained constant until the heat stress was removed. These studies have been used as a demonstration of the concept of a body weight set-point lowering effect due to a hot environment (Thompson, 1980).
The "thermostatic" theory of food intake (a decrease in the body weight or fat set-point in response to hot environments) has been proposed as the method by which the body may thermoregulate, in part by decreasing the insulating amount of body fat (Brobeck, 1948). Studies in a number of experimental animals demonstrate a cessation of eating at high temperatures, which indicates that continued eating would probably lead to hyperthermia. The decrease in food intake is thus followed by a decrease in body weight and fat (Jakubczak, 1976).
In investigations of the theory that animals stop eating to prevent hyperthermia, the differences in the resulting thermic effect of food ingested (specific dynamic action) have been implicated. In the series of experiments by Hamilton (1963a), calorie intake of rats fed special diets during mild heat stress was inversely related to the thermogenic effect of the diet selected. It appears that fats may be the preferred energy source in heat stress (Salganik, 1956), and that in severe heat stress, protein is avoided due to the comparatively high amount of heat created (Hamilton, 1963a). With this theory, body temperature should be highly correlated with hunger and satiety. However, there is no consistent observed relationship between the two. Although in 1936 Booth and Strang reported that skin temperature in adults increased 2°C within 10 minutes of eating a high protein meal, a postprandial increase in skin temperature was not found by Stunkard et al. (1962). In dogs, Passmore and Ritchie (1957) found an extremely small rise in skin temperature after a high protein meal while Hamilton (1963a) determined that food consumption and rectal temperature in rats decreased incrementally with temperatures between 7° and 32 °C, but at 35°C, rectal temperature became elevated, while food intake continued to decrease.
Andersson and Larsson (1961) have shown that heating of the preoptic and anterior hypothalamic regions of the brain (areas that are known to be involved in regulating body temperature) inhibits feeding in animals. Opposite results, however, were obtained by Spector et al. (1968) in rats, when heating of the preoptic medialis region caused increased eating when the temperature of the area was raised to 43°C. Decreased eating occurred in their study when the ambient temperature was raised to 35°C. Local temperature in the anterior hypothalamic area has been reported to drop at the onset of feeding, which is opposite of what would be expected (Hamilton, 1963b). Thus it appears that the effect of brain temperature on feeding may
be more a result of external ambient temperature than of localized temperature changes and may be due to the rate of heat flow from the core to the periphery or vice versa, as no single temperature appears to uniquely govern the level of food intake (Spector et al., 1968).
Osmotic factors have also been shown to affect food intake in animals. Ingestion or intubation of hypertonic saline or glucose solutions results in decreased food intake in rats (Ehman et al., 1972; Kozub, 1972). However, intravenous administration of hypertonic infusions resulted in decreased food intake in rats only when the hyperosmolar solution was sodium chloride, but did not affect food intake when the solution was glucose or xylose (Yin and Tsai, 1973). As reviewed by Thompson (1980), this observed decrease in food intake serves as a protective mechanism that is demonstrated under conditions of total water deprivation, which significantly reduces food intake in most species studied, including pigeons (Ziegler et al., 1972). Ad lib food intake dropped to half in rats during a 24-hour period without water (Cizek and Nocenti, 1965), thus demonstrating that food intake and fluid balance are directly related. It appears that observations of decreased food intake in unacclimatized people in tropical climates may, to a large extent, be mediated by hypertonicity associated with initial dehydration, and improve as acclimatization occurs (Bass et al., 1955).
Not only is the stress of a hot environment due to thermoregulation and maintenance hydration, but it may be due to psychic stress as well. Such stress may be initiated by the degree of mental discomfort caused by the heat. Thus the impact of the need to (a) physiologically maintain thermoneutrality, (b) maintain normal hydration in spite of profuse sweating, and (c) feel comfortable in the heat may each affect the individual's appetitie and his/her perceived hunger to a different degree. Researchers cannot distinguish the difference between appetite and hunger in animals due to the lack of methods to communicate feelings; in humans, such information may be important in determining appropriate mechanisms for maintaining body weight and health status in prolonged exposure to heat. If an additional stress due to the situation occurs, such as that resulting from fear of death (as found in war or military conflict), then there may be additive effects on the desire to eat (appetite) or the perceived need to eat (hunger).
OBSERVATIONAL DATA ON INTAKE
A few studies (conducted primarily in foreign countries) do exist in which food intake of adults in hot and/or humid environments has been studied in isolated work environments. Edholm and Goldsmith (1966) reported their study in Bahrain and in the United Kingdom in which two groups of military men were followed in a carefully controlled environment. One group had spent a year in Bahrain prior to the experiment, while
the second group was first studied for 12 days in the United Kingdom and then flown to Bahrain where it was joined by the first group. All men then spent the first 4 days in hard work, the next 4 days in mainly sedentary work, and the final 4 days in hard work in tents and outdoors. Both groups then returned to the United Kingdom for a repeat of the 12-day protocol. The daytime temperature in Bahrain rarely fell below 30°C, with a relative humidity of 40 to 90 percent. Energy balance was measured, and similar food was provided to both groups in all settings. The mean food intake in Bahrain was approximately 25 percent less than in the United Kingdom; however, the percentage of calories from fat and carbohydrate was similar, as was the percentage of calories from protein. Both groups lost weight during the 12 days in Bahrain, with the unacclimatized group losing 2.5 kg in 12 days and the acclimatized group losing 1.1 kg. Because weight loss was not quickly recovered upon returning to the United Kingdom, it was thought that the caloric deficit was responsible for the majority of the weight lost in the hot environment. It is apparent, though, that those men previously adapted to the hot environment were less affected by the work schedule, perhaps due to decreased acute dehydration.
Balance studies conducted on an oil tanker in the South Atlantic and the Persian Gulf during the summer season with six male subjects, two of whom were crew members that were involved in heavy work, did not show any directional changes in food intake as a result of heat or acclimatization (Collins et al., 1971; Eddy et al., 1971a,b). However, since the protocol was changed during the study to increase the exercise levels of those who did not initially participate in heavy work, it is difficult to determine whether a decrease in food intake was masked by the increase in energy expenditure in three of the subjects. A number of military studies conducted by the U.S. Department of Defense have looked at garrison and field feeding, food choices and food waste, in addition to tests of the rations developed (Consolazio et al., 1960; Hirsch et al., 1984; Johnson et al., 1947, 1983; Kretsch et al., 1979, 1984, 1986a,b; Richardson et al., 1979). These have all been conducted in only one season, usually fall or spring; thus comparative information regarding summer food choices is not available. A confounding factor in such studies is the presence of air conditioning, which might in itself alter food preferences and intake depending on the length of time the individual is in a conditioned environment where meals are consumed.
A few studies have looked at seasonal body compositional changes and found that there is a decrease in caloric intake and a corresponding decrease in body weight/fat during the summer season as compared to winter. Some studies have also evaluated nutrient intake in adults based on season of the year in hot environments; decreased intake of some vitamins has been reported, such as vitamin A and C (Aldashev et al., 1986), and protein, vita-
min C, and total energy (Mommadov and Grafova, 1983), but such studies did not evaluate changes in food preferences or appetite.
Empirical data, based on observations and practices in food service, in both the military and the commercial sectors, indicate a change in food preferences during seasons associated with elevated mean temperature. Few basic studies have attempted specifically to address changing food patterns in self-selected diets due to season. In a study of seasonal variations in self-selected lunches in a large employee cafeteria in Maryland, Zifferblatt et al. (1980) found decreased selection of starches and cooked vegetables, with increased purchases of fruits, salads, yogurt, and cottage cheese, as the noon-time temperature rose (significant at p < 0.05). As the temperature increased, average caloric purchases tended to decrease (p < 0.0529). It should be noted that the workplace cafeteria, along with the work areas of most of the employees, was kept at 22.2°C (72°F). Therefore, the external temperature may have only moderately influenced appetite.
National surveys have been conducted on food consumption patterns of Americans, but they have not recently gathered data on the same individuals or individuals in similar geographic, and thus environmental, areas at different times of the year to determine if seasonal variation, and consequently changes in temperature, affects appetite (resulting in changes in food intake) or food selection patterns. The 1965–1966 Food Consumption Survey (USDA, 1972) compared household food purchases by season; foods that increased in the summer survey included fresh salad ingredients (tomatoes, lettuce, cucumbers, and so on), salad dressings, cookies, and frozen milk desserts, rice, bread, ground beef, lunch meat, chicken, shellfish, sugar, fresh corn, fresh cantaloupes, other fresh fruits, carbonated beverages, fruit drinks, and alcoholic beverages. Decreased food purchases included fresh milk, table fats, flour, hot cereal, beef roast, sausage, potatoes, fresh dark green leafy vegetables, fresh deep yellow vegetables, oranges, canned vegetables, canned fruit, and soup. Accordingly individuals alter their purchasing behavior during the year, to some extent based on availability and price of food items. Whether appetite (the desire to eat) also changes is unknown from these data.
CONCLUSIONS AND RESEARCH RECOMMENDATIONS
Based on human studies that have documented voluntary decreased food intake in individuals in hot environments and animal studies that have supported the concept of decreased food intake as an adaptive mechanism to ameliorate the increased need for thermoregulation, optimal nutrition is compromised if intake decreases to the extent that inadequate levels of key nutrients are consumed. The following areas of study are recommended in order to determine the exact impact of hot environments on appetite:
Studies regarding self-selected food patterns of individuals engaged in similar activity in hot versus temperate climates
Studies that determine the effect of stress on appetite, with temperature as a major variable
When such information is available, then it should be possible to
develop basic recommendations for types of foods that should be part of rations in hot environments, and
determine if specific supplements with improved palatability should be used when troops are in hot environments where depressed appetite for prolonged periods may prevent adequate nutrient intake.
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