Ambient temperature and survival on a protein-deficient diet

What does self-selection of dietary proteins in rats tell us about protein requirements and body weight control?

Omnivores are able to correctly select adequate amounts of macronutrients from natural foods as well as purified macronutrients. In the rat model, the selected protein levels are often well above the requirements estimated from the nitrogen balance. These high intake levels were initially interpreted as reflecting poor control of protein intake, but the selected levels were later found to be precisely controlled for changes in dietary protein quality and adjusted for cold, exercise, pregnancy, lactation, age, etc. and therefore met physiological requirements. Several authors have also suggested that instead of a given level of protein intake, rodents regulate a ratio of protein to dietary carbohydrates in order to achieve metabolic benefits such as reduced insulin levels, improved blood glucose control, and, in the long term, reduced weight and fat gain. The objective of this review was to analyze the most significant results of studies carried out on rats and mice since the beginning of the 20th century, to consider what these results can bring us to interpret the current causes of the obesity pandemic and to anticipate the possible consequences of policies aimed at reducing the contribution of animal proteins in the human diet.

Low-protein diet-induced hyperphagia and adiposity are modulated through interactions involving thermoregulation, motor activity, and protein quality in mice

Low protein (LP)-containing diets can induce overeating in rodents and possibly in humans in an effort to meet protein requirement, but the effects on energy expenditure (EE) are unclear. The present study evaluated the changes induced by reducing dietary protein from 20% to 6%-using either soy protein or casein-on energy intake, body composition, and EE in mice housed at 22°C or at 30°C (thermal neutrality). LP feeding increased energy intake and adiposity, more in soy-fed than in casein-fed mice, but also increased EE, thus limiting fat accumulation. The increase in EE was due mainly to an increase in spontaneous motor activity related to EE and not to thermoregulation. However, the high cost of thermoregulation at 22°C and the subsequent heat exchanges between nonshivering thermogenesis, motor activity, and feeding induced large differences in adaptation between mice housed at 22°C and at 30°C.

Szilárd Donhoffer (1902–1999)

The authors summarize the main events in the long life of Szilárd Donhoffer and his importance in founding thermoregulatory research at Pécs, Hungary.

Standing on the shoulders of giants

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Growth, food intake and cold exposure in mice. 1. Cold exposure of adolescent mice

1. Adolescent mice were exposed from 33 to 84 days of age to a warm (30°C) or to one of four cold (15°C, 10°C, 5°C or 1°C) environments. Before and after exposure the mice were kept at a normal mouseroom temperature of 23°C. Controls remained at this temperature throughout. Voluntary food intake, live weight and growth efficiency were individually monitored before, during and after exposure, i.e. from 19 to 112 days of age.

2. Food intake during exposure was linearly related to temperature, ranging from 4 g/day at 30°C to 14 g/day at 1 °C. Intakes differed only slightly between males and females. Food intake at 10°C was 70% above controls. Oxygen consumption in adults at 10°C was 74% above controls.

3. Male growth was unaffected by temperature. Superiority over females (56% at 30°C) diminished at lower temperatures (12% at 1 °C) because females grew 41% faster at 1 °C than at 30°C. Females whose growth was depressed at 30°C, showed compensatory growth on returning to 23°C.

4. Growth efficiency was strongly, but not entirely linearly, related to temperature, and increased approximately three-fold between 1°C and 30°C. Efficiency was maximal at 30°C in males and at 23 °C in females. Males grew 76% more efficiently than females at 30°C and only 12% more efficiently at 1°C.

5. Increased food intake in the cold satisfied energy requirements and allowed females to grow faster.

6. Individual differences in appetite persisted throughout life, but rankings in growth and efficiency changed with age. Food intake was strongly correlated with growth rate in weanlings, but growth rate was always the main determinant of growth efficiency.

7. The variance of adolescent growth rate was unaffected by the treatments.

Selected contribution: ambient temperature for experiments in rats: a new method for determining the zone of thermal neutrality

There is a misbelief that the same animal has the same thermoneutral zone (TNZ) in different experimental setups. In reality, TNZ strongly depends on the physical environment and varies widely across setups. Current methods for determining TNZ require elaborate equipment and can be applied only to a limited set of experimental conditions. A new, broadly applicable approach that rapidly determines whether given conditions are neutral for a given animal is needed. Consistent with the definition of TNZ [the range of ambient temperature (T(a)) at which body core temperature (T(c)) regulation is achieved only by control of sensible heat loss], we propose three criteria of thermoneutrality: 1) the presence of high-magnitude fluctuations in skin temperature (T(sk)) of body parts serving as specialized heat exchangers with the environment (e.g., rat tail), 2) the closeness of T(sk) to the median of its operational range, and 3) a strong negative correlation between T(sk) and T(c). Thermocouple thermometry and liquid crystal thermography were performed in five rat strains at 13 T(a). Under the conditions tested (no bedding or filter tops, no group thermoregulation), the T(a) range of 29.5-30.5 degrees C satisfied all three TNZ criteria in Wistar, BDIX, Long-Evans, and Zucker lean rats; Zucker fatty rats had a slightly lower TNZ (28.0-29.0 degrees C). Skin thermometry or thermography is a definition-based, simple, and inexpensive technique to determine whether experimental or housing conditions are neutral, subneutral, or supraneutral for a given animal.

Developmental consequences of diet and activity

The effect of a protein-deficient and a protein-surfeit diet and continuous access to an activity wheel on food intake, growth, and body temperatures of behaviorally thermoregulating White Leghorn chicks was assessed in two experiments. In Experiment 1, both imbalanced-protein diets depressed intake and growth and differentially affected activity relative to a control diet, but activity did not ameliorate the deleterious effect of a high-protein diet on growth. Diet groups with continuous access to a running wheel did not differ on any measure from corresponding inactive dietary control groups. In Experiment 2, these results were replicated in a lower ambient temperature, and an effect of diet on body temperature emerged. Diets that affected spontaneous activity or body temperature also affected death feigning, a predation defense behavior. The data from behaviorally thermoregulating chicks are consistent with previous findings that activity does not depress growth rate in animals who cannot convert a portion of their intake into adipose tissue.

Interactions between major nutrients in the diet and the lactational performance of rats

The effect on lactational performance of replacing feed carbohydrate with fat at two different protein levels was studied. Lactating Sprague-Dawley rats with a standardized litter size of thirteen pups were allocated one of eight feeds containing either 300 or 150 g protein/kg organic matter (OM) and ranging in fat content from 100 to 550 g/kg OM from day 2 until day 14 of lactation. Daily food intake, live-weight gains, and changes in body composition of both dams and litters were measured. Feeds of low protein content resulted in a significant decline (P < 0.001) in lactational performance despite a significant increase (P < 0.001) in maternal protein mobilization. Maternal lipid mobilization was not significantly affected by feed composition. Litter lipid gain was significantly increased (P < 0.05) as fat replaced carbohydrate in the high-protein feeds, due to an increase in maternal energy intake. In contrast, lactational performance was severely depressed (P < 0.001) as fat replaced carbohydrate in the low-protein feeds. This interaction between feed components on lactational performance was in accordance with the hypothesis that the heat production of lactating rats is maximal and, hence, constraining intake.

The effect of constant ambient temperature and ration on the performance of sexed broilers

Two trials involving 480 Cobb color-sexed broiler chicks were conducted to determine the effect of various constant ambient temperatures on the performance of broilers. Temperatures in Trial 1 were 18 and 29 C and in Trial 2 were 24 and 35 C. The interacting effect of dietary energy (3.142 or 3.252 kcal ME/g of diet) and protein (16, 19, or 22%) on performance criteria was also examined within each trial. There was no indication of selective consumption of any of the ratios at any temperature. Differences in feed consumption observed in either trial were totally contributed by temperature effect. Within a trial, and irrespective of temperature treatment, the rate of growth and feed consumption of the females were less than that of the males. Males and females responded equally to the ambient temperature; there was no significant sex X temperature interaction in Trials 1 or 2.

The effect of a low-protein diet and a cold environment on calorie intake and body composition in the rat

1. Rats 10 weeks old were fed for 9 weeks either on a stock diet containing 17% protein, or on a low-protein diet prepared from the stock diet with added glucose, minerals and vitamins. Half the animals on each diet were kept at room temperature (21°) and half in a cold environment (5°).

2. The calorie intake of the animals kept at 5° on both diets was 60–70% higher than that of the corresponding group at 21°. The animals on the stock diet and kept at 5° gained weight but not so much as those on the same diet at 21°. The animals kept on the low-protein diet at 21° lost weight, while those on the same diet at 5° lost only a little weight initially and none thereafter.

3. On both types of diet the liver, kidneys and gastro-intestinal tract weighed more per 100 g body-weight in animals kept in the colder of the two environments; the small intestine was conspicuous in this respect.

4. The weight of the fur was greater, and the weight of the skin less per 100 g body-weight at 5° than at 21°.

5. The animals on the stock diet at 21° had most fat in their bodies, both in absolute terms and per 100 g body-weight. There were no significant differences between the other three groups.

6. The skin of the animals kept at 5° had a significantly higher collagen to N ratio than the skin of those having the same diet at 21°.