Responses to protein dilution in the adult female rat

Obesity in C57BL/6J mice fed diets differing in carbohydrate and fat but not energy content

To investigate the contributions of carbohydrate and fat to obesity we measured the body weight, body composition and food intake of adult C57BL/6J mice fed ad libitum with various combinations of two semisynthetic diets that differed in carbohydrate and fat but not in protein, micronutrient or energy content. In Experiment 1, involving male mice, body weights were similar in groups fed diets comprised of (by energy) 20% protein, 75% carbohydrate and 5% fat (C75-F5) or 20% protein, 5% carbohydrate and 75% fat (C5-F75). However, mice fed a 50:50 composite mixture of the C75-F5 and C5-F75 diets (i.e., a C40-F40 diet) became substantially more obese. Mice that could choose between the C75-F5 and C5-F75 diets ate equal amounts of each diet and gained almost as much weight as did the group fed C40-F40 diet. Mice switched every day between the C75-F5 and C5-F75 diets gained no more weight than did those fed either diet exclusively. In Experiment 2, male and female mice were fed chow or one of 8 isocaloric diets that differed parametrically in carbohydrate and fat content. Groups fed diets in the middle of the range (i.e., C35-F45 or C45-F35) weighed significantly more and were significantly fatter than were those fed diets with more extreme proportions of carbohydrate and fat (e.g., C75-F5, C5-F75), an effect that was more pronounced in males than females. In Experiment 3 and 4, male mice fed versions of the C40-F40 formulation gained more weight than did those fed the C75-F5 or C5-F75 formulations irrespective of whether the carbohydrate was predominantly sucrose or predominantly starch, or whether the fat was vegetable shortening, corn oil, palm oil or canola oil; the type of carbohydrate or fat had little or no impact on body weight. In all four experiments, energy intakes differed among the diet groups but could not account for the differences in body weight. These results demonstrate that the proportion of carbohydrate and fat in the diet influences body weight independently of energy content, and that the type of carbohydrate or fat has little impact on body weight. Consuming carbohydrate and fat simultaneously or in close temporal proximity exacerbates obesity.

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.

Putting behavior back into feeding behavior: a tribute to George Collier

In an attempt to explain my failure to find any correlation between meal size and intermeal intervals in free-feeding rats, George Collier, my mentor, suggested that perhaps eating behavior had strong extra-physiological determinants, a heretical idea that had an enormous impact on my subsequent research. After moving to Cornell University, my students and I began a series of studies, first with animals, then with humans, all of which supported George's idea. We observed that, under our test conditions, humans (a) do not respond to overfeeding by reducing their food intake, (b) do not change the amount they consume at meals when snacks or breakfasts are added or subtracted from their daily intake, (c) do not change the amount they eat when the energy density of their food is changed, (d) nor do they increase their intake following a semi or total fast for 24 h. They also increase the amount they consume proportionally to the amount of food they are served, the variety of foods offered, and the number of people with whom they eat. The combination of these data with George's insightful idea, has merged into a modification of the popular Set-Point Theory of the regulation of body weight. The alternative "Settling Zone" Theory suggests that whereas biology may determine a range of body weights (adiposity) that are maintained fairly constant for long periods of time, within this "zone", the behaviors responsible for controlling energy intake and energy expenditure are influenced primarily by environmental and cognitive stimuli. The size of the "Settling Zone" is not currently known, but if it is 10% or greater, then efforts to identify and understand the environmental and cognitive stimuli that influence body weight may produce advances that will reduce our high rates of overweight and obesity.

Effects of pregnancy, sucrose, and various low-protein diets on the eating behavior of rats

To assess the effects of various low-protein diets on the eating behavior of female rats, 280 rats were assigned to one of 20 groups, according to a 5 x 2 x 2 (Protein Level x Pregnancy x Sugar) design. Each rat was offered one of five isocaloric, soy-based diets: 25%, 8%, 4%, 2% or 1% protein diet. The subjects' intakes and weights were recorded daily for 3 weeks. Protein content of the diet, pregnancy, and the presence of dietary sucrose all had significant effects on diet, sucrose, and protein intake. By the third week of the experiment, pregnant rats offered 1%, 2% or 4% protein diets consumed significantly less of their diet than did their nonpregnant controls, whereas pregnant rats in the 25% protein group consumed significantly more of their diet than did nonpregnant controls. The eating behavior of the rats offered the 8% protein diet was most affected by the presence of dietary sucrose. Pregnant rats in the 8% protein group consumed less of their diet than did nonpregnant controls when sucrose was also offered, but they consumed more of their diet than did nonpregnant controls when sucrose was not offered. A learned food aversion is probably the best explanation for the severe reduction of diet intake observed in pregnant rats in all of the 1%, 2%, and 4% protein groups, as well as in pregnant rats offered the 8% protein diet and sucrose.

Foraging for macronutrients: effects of protein availability and abundance

A laboratory simulation of foraging required rats to search for opportunities to obtain meals of pure-macronutrient foods (separate sources of carbohydrate, fat, and protein). Reductions in protein availability (increases in the cost of procuring protein meals) and reductions in protein abundance (decreases in the likelihood of encountering protein meal opportunities) were examined in separate phases. Rats foraged selectively, composing high-fat, adequate-protein diets; they tended to alternate between protein and nonprotein meals when protein was accessible. When protein became less available or relatively less abundant, meal patterns changed, and the proportional intake of protein fell. The tactic of shifting relative nutrient intake was used to a greater extent than the tactic of increasing foraging behavior. The foraging constraints revealed a flexible nutritional strategy that balanced responses to internal and external demands.

Selection of protein and fat by diabetic rats following separate dilution of the dietary sources

Diabetic and normal rats were allowed to select their diets from separate sources of protein, carbohydrate and fat. Following the determination of baseline intakes, diabetic and normal rats received dietary components in which either the protein (Experiment 1) or fat source (Experiment 2) was diluted by 25% or 50% by the addition of cellulose. Diabetic rats failed to maintain protein intake at both dilution levels, but made up for the loss of protein-derived calories by consuming more fat. Diabetic rats maintained fat intake at both dilution levels. Dietary dilutions had no effect on total caloric intakes or body weight gain of diabetic rats. Diabetic status, measured by fasting plasma glucose levels and urinary glucose excretion rates, also was unaffected by diet dilutions. These data suggest that diabetic rats maintain total caloric intake following dilution of either the protein or fat source of their diets, but defend intake of fat-derived calories more readily than protein-derived calories. Normal rats maintained both protein and fat intake at the 25% but not at the 50% dilution level. These findings are discussed in terms of the ability of diabetic rats to solve the metabolic problems associated with their diabetic condition.

Severe reduction in food intake by pregnant rats resembles a learned food aversion

To assess the effects of sucrose, low levels of dietary protein, and pregnancy on food intake, sixty-four female rats were assigned to eight groups in a 2 X 2 X 2 design. Food, water, and sucrose intake and rats' weights were measured daily. Pregnant rats offered sucrose with low protein (8%) diets did not maintain adequate weight gain during pregnancy; 25% of these subjects perished during the last days of pregnancy. These results have implications for human populations that have high birth rates and diets that are deficient in protein. A hormonal mechanism which regulates appetite or taste preferences during pregnancy is postulated and a "learned food aversion" interpretation is given for the data.

Self-selected intake of carbohydrate, fat, and protein by obese (ob/ob) and lean mice

Obese mice, self-selecting from three food cups containing carbohydrate, fat and protein supplemented with minerals and vitamins, selected the same proportion of energy from protein (approximately 20%) as did lean mice. Young obese mice also selected the same proportion of carbohydrate and fat as selected by lean mice, but adult obese mice self-selected a higher proportion of energy from fat and a lower proportion from carbohydrate than did lean mice. The high preference by adult obese mice for fat was confirmed in additional studies employing two food cups, one containing a high-carbohydrate diet and the other a high-fat diet. We speculate that the preference by adult obese mice for fat is related to their insulin resistance.