Dietary self-selection in cycling and neonatally ovariectomized rats

The Metabolic Benefits of Menopausal Hormone Therapy Are Not Mediated by Improved Nutritional Habits. The OsteoLaus Cohort

Menopause alters body composition by increasing fat mass. Menopausal hormone therapy (MHT) is associated with decreased total and visceral adiposity. It is unclear whether MHT favorably affects energy intake. We aimed to assess in the OsteoLaus cohort whether total energy intake (TEI) and/or diet quality (macro- and micronutrients, dietary patterns, dietary scores, dietary recommendations)—evaluated by a validated food frequency questionnaire—differ in 839 postmenopausal women classified as current, past or never MHT users. There was no difference between groups regarding TEI or consumption of macronutrients. After multivariable adjustment, MHT users were less likely to adhere to the unhealthy pattern ‘fat and sugar: Current vs. never users [OR (95% CI): 0.48 (0.28–0.82)]; past vs. never users [OR (95% CI): 0.47 (0.27–0.78)]. Past users exhibited a better performance in the revised score for Mediterranean diet than never users (5.00 ± 0.12 vs. 4.63 ± 0.08, p < 0.04). Differences regarding compliance with dietary recommendations were no longer significant after adjustment for covariates. Overall, these results argue against a major role of TEI and diet quality as possible mediators of the MHT metabolic benefits. Future research on this relationship should focus on other potential targets of MHT, such as resting energy expenditure and physical activity.

Effects of Estrogens on Central Nervous System Neurotransmission: Implications for Sex Differences in Mental Disorders

Nearly one of every five US individuals aged 12 years old or older lives with certain types of mental disorders. Men are more likely to use various types of substances, while women tend to be more susceptible to mood disorders, addiction, and eating disorders, all of which are risks associated with suicidal attempts. Fundamental sex differences exist in multiple aspects of the functions and activities of neurotransmitter-mediated neural circuits in the central nervous system (CNS). Dysregulation of these neural circuits leads to various types of mental disorders. The potential mechanisms of sex differences in the CNS neural circuitry regulating mood, reward, and motivation are only beginning to be understood, although they have been largely attributed to the effects of sex hormones on CNS neurotransmission pathways. Understanding this topic is important for developing prevention and treatment of mental disorders that should be tailored differently for men and women. Studies using animal models have provided important insights into pathogenesis, mechanisms, and new therapeutic approaches of human diseases, but some concerns remain to be addressed. The purpose of this chapter is to integrate human and animal studies involving the effects of the sex hormones, estrogens, on CNS neurotransmission, reward processing, and associated mental disorders. We provide an overview of existing evidence for the physiological, behavioral, cellular, and molecular actions of estrogens in the context of controlling neurotransmission in the CNS circuits regulating mood, reward, and motivation and discuss related pathology that leads to mental disorders.

Effects of the menopausal transition on dietary intake and appetite: a MONET Group Study

Objective

The purpose of this study was to investigate changes in dietary intake and appetite across the menopausal transition.

Methods

This was a 5-year observational, longitudinal study on the menopausal transition. The study included 94 premenopausal women at baseline (age: 49.9 ± 1.9 yrs; BMI: 23.3 ± 2.3 kg/m²). Body composition (DXA), appetite (visual analogue scale), eating frequency, energy intake (EI) and macronutrient composition (7-day food diary and buffet-type meal) were measured annually.

Results

Repeated-measures analyses revealed that total EI and carbohydrate intake from food diary decreased significantly over time in women who became postmenopausal by year 5 (P > 0.05) compared to women in the menopausal transition. In women who became postmenopausal by year 5, fat and protein intakes decreased across the menopausal transition (0.05 > P < 0.01). Although a decrease in % fat intake was observed during the menopausal transition (P < 0.05), this variable was significantly increased in the postmenopausal years (P < 0.05). Spontaneous EI and protein intake also declined over time and were higher in the years preceding menopause onset (P < 0.05). Desire to eat, hunger and prospective food consumption increased during the menopausal transition and remained at this higher level in the postmenopausal years (0.05 > P< 0.001). Fasting fullness decreased across the menopausal transition (P < 0.05).

Conclusion

These results suggest that menopausal transition is accompanied with a decrease in food intake and an increase in appetite.

Sex differences in the physiology of eating

Hypothalamic-pituitary-gonadal (HPG) axis function fundamentally affects the physiology of eating. We review sex differences in the physiological and pathophysiological controls of amounts eaten in rats, mice, monkeys, and humans. These controls result from interactions among genetic effects, organizational effects of reproductive hormones (i.e., permanent early developmental effects), and activational effects of these hormones (i.e., effects dependent on hormone levels). Male-female sex differences in the physiology of eating involve both organizational and activational effects of androgens and estrogens. An activational effect of estrogens decreases eating 1) during the periovulatory period of the ovarian cycle in rats, mice, monkeys, and women and 2) tonically between puberty and reproductive senescence or ovariectomy in rats and monkeys, sometimes in mice, and possibly in women. Estrogens acting on estrogen receptor-α (ERα) in the caudal medial nucleus of the solitary tract appear to mediate these effects in rats. Androgens, prolactin, and other reproductive hormones also affect eating in rats. Sex differences in eating are mediated by alterations in orosensory capacity and hedonics, gastric mechanoreception, ghrelin, CCK, glucagon-like peptide-1 (GLP-1), glucagon, insulin, amylin, apolipoprotein A-IV, fatty-acid oxidation, and leptin. The control of eating by central neurochemical signaling via serotonin, MSH, neuropeptide Y, Agouti-related peptide (AgRP), melanin-concentrating hormone, and dopamine is modulated by HPG function. Finally, sex differences in the physiology of eating may contribute to human obesity, anorexia nervosa, and binge eating. The variety and physiological importance of what has been learned so far warrant intensifying basic, translational, and clinical research on sex differences in eating.

Increased visceral fat and decreased energy expenditure during the menopausal transition

OBJECTIVE

This study assessed longitudinal changes in body composition, fat distribution and energy balance in perimenopausal women. We hypothesized that total fat and abdominal body fat would increase at menopause due to decreased energy expenditure (EE) and declining estrogen, respectively.

DESIGN

Observational, longitudinal study with annual measurements for 4 years.

SUBJECTS

Healthy women (103 Caucasian; 53 African-American), initially premenopausal. During follow-up, lack of menstruation for 1 year and follicle-stimulating hormone >30 mIU ml(-1) defined a subject as postmenopausal.

MEASUREMENTS

Fat and lean mass (dual-energy X-ray absorptiometry), visceral (VAT) and subcutaneous abdominal fat (SAT) (computed tomography), dietary intake (4-day food record), serum sex hormones and physical activity (tri-axial accelerometry). Twenty-four hour EE was measured by whole-room calorimeter in a subset of 34 women at baseline and at year 4.

RESULTS

Body fat and weight increased significantly over time only in those women who became postmenopausal by year 4 (n=51). All women gained SAT over time; however, only those who became postmenopausal had a significant increase in VAT. The postmenopausal group also exhibited a significant decrease in serum estradiol. Physical activity decreased significantly 2 years before menopause and remained low. Dietary energy, protein, carbohydrate and fiber intake were significantly higher 3-4 years before the onset of menopause compared with menopause onset. Twenty-four hour EE and sleeping EE decreased significantly with age; however, the decrease in sleeping EE was 1.5-fold greater in women who became postmenopausal compared with premenopausal controls (-7.9 vs -5.3%). Fat oxidation decreased by 32% in women who became postmenopausal (P<0.05), but did not change in those who remained premenopausal.

CONCLUSION

Middle-aged women gained SAT with age, whereas menopause per se was associated with an increase in total body fat and VAT. Menopause onset is associated with decreased EE and fat oxidation that can predispose to obesity if lifestyle changes are not made.

Increased caloric intake on a fat-rich diet: role of ovarian steroids and galanin in the medial preoptic and paraventricular nuclei and anterior pituitary of female rats

Previous studies in male rats have demonstrated that the orexigenic peptide galanin (GAL), in neurones of the anterior parvocellular region of the paraventricular nucleus (aPVN) projecting to the median eminence (ME), is stimulated by consumption of a high-fat diet and may have a role in the hyperphagia induced by fat. In addition to confirming this relationship in female rats and distinguishing the aPVN-ME from other hypothalamic areas, the present study identified two additional extra-hypothalamic sites where GAL is stimulated by dietary fat in females but not males. These sites were the medial preoptic nucleus (MPN), located immediately rostral to the aPVN, and the anterior pituitary (AP). The involvement of ovarian steroids, oestradiol (E(2)) and progesterone (PROG), in this phenomenon was suggested by an observed increase in circulating levels of these hormones and GAL in MPN and AP with fat consumption and an attenuation of this effect on GAL in ovariectomised (OVX) rats. Furthermore, in the same four areas affected by dietary fat, levels of GAL mRNA and peptide immunoreactivity were stimulated by E(2) and further by PROG replacement in E(2)-primed OVX rats and were higher in females compared to males. Because both GAL and PROG stimulate feeding, their increase on a fat-rich diet may have functional consequences in females, possibly contributing to the increased caloric intake induced by dietary fat. This is supported by the findings that PROG administration in E(2)-primed OVX rats reverses the inhibitory effect of E(2) on total caloric intake while increasing voluntary fat ingestion, and that female rats with higher GAL exhibit increased preference for fat compared to males. Thus, ovarian steroids may function together with GAL in a neurocircuit, involving the MPN, aPVN, ME and AP, which coordinate feeding behaviour with reproductive function to promote consumption of a fat-rich diet at times of increased energy demand.

Modulation of appetite by gonadal steroid hormones

Several sex differences in eating, their control by gonadal steroid hormones and their peripheral and central mediating mechanisms are reviewed. Adult female rats and mice as well as women eat less during the peri-ovulatory phase of the ovarian cycle (estrus in rats and mice) than other phases, an effect under the control of cyclic changes in estradiol secretion. Women also appear to eat more sweets during the luteal phase of the cycle than other phases, possibly due to simultaneous increases in estradiol and progesterone. In rats and mice, gonadectomy reveals further sex differences: orchiectomy decreases food intake by decreasing meal frequency and ovariectomy increases food intake by increasing meal size. These changes are reversed by testosterone and estradiol treatment, respectively. A variety of peripheral feedback controls of eating, including ghrelin, cholecystokinin (CCK), glucagon, hepatic fatty acid oxidation, insulin and leptin, has been shown to be estradiol-sensitive under at least some conditions and may mediate the estrogenic inhibition of eating. Of these, most progress has been made in the case of CCK. Neurons expressing estrogen receptor-alpha in the nucleus tractus solitarius of the brainstem appear to increase their sensitivity to CCK-induced vagal afferent input so as to lead to an increase in the satiating potency of CCK, and consequently decreased food intake, during the peri-ovulatory period in rats. Central serotonergic mechanisms also appear to be part of the effect of estradiol on eating. The physiological roles of other peripheral feedback controls of eating and their central mediators remain to be established.

Is the control of fat ingestion sexually differentiated?

Sexual differentiation is a fundamental aspect of human physiology [Wizemann TM, Pardue M-L, editors. Exploring the biological contributions to human health: does sex matter? Washington DC, National Academy Press, 2001]. Therefore, this review considers whether the physiological control of eating, as related to dietary fat, is sexually differentiated. The effects of dietary fat are considered from the perspective of stimuli controlling eating that arise from oral, gastric, intestinal, hepatic, and adipose sites. The data reviewed provide substantial support for hypothesis that many such controls of fat ingestion are sexually differentiated in both humans and laboratory animals. Because as yet little is established definitively, however, the apparently most promising questions and methodologies for future work are emphasized.

Validity of test meals for determining binge eating

Assessment of binge eating has been criticized because of serious doubts concerning the accuracy of self-report. This experiment tested the validity of a laboratory test meal as an indicator of binge eating. Eight individuals diagnosed with binge-eating disorder (BED), eight obese non-binge-eaters, and eight normal-weight non-binge-eaters ate a test meal under conditions designed to increase the likelihood of inducing a binge episode. Non-binge-eaters, regardless of weight, felt in control of their eating and ate a relatively small amount of the test meal, while participants with BED ate significantly more food and felt significantly more out of control. Eating behavior during test meals can be a useful indicator of BED diagnostic status and may be a useful method for objectively defining binge eating.

Estradiol, CCK and satiation

Estradiol has long been known to inhibit feeding in animals, but the mechanism(s) mediating its effects have not been clear. Demonstrations that estradiol's feeding effects are expressed as decreases in meal size coupled with the emerging consensus that cholecystokinin (CCK) released from the small intestines during meals is a physiological negative-feedback signal controlling meal size (i.e. satiation) suggested a new approach to the problem of the mechanisms of estradiol's inhibitory effect on feeding. Progress on this approach is reviewed here. Experimental manipulations of exogenous and endogenous CCK and estradiol have produced converging evidence that estradiol cyclically increases the activity of the CCK satiation-signaling pathway so that meal size and food intake decrease during the ovulatory or estrus phase of the ovarian cycle. This is a striking example of the modulation of the operation of a control of meal size by the physiological context in which the meal occurs. Estradiol also produces a tonic decrease in meal size, but this apparently does not involve the CCK satiation-signaling pathway. Where and how estradiol acts to increase the potency of the CCK satiating-signaling pathway are not known. Several possible sites are suggested by the observations that estradiol treatment increases feeding- and CCK-induced expression of c-Fos in ovariectomized animals in brain areas including the nucleus tractus solitarius, paraventricular nucleus of the hypothalamus, and central nucleus of the amygdala. Tests with null mutation mice indicate that estrogen receptor-alpha is necessary for estradiol's feeding effects. Finally, the possibilities that estradiol exerts important influences on normal or disordered eating in women are discussed. It is concluded that estradiol exerts a biologically significant action on CCK satiation in animals. Further research to determine whether this action of estradiol has a role in the pathogenesis, course, or treatment of disordered eating in women is indicated.

Reduction of fat and protein intakes but not carbohydrate intake following acute and chronic fluoxetine in female rats

Fluoxetine hydrochloride, a selective serotonin reuptake inhibitor, leads to reductions in food intake and body weight and is under investigation as a possible treatment for obesity. Additionally, it has been suggested that fluoxetine administration could lead to a selective suppression in carbohydrate consumption. Because women more often than men seek weight reduction treatment, the present study examined the acute and chronic effects of fluoxetine on food intake, macronutrient selection, body weight, estrous cycle, and motor activity in female rats. Female Long-Evans rats were provided with separate sources of protein, fat and carbohydrate, and nutrient intakes were recorded following single (5.0, 10.0, and 20.0 mg/kg, IP) and chronic daily (10 mg/kg for 28 days) injections of fluoxetine. Acute and chronic administration of fluoxetine significantly reduced total caloric intake when compared to vehicle treatment. Moreover, fluoxetine significantly suppressed fat and protein intakes, but not carbohydrate intake following both acute and chronic drug administration. Animals chronically treated with fluoxetine gained significantly less weight than animals treated with vehicle. Chronic fluoxetine treatment did not significantly alter estrous cycle. However, in both fluoxetine- and vehicle-treated animals, total caloric intake, and carbohydrate and protein intakes were reduced and fat intake was increased when estrogen levels were high. Fluoxetine significantly reduced motor activity up to 4 h postinjection, and increased motor activity 24 h postinjection.

The influence of sex hormones on obesity across the female life span

Women have a higher prevalence of obesity than men in most developed countries. Obesity affects many aspects of women's health by increasing risk for heart disease, diabetes, breast cancer, and infertility. One reason for the gender difference in obesity may be that fluctuations in reproductive hormone concentrations throughout women's lives uniquely predispose them to excess weight gain. Studies in experimental animals and women have shown that hormonal changes across the menstrual cycle affect calorie and macronutrient intake and alter 24-hour energy expenditure. Pregnancy is a significant factor in the development of obesity for many women. Various factors are associated with excess weight retention following pregnancy, including weight gain during pregnancy, ethnicity, dietary patterns, and interval between pregnancies. There is a need to tailor recommendations for energy intake during pregnancy to individual women, and recent evidence also suggests that the timing of weight gain during pregnancy is a critical factor. Menopause is also a high-risk time for weight gain in women. Although the average woman gains 2-5 pounds during menopausal transition, some women are at risk for greater weight gains. There is also a hormonally driven shift in body fat distribution from peripheral to abdominal at menopause, which may increase health risks in older women. Hormone therapies have varying impacts on body weight and fat distribution. In summary, hormonal fluctuations across the female life span may explain the increased risk for obesity in women. Awareness of these factors allows development of targets for prevention and early intervention.

Cholecystokinin and estradiol synergistically potentiate satiety in rats

The ability of octapeptide cholecystokinin (CCK), in interaction with ovarian steroid conditions, to decrease 1-h feeding was studied in 5-h food-deprived Sprague-Dawley rats. In Experiment 1, intact, unilaterally ovariectomized, and bilaterally ovariectomized females and intact males were given IP injections of 0, 0.25, 0.50, and 1.0 microgram/kg b.wt. CCK and were assessed for food ingestion at 1, 3, and 19 h. Food intake at 1 h was suppressed in animals receiving CCK compared to saline (0 dose); the threshold dosage was 1 microgram/kg b.wt. (p < 0.01) in this paradigm. No significant sex difference was observed between the four groups; however, animals with decreased ovarian steroids (intact males and bilaterally ovariectomized females) suppressed ingestion less than animals with greater ovarian steroid levels (intact and unilaterally ovariectomized females) at both the 0.25 and 1.0 microgram/kg b.wt. dosages (p < 0.01). Therefore, in a second experiment, sensitivity to CCK was compared in females in early metestrus, when estrogen levels are decreased, and during late diestrus, when estrogen levels are high, using dosages of 0, 0.25, 1, and 2.5 micrograms/kg b.wt. A statistically significant difference was found between sensitivity at early metestrus and late diestrus at the 2.5 micrograms/kg b.wt. dose only, with food ingestion more reliably depressed during periods of increased estrogen (p < 0.05). These results suggest that estradiol and CCK can have a synergistic effect on satiety.

Metabolic fuels and reproduction in female mammals

A complete reproductive cycle of ovulation, conception, pregnancy, and lactation is one of the most energetically expensive activities that a female mammal can undertake. A reproductive attempt at a time when calories are not sufficiently available can result in a reduced return on the maternal energetic investment or even in the death of the mother and her offspring. Numerous physiological and behavioral mechanisms link reproduction and energy metabolism. Reproductive attempts may be interrupted or deferred when food is scarce or when other physiological processes, such as thermoregulation or fattening, make extraordinary energetic demands. Food deprivation suppresses both ovulation and estrous behavior. The neural mechanisms controlling pulsatile release of gonadotropin-releasing hormone (GnRH) and, consequently, luteinizing hormone secretion and ovarian function appear to respond to minute-to-minute changes in the availability of metabolic fuels. It is not clear whether GnRH-secreting neurons are able to detect the availability of metabolic fuels directly or whether this information is relayed from detectors elsewhere in the brain. Although pregnancy is less affected by fuel availability, both lactational performance and maternal behaviors are highly responsive to the energy supply. When a reproductive attempt is made, changes in hormone secretion have dramatic effects on the partitioning and utilization of metabolic fuels. During ovulatory cycles and pregnancy, the ovarian steroids, estradiol and progesterone, induce coordinated changes in the procurement, ingestion, metabolism, storage, and expenditure of metabolic fuels. Estradiol can act in the brain to alter regulatory behaviors, such as food intake and voluntary exercise, as well as adenohypophyseal and autonomic outputs. At the same time, ovarian hormones act on peripheral tissues such as adipose tissue, muscle, and liver to influence the metabolism, partitioning and storage of metabolic fuels. During lactation, the peptide hormones, prolactin and growth hormone, rather than estradiol and progesterone, are the principal hormones controlling partitioning and utilization of metabolic fuels. The interactions between metabolic fuels and reproduction are reciprocal, redundant, and ubiquitous; both behaviors and physiological processes play vital roles. Although there are species differences in the particular physiological and behavioral mechanisms mediating nutrition-reproduction interactions, two findings are consistent across species: 1) Reproductive physiology and behaviors are sensitive to the availability of oxidizable metabolic fuels. 2) When reproductive attempts are made, ovarian hormones play a major role in the changes in ingestion, partitioning, and utilization of metabolic fuels.

Developmental patterns of macronutrient intake in female and male rats from weaning to maturity

Male and female Sprague-Dawley rats had available pure macronutrient diets, protein, carbohydrate and fat, from birth to day 77 of age. Analyses of their intake of these nutrients, as a function of age, demonstrate that, in both sexes, daily protein intake and preference for this nutrient relative to the other macronutrients rise steadily from weaning and peak precisely at the time of puberty (day 37-44 for females and days 42-49 for males), when there is also a peak in body weight gain. This is in contrast to daily carbohydrate intake, which peaks 2 weeks after puberty in males, and also to the female and male rats' preference for carbohydrate, which remains relatively stable from weaning to maturity. These patterns also differ from those observed for daily fat intake and fat preference for females and males, which are relatively high during the first postweaning week and then decline and remain very low until shortly after puberty (day 54), when there occurs a sharp burst in fat intake. Comparisons between the females and males reveal a significantly stronger preference for carbohydrate in the females, exhibited as early as 23 days of age; a stronger preference for protein and fat in the males, evident after day 28; and greater light-period feeding of carbohydrate and fat by females compared to males, apparent after puberty. Correlational analyses demonstrate that body weight and total kcal intake are closely related to daily protein consumption, more strongly in females compared to males; are strongly related to daily fat intake only in males; and are unrelated to intake of carbohydrate, at any age and in either sex.

Sex differences in polysaccharide and sugar preferences in rats

The preferences of adult male and female rats for sucrose and polysaccharide (Polycose) solutions were measured in three experiments. In Experiment 1, male and female rats did not differ in their preference for molar solutions of sucrose or Polycose as measured by 24-hr solution vs. water tests. In Experiment 2, however, male rats displayed a greater preference for Polycose, and females a greater preference for sucrose as measured in 3-min and 30-min/day sucrose vs. Polycose tests. Both sexes preferred Polycose to sucrose at low concentrations, but switched their preference at higher concentrations. Experiment 3 revealed that both male and female rats strongly preferred 32% sucrose to 32% Polycose in 30-min/day two-solution tests, but this preference did not persist when the rats were given 24-hr/day access to the two solutions. Male rats lost their sucrose preference, while female rats developed a preference for Polycose over sucrose. The sexes also differed in that the female rats increased their total caloric intake more and consumed proportionately more carbohydrate than did the male rats during the 24-hr/day test. The findings demonstrate that while both male and female rats have an avid appetite for polysaccharide and sucrose solutions, they differ in their short-term and long-term relative preferences for the two carbohydrates, as well as in their caloric and diet selection responses to the carbohydrate solutions. The possible functional significance of these sex differences are discussed.

Dietary self-selection and intake of protein and energy is altered by the form of the diets

The technique of dietary self-selection has been widely used to investigate proposed mechanisms of protein and carbohydrate regulation and to substantiate theories relating nutrients, brain chemistry and behaviour. These experiments have produced conflicting results which might be accounted for by differences in experimental variables or in the different techniques used to measure self-selection. The possibility that the sensory qualities of diets might alter dietary self-selection was examined in three groups of rats self-selecting protein and carbohydrate from pairs of isocaloric diets in which the physical form (finely-milled casein/corn starch powder, granular casein/corn starch powder and gelled finely-milled casein/corn starch), but not the nutrient composition of each pair was varied. In addition, since the serotonergic drug fenfluramine has been frequently used to influence dietary selection, the effects of this drug on the selection of protein and carbohydrate from the three diets were also examined. The selection of particular diets and consequently protein and carbohydrate intake was markedly altered by the form of the diet. Furthermore, the effects of fenfluramine on diet selection were also modified significantly by the form of the diet. These results suggest that intake of certain nutrients is not strictly regulated at a particular fixed value and that drug-induced changes of self-selection behaviour must be interpreted cautiously.

Protein and carbohydrate self-selection: modification of the effects of fenfluramine and amphetamine by age and feeding regimen

Age and daily restricted feeding are two frequently manipulated procedural variables that have been shown to alter dietary self-selection of protein and carbohydrate in rats. This study examined whether age and restricted feeding could further interact with drugs such as fenfluramine and amphetamine that are used to manipulate dietary self-selection. At the time of peak blood levels, fenfluramine spared relative intake of protein (as reflected in an increase in %P-E ratio of protein to total energy consumed). This effect, however, was significant at only some doses of fenfluramine and in some groups. At the times of peak blood levels of amphetamine, sparing of protein intake was never observed. In contrast, amphetamine tended to suppress protein intake (decrease in %P-E). This effect was dose-related (0.5, 1.0 and 2.0 mg/kg) in the adult, free-feeding rats, but not apparent in food-restricted rats. Consequently the effect of drugs on the self-selection of protein and carbohydrate diets is influenced by both age and feeding regimen. These results have implications for proposed neurochemical and hormonal hypotheses of protein and carbohydrate regulation as well as the design and interpretation of self-selection experiments.

Variation of fat intake with estrous cycle, ovariectomy and estradiol replacement in hamsters (Mesocricetus auratus) eating a fractionated diet

Intakes of fats, proteins, and carbohydrates were measured daily for a 3 month period in female hamsters maintained on a fractionated diet consisting of the dietary constituents, dextrose, fat and soybean meal. In the first study, hamsters showed variations in body weight and fat intake across the estrous cycle and following ovariectomy, with elevations during diestrous and after ovariectomy when endogenous estrogens are reduced. In the second study, the effects of exogenous estradiol or cholesterol on intake patterns of ovariectomized hamsters were determined. Hamsters lost weight and decreased fat intake when implanted with a silastic containing estradiol and gained weight and increased fat intake when exposed to cholesterol treatment. In neither of the two studies were the intakes of protein or carbohydrates significantly affected by the animals' hormonal status. These results suggest that in the hamster, estrogenic effects on food intake are specific to some dietary constituents and not to others.