Protein and carbohydrate selection respond to changes in dietary saturated fatty acids but not to changes in essential fatty acids

Divergence in proportional fat intake in AKR/J and SWR/J mice endures across diet paradigms

These experiments were designed to test the hypothesis that the contrasting patterns of macronutrient selection described previously in AKR/J (fat preference) and SWR/J (carbohydrate preference) mice are not dependent on a single diet paradigm. The effect of mouse strain on proportional fat intake was tested in naive mice by presenting two-choice diets possessing a variety of physical, sensory, and nutritive properties. In three separate experiments, AKR/J mice preferentially selected and consumed a higher proportion of energy from the high-fat diet than SWR/J mice. Specifically, this phenotypic difference was observed with 1) fat-protein vs. carbohydrate-protein diets, independent of fat type (vegetable shortening or lard), 2) isocaloric, high- vs. low-fat liquid diet preparations, and 3) high- vs. low-fat powdered-granular diets. These results confirm our previous observation of a higher proportional fat intake by AKR/J compared with SWR/J mice using the three-choice macronutrient selection diet and show that this strain difference generalizes across several diet paradigms. This strain difference is due largely to the robust and reliable fat preference of the AKR/J mice. In contrast, macronutrient preference in SWR/J mice varied across paradigms, suggesting a differential response by this strain to some orosensory or postingestive factor(s).

The effects of high fat diets and environmental influences on cognitive performance in rats

As part of a continuing investigation of the relationship between dietary factors and cognitive function, the present study examined the combined effects of environmental influences and high-fat diets on learning and memory. Following 3 months of dietary (20% by weight fat diets, composed primarily of either beef tallow or soybean oil versus standard laboratory chow) and environmental treatments (standard, enriched or impoverished), subjects were tested on a variable interval delayed alternation (VIDA) task which measures learning and memory functions that differentially involve specific brain regions. The results confirmed the negative effects of high fat diets, relative to chow, on all aspects of VIDA performance and showed that environmental enrichment overcame deficits associated with dietary fat. Housing rats fed high-fat diets in an impoverished environment did not further exacerbate cognitive deficits observed in such rats living under standard conditions. By comparison, chow-fed rats exhibited no benefit associated with the enriched environment on any aspect of task performance, and only a transitory learning impairment when housed in an impoverished environment. The results show that high fat diets and environmental conditions influence cognitive function and that these two factors interact with one another to produce different profiles of benefits and impairments.

Dietary saturated fatty acids and brain function

The degree to which fatty acids modulate brain function beyond periods of rapid brain growth is poorly understood. Nevertheless, recent evidence suggests that dietary fatty acid composition influences numerous behaviors including body temperature regulation, pain sensitivity, feeding behavior including macronutrient selection, and cognitive performance. Importantly, alterations are observed in the absence of essential fatty acid (EFA) deficiency, beyond periods of rapid brain development, and at levels similar to those consumed by the North American population. Data suggest that the content of saturated fatty acids (SFAs), and not that of the EFAs, may be the important component of dietary fat mediating macronutrient selection and cognition under these experimental conditions. Yet, a direct role of SFAs in modulating brain functions has not been elucidated. A discussion of potential mechanisms which may directly involve the central nervous system, or may indirectly influence central processes via peripheral pathway(s) is presented.

Effects of dietary preference and galanin administration in the paraventricular or amygdaloid nucleus on diet self-selection

Microinjection of galanin into the paraventricular nucleus (PVN) of the hypothalamus has been reported to predominantly increase dietary fat consumption during the light or late dark periods. However, there have been no studies of the macronutrient-specific effects of galanin administered into the amygdala (AMY). We used male Sprague-Dawley rats to test the effect of galanin administered into the PVN or central nucleus of the AMY on diet selection using two different protocols: the two-choice composite diet (10% or 55% energy as fat) (Study 1) or the three-choice macronutrient diet (Study 2). In the first study, total food intake was significantly increased 60 min after galanin injection (0.3 nmol) into either the PVN or AMY, but there was no difference in intake between the low- and high-fat diets. In Study 2, using the three-choice macronutrient diet, the feeding stimulation produced by galanin in the PVN also was not diet-dependent. In the AMY, there was a significant effect of diet on the feeding response to galanin, due primarily to the lack of stimulation of protein intake, whereas the intakes of fat and carbohydrate were not significantly different. In summary, there was no difference in the effect of galanin on fat versus carbohydrate intake for either nuclei, whether animals were injected during the light or the late-dark phase. Because rats often display preferences when allowed to choose among individual macronutrient sources, we also examined the relationship between baseline macronutrient preference and the feeding response to galanin in groups of highly fat-preferring and carbohydrate-preferring rats (Study 3). After PVN galanin injection, fat-preferring rats demonstrated significant increases in their consumption of both carbohydrate and fat, whereas carbohydrate-preferring rats responded by eating carbohydrate almost exclusively. There were significant positive associations between baseline macronutrient intake and galanin-induced intake. The results of these studies combined suggest that the effect of galanin on macronutrient selection is not specific for fat and that underlying macronutrient preferences are important in determining the feeding response to galanin.

Effects of dietary fat on food intake and brain uptake and oxidation of fatty acids

The present study was designed to investigate (a) whether dietary fat manipulation, quantitatively and/or qualitatively, may influence the transport of fatty acids into the brain and oxidation of fatty acids in the hypothalamus; and (b) if an inhibitor of fatty acid oxidation changed food intake on these diets. Rats were fed for 4 wk 5% or 34% corn oil or tallow diets that were isocaloric and isonitrogenous. It was found that rats fed corn oil diets had significantly higher brain uptake index of palmitate than did rats fed tallow diets. In a second experiment, rats were fed either 30% corn oil or tallow diets and injected with either saline or mercaptoacetate (MA). Both saturated fat diets and mercaptoacetate injection reduced lateral hypothalamic, but not ventromedial hypothalamic fatty acid oxidation. Mercaptoacetate increased food intake only in the corn oil fed rats. It is proposed that these uptake and metabolic changes contribute to alterations in either energy expenditure or feeding behavior.

Neural 22-carbon fatty acids in the weanling rat respond rapidly and specifically to a range of dietary linoleic to alpha-linolenic fatty acid ratios

Changing the dietary ratio of the essential fatty acids (EFA), 18:2n6 and 18:3n3, while keeping the amounts of other fatty acids in the diet constant can rapidly and specifically alter the proportions of n6 and n3 22-carbon fatty acids in the brain of the weanling rat. A dietary 18:2n6/18:3n3 ratio of 165 versus 1.8 caused higher n6 and lower n3 22-carbon fatty acid levels, without changing total 22-carbon fatty acid levels, in phosphatidylethanolamine and phosphatidylcholine from several neural membrane fractions. This was apparent after only 2 weeks and showed no sign of plateauing after 12 weeks. Other neural fatty acids were essentially unaffected. The three most abundant 22-carbon fatty acids responded somewhat differently to increments in the dietary 18:2n6/18:3n3 ratio (1.8, 9, 36, and 165). Levels of 22:4n6 increased by similar absolute amounts for each four-fold increase in dietary 18:2n6/18:3n3 ratio; in contrast, the largest absolute changes in 22:5n6 and 22:6n3 levels occurred as the 18:2n6/18:3n3 ratio increased from 36 to 165. This study shows that the 18:2n6/18:3n3 ratio of diets high in fat (40% of energy) and adequate in EFA, both typical of diets in developed countries, can substantially and relatively quickly affect the 22-carbon fatty acids in the brain, even after the rapid accumulation of these fatty acids during neural growth has ceased.