Genetic variance contributes to ingestive processes: A survey of eleven inbred mouse strains for fat (Intralipid) intake

Abstract

Genetic variation across inbred and outbred mouse strains have been observed for intake of sweet solutions, salts, bitter tastants and a high-fat diet. Our laboratory recently reported marked strain differences in the amounts and/or percentages of kilocalories of sucrose consumed among 11 inbred and one outbred mouse strains exposed to a wide range of nine sucrose concentrations (0.0001–5%) in two-bottle 24-h preference tests. To assess whether differences in fat intake were similarly associated with genetic variation, the present study examined intake of chow, water and an emulsified fat source (Intralipid) across nine different concentrations (0.00001–5%) in the same 11 inbred and 1 outbred mouse strains using two-bottle 24-h preference tests, which controlled for Intralipid concentration presentation effects, Intralipid and water bottle positions, and measurement of kilocalorie intake consumed as Intralipid or chow. Strains displayed differential increases in Intralipid intake relative to corresponding water with significant effects observed at the seven (BALB/cJ: 0.001% threshold sensitivity), four (AKR/J, C57BL/6J, DBA/2J, SWR/J: 0.5% threshold sensitivity), three (CD-1, C57BL/10J, SJL/J: 1% threshold sensitivity) and two (A/J, CBA/J, C3H/HeJ, 129P3/J: 2% threshold sensitivity) highest concentrations. In assessing the percentage of kilocalories consumed as Intralipid, SWR/J mice consumed significantly more at the three highest concentrations to a greater degree than BALB/cJ, C57BL/6J, CD-1, C3H/HeJ, DBA/J and 129P3/J strains which in turn consumed more than A/J, AKR/J, CBA/J, C57BL/10J and SJL/J mice. Relatively strong (h² = 0.73–0.79) heritability estimates were obtained for weight-adjusted Intralipid intake at those concentrations (0.001–1%) that displayed the largest strain-specific effects in sensitivity to Intralipid. The identification of strains with diverging abilities to regulate kilocalorie intake when presented with high Intralipid concentrations may lead to the successful mapping of genes related to hedonics and obesity.

Introduction

Systematic analyses of rodent strain differences are important sources regarding the genetic control of all aspects of ingestive behavior (see review: [32]). These studies indicate widespread strain-dependent (i.e., genetic) variance in food, water and mineral intake as well as spout side preference [2], [3]. Particular orosensory stimuli such as salts (e.g., [2], [6], [7], [8], [39]), bitter tastants (e.g., [6], [9], [11], [14], [16], [25], [39]), saccharin (e.g., [9], [10], [13], [20], [26], [27], [28], [29], [33], [39]), and sucrose (e.g., [4], [9], [20], [23], [26], [30], [31], [38]) are also subject to intake differences among strains. In addition, these studies help to identify strains with divergent sensitivities for subsequent QTL analyses to localize chromosomal regions, and ultimately genes, critically involved in such differences [17].

Differences in dietary fat intake are also associated with genetic variation (see review: [45]) and have led to identification of dietary resistance and susceptibility in inbred and outbred strains of rats (e.g., [22], [34]) and mice (e.g., [41], [43]). The latter studies identified particular strains in which moderate intake of a high-fat diet promoted weight gain and obesity (e.g., AKR/J mice), and other strains in which large intake of the high-fat diet was not accompanied by weight gain (e.g., SWR/J). Moreover, such weight effects were largely due to variation in the dietary fat content, but this variable weakly correlated with total energy intake. These particular strains displayed similar effects whether the fat source was shortening, lard or granular, and whether the high- and low-fat diets were isocaloric [36]. Indeed, whereas AKR/J and C57BL/6J mice self-selected the highest proportion of fat in macronutrient diet selection with epididymal fat correlated with fat consumption, SWR/J and CAST/Ei strains consumed fat that was inversely correlated with epididymal fat [35]. Moreover, whereas the diet-sensitive AKR/J and DBA/2J strains consumed more fat, displayed more adiposity and displayed elevated levels of leptin and insulin, the C57BL/6J strain showed an equal preference between protein and fat and displayed normal insulin and leptin levels [1]. In contrast, obesity-resistant SWR/J and A/J mice consume more fat than carbohydrate, but fail to gain weight, potentially because of lower insulin levels, increased capacity of skeletal muscle to metabolize fat, enhanced paraventricular galanin and reduced arcuate NPY [21]. Our laboratory [24] found genetic variance in the sensitivity and magnitude of feeding responses of mouse strains exposed to the free fatty acid oxidation inhibitor, mercaptoacetate. Thus, inbred DBA/J and outbred CD-1 mice were the most sensitive to mercaptoacetate-induced feeding, whereas mercaptoacetate failed to significantly increase food intake in A/J, C57BL/10J and 129P/3J mice. A series of genetic loci were mapped to explain some of these genetic variations for fat and obesity (e.g., [5], [12], [37], [40], [42], [44]). Because the above studies used solid fat sources (e.g., shortening, lard, granular), it is therefore relatively difficult to systematically manipulate the amount of fat in the diet over a wide concentration range in short-term intake tests to determine if differences in sensitivity may account for some of the observed genetic variance.

Our laboratory [23] recently reported on a number of factors potentially involved in murine genetic variance in sucrose intake among 11 inbred (A/J, AKR/J, BALB/cJ, CBA/J, C3H/HeJ, C57BL6/J, C57BL10/J, DBA/2J, SJL/J, SWR/J, 129P3/J) and one outbred (CD-1) strains, thereby allowing for the valid estimation of genetic correlations [16]. Intake across a range of nine different sucrose concentrations (0.0001–20%) was compared with water intake in two-bottle 24-h preference tests which importantly controlled for order effects by exposing half of the mice to an ascending concentration order and the remainder to a descending concentration order [15]. Bottle positions of the sucrose and water bottles were also systematically switched across animals and across strains, another demonstrably important variable [3]. Additionally, chow intake was measured in order to determine strain differences in kilocalorie intake as a function of sucrose relative to chow. A/J, C57BL/6J, CD-1 and SWR/J strains consumed the greatest (11.6–22 ml) amount of sucrose, whereas the A/J, C57BL/10J, SJL/J and SWR/J strains consumed the greatest (44–56%) percentages of kilocalories as sucrose. The AKR/J, CBA/J, C3H/HeJ and DBA/2J strains consumed the least (6.9–7.9 ml) amount of sucrose and displayed lower (20–30%) percentages of kilocalories consumed as sucrose. Whereas A/J, C57BL/6J, C57BL/10J, CD-1, SWR/J and SJL/J strains all displayed the most pronounced compensatory decreases in chow intake as the percentage of kilocalories consumed as sucrose increased, the AKR/J, C3H/HeJ and DBA/2J strains failed to significantly alter chow intake even at high sucrose concentrations. Therefore, in this study, the use of liquid sucrose solutions at a wide range of different concentrations allowed for analyses of concentration-dependent differences in sensitivities as a function of murine strain.

Typical difficulties in using different liquefied fat sources presented at different concentrations are their inability to stay in solution over a time course (e.g., 24 h) that is reasonable to study murine intake. Intralipid (Baxter Healthcare Corporation, Deerfield, Illinois) is an emulsified fat solution (20%) made almost exclusively from soybean oil (20 g in 100 ml) and is used clinically for delivery of a fat source to patients. Therefore, the use of Intralipid insures that the fat is equally distributed in solution across a wide range of concentrations, and indeed, Intralipid solutions are readily consumed in a manner similar to sucrose and other palatable solutions (e.g., [18], [19]). Therefore, analysis of Intralipid intake across concentrations (0.00001–5%) can potentially parallel our previous use [23] of different sucrose concentrations (0.0001–20%) and can be compared to water intake in two-bottle 24-h preference tests for the study of genetic variance in fat intake. Given that a number of strains evaluated in our previous study [23] display high levels of fat intake with weight gain (e.g., AKR/J, C57BL/6J, DBA/2J: [1], [36], [41], [43]), high levels of fat intake without weight gain (e.g., A/J, SWR/J: [21], [35]), and lower levels of fat intake (BALB/cJ, C3H/HeJ SJL/J, 129/J: 1,35) in prior studies, the present study examined these same strains for Intralipid intake across a wide range of concentrations in two-bottle 24 h preference tests using all of the dependent measures assessed previously [23] for sucrose.