Nutritional properties of protein sources on diet-induced obesity, hepatic lipid accumulation, and fatty acid composition in mice
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The prevalence of obesity has increased rapidly, and excess adiposity is associated with comorbidities that increase the risk of mortality. An energy-dense diet that is high in sugar and fat is one of the major explanations for the increased levels of obesity. Importantly, obesity is preventable, and diet represents one significant tool for curbing the development of excess adiposity and its related comorbidities. Epidemiological studies have indicated that the intake of various seafood, dairy, and plant-derived protein-rich food is associated with protection against long-term weight gain. In addition, cod contains the majority of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) as phospholipids (PLs), which, it has been suggested, are more bioavailable and can promote higher anti-obesogenic effects than triacylglycerolbound EPA and DHA. In line with this, it is important to assess the role of food that could influence primary prevention of obesity. The overall aim of this thesis is to determine the impact of different sources of dietary protein for preventing diet-induced obesity and influence on metabolic changes associated with obesity development in mice.
The studies were conducted by performing dietary trials with the obesity-prone C57BL/6J mice. Various protein sources were blended in high-protein diets, while cod fillets and pork sirloins were blended in Western diets. In addition, the nutritional effects of EPA and DHA in cod were investigated by supplementing pork-containing Western diets with n-3 PUFAs to match the levels of the cod-containing diet. Body weight, energy intake, body composition, and glucose tolerance testing were measured during the experiments. The mice were sacrificed, and adipose tissue depots and liver were dissected out. Analyzes of gene and protein expression, histological investigation, and lipid and fatty acid analyses were performed in the tissues.
By comparing proteins from casein, soy, cod, beef, chicken, and pork in a highfat/ high-protein diet, we observed that casein was the only protein that completely protected against high-fat diet-induced obesity in comparison with mice fed on a lowfat diet. Casein was particularly effective in preventing the whitening of interscapular brown adipose tissue (iBAT) and maintaining Uncoupling Protein 1 (UCP1) expression. The terrestrial proteins from pork, chicken, and meat promoted fat mass gain more than proteins from cod and soya. In addition, cod was the only protein source that significantly decreased energy intake. Furthermore, in a Western diet, we reported that exchanging pork with cod as protein source reduced energy intake by 6%, attenuated adiposity and hepatic lipid accumulation, and lowered plasma levels of 2- AG and AEA as well as the n-6:n-3 ratio in RBCs and hepatic polar lipids. Frozen storage decreased the PLs’ content in the cod fillets and thus could affect the ability of cod to attenuate obesity. Compared with mice fed on a casein-based Western diet, mice fed on fresh cod, but not frozen cod with a reduced PL content, increased obesity development. In addition, exchanging pork with fresh cod did not protect against dietinduced obesity or reduce energy intake. Supplementing a pork-containing diet with PL-bound but not TAG-bound EPA and DHA to the level of a fresh cod diet led to significantly higher body weight, fat mass, and liver mass in mice compared to a diet without supplementation.
Based on the data of this research, the protein source exerted pronounced effects in attenuating obesity in the intake of high-protein diets and Western diets. Casein effectively protected against high-fat induced obesity. Despite the low content, the level of EPA and DHA in the cod-containing Western diet was sufficient to replace ARA in RBCs and hepatic polar lipids and to modulate the endocannabinoid profile. However, the frozen storage of cod seems to modulate the ability of cod intake to attenuate obesity. The combined effect of PL-bound n-3 PUFA and pork in promoting obesity needs further investigation since pork is a major component of the Western diet.