Mixed fish/flaxseed oil diets promote body weight gain by lowering whole body energy expenditure in a dose‐dependent manner

We previously discovered that a high fat (HF) n‐3 polyunsaturated fatty acid (PUFA)‐rich diet made of fish/flaxseed oils increased body weight gain in C57BL/6 mice relative to a low fat (LF) normal diet (ND) control. Here we tested if the increase in body weight with the HF n‐3 PUFA diet was due to a reduction in energy expenditure compared to the ND control. In addition, we also tested the effects of a LF n‐3 PUFA diet, also made of fish/flaxseed oils. Metabolic cage and echo‐MRI studies showed the HF n‐3 PUFA diet promoted a dramatic reduction in running wheel and rearing activities after 3 weeks of feeding the mice, relative to the ND control. Furthermore, the HF n‐3 PUFA diet had a tendency to lower ambulatory activity and oxygen consumption. In comparison, the LF n‐3 PUFA diet, relative to the control, also had a tendency to promote body weight gain and lower energy expenditure, albeit not as pronounced as the HF n‐3 PUFA diet. Despite the propensity to increase body weights, both n‐3 PUFA diets lowered serum and liver triglycerides, consistent with the reported effects of n‐3 PUFAs. Taken together, we conclude that short‐term administration of mixed fish and flaxseed oil diets lowers whole body energy expenditure, which can promote body weight gain. Supported by NIH R15AT006122 (to SRS)