Protein restricted, high carbohydrate diets improve metabolic health in rodents, yet the precise dietary components that are responsible for these effects have not been identified. Further, the applicability of these studies to humans is unclear. Here, we demonstrate in a randomized controlled trial that a moderately protein restricted (PR) diet also improves markers of metabolic health in humans. Intriguingly, we find that feeding mice a diet specifically reduced in branched chain amino acids (BCAAs) is sufficient to improve glucose tolerance and body composition equivalently to a PR diet, via metabolically distinct pathways. Our results highlight a critical role for dietary quality at the level of amino acids in the maintenance of metabolic health, and suggest that diets specifically reduced in BCAAs, or pharmacological interventions in this pathway, may offer a translatable way to achieve many of the metabolic benefits of a PR diet.

Key pointsr We recently found that feeding healthy mice a diet with reduced levels of branched-chain amino acids (BCAAs), which are associated with insulin resistance in both humans and rodents, modestly improves glucose tolerance and slows fat mass gain.r In the present study, we show that a reduced BCAA diet promotes rapid fat mass loss without calorie restriction in obese mice.r Selective reduction of dietary BCAAs also restores glucose tolerance and insulin sensitivity to obese mice, even as they continue to consume a high-fat, high-sugar diet.r A low BCAA diet transiently induces FGF21 (fibroblast growth factor 21) and increases energy expenditure.r We suggest that dietary protein quality (i.e. the precise macronutrient composition of dietary protein) may impact the effectiveness of weight loss diets.Abstract Obesity and diabetes are increasing problems around the world, and although even moderate weight loss can improve metabolic health, reduced calorie diets are notoriously difficult to sustain. Branched-chain amino acids (BCAAs; leucine, isoleucine and valine) are elevated in the blood of obese, insulin-resistant humans and rodents. We recently demonstrated that specifically reducing dietary levels of BCAAs has beneficial effects on the metabolic health of young, growing mice, improving glucose tolerance and modestly slowing fat mass gain. In the present study, we examine the hypothesis that reducing dietary BCAAs will promote weight loss, reduce adiposity, and improve blood glucose control in diet-induced obese mice with pre-existing metabolic syndrome. We find that specifically reducing dietary BCAAs rapidly reverses diet-induced obesity and improves glucoregulatory control in diet-induced obese mice. Most dramatically, mice eating an otherwise unhealthy high-calorie, high-sugar Western diet with reduced levels of BCAAs lost weight and fat mass rapidly until regaining a normal weight. Importantly, this normalization of weight was mediated not by caloric restriction or increased activity, but by increased energy expenditure, and was accompanied by a transient induction of the energy balance regulating hormone FGF21 (fibroblast growth factor 21). Consumption of a Western diet reduced in BCAAs was also accompanied by a dramatic improvement in glucose tolerance and insulin resistance. Our results link dietary BCAAs with the regulation of metabolic health and energy balance in obese animals, and suggest that specifically reducing dietary BCAAs may represent a highly translatable option for the treatment of obesity and insulin resistance.

There is growing concern over confounding artifacts associated with β-cell–specific Cre-recombinase transgenic models, raising questions about their general usefulness in research. The inducible β-cell–specific transgenic (MIP-CreERT1Lphi) mouse was designed to circumvent many of these issues, and we investigated whether this tool effectively addressed concerns of ectopic expression and disruption of glucose metabolism. Recombinase activity was absent from the central nervous system using a reporter line and high-resolution microscopy. Despite increased pancreatic insulin content, MIP-CreERT mice on a chow diet exhibited normal ambient glycemia, glucose tolerance and insulin sensitivity, and appropriate insulin secretion in response to glucose in vivo and in vitro. However, MIP-CreERT mice on different genetic backgrounds were protected from high-fat/ streptozotocin (STZ)-induced hyperglycemia that was accompanied by increased insulin content and islet density. Ectopic human growth hormone (hGH) was highly expressed in MIP-CreERT islets independent of tamoxifen administration. Circulating insulin levels remained similar to wild-type controls, whereas STZ-associated increases in α-cell number and serum glucagon were significantly blunted in MIP-CreERT1Lphi mice, possibly due to paracrine effects of hGH-induced serotonin expression. These studies reveal important new insight into the strengths and limitations of the MIP-CreERT mouse line for β-cell research.

Prostaglandin E2 (PGE2) is derived from arachidonic acid, whereas PGE3 is derived from eicosapentaenoic acid (EPA) using the same downstream metabolic enzymes. Little is known about the impact of EPA and PGE3 on β-cell function, particularly in the diabetic state. In this work, we determined that PGE3 elicits a 10-fold weaker reduction in glucose-stimulated insulin secretion through the EP3 receptor as compared with PGE2. We tested the hypothesis that enriching pancreatic islet cell membranes with EPA, thereby reducing arachidonic acid abundance, would positively impact β-cell function in the diabetic state. EPA-enriched islets isolated from diabetic BTBR Leptinob/ob mice produced significantly less PGE2 and more PGE3 than controls, correlating with improved glucose-stimulated insulin secretion. NAD(P)H fluorescence lifetime imaging showed that EPA acts downstream and independently of mitochondrial function. EPA treatment also reduced islet interleukin-1β expression, a proinflammatory cytokine known to stimulate prostaglandin production and EP3 expression. Finally, EPA feeding improved glucose tolerance and β-cell function in a mouse model of diabetes that incorporates a strong immune phenotype: the NOD mouse. In sum, increasing pancreatic islet EPA abundance improves diabetic β-cell function through both direct and indirect mechanisms that converge on reduced EP3 signaling.