While macronutrients like fats and carbohydrates have traditionally been the focus of diet design, the new research suggests that micronutrients like specific amino acids could be used in the future.

“Rather than basing our diets only on macronutrients like fat and carbohydrates, we might one day be designing diets based on micronutrients like amino acids,”said lead researcher Randy Seeley in a statement.

Scientists from the University of Cincinnati's Genome Research Institute have reported that a signaling pathway of the protein, mammalian Target of Rapamycin (mTOR), previously linked to the development of diabetes and cancer, could play a key role in food intake.

“We take in calories to maintain levels of stored fuel (energy) and adequate available fuel,” explained Seeley. “The signals that tell our brain about both the stored and the available energy in our body can activate the mTOR pathway in key parts of the brain that control appetite.”

The researchers found that injection of the amino acid L-leucine into the hypothalamus of rats, the animals ate less. In fact, the researchers found that L-leucine (1.1 micrograms in 2 microlitres of a phosphate-buffered saline solution) caused a decrease in food intake four hours after administration, and lasted for 24 hours. Interestingly, similar results were not observed when similar branched chain amino acids like L-valine were used.

“The ability of L-leucine to activate mTOR in the hypothalamus and to inhibit food intake may be an example of central nervous system circuits using an evolutionary conserved signalling mechanism as a fuel sensor rather than as an amino acid sensor,” wrote Seeley and colleagues in the 12 May issue of Science (Vol. 312, pp. 927-930).

This fuel-sensitive pathway, said the researchers, could eventually lead to a better understanding of obesity and type 2 diabetes.

In an accompanying editorial, Jeffrey Flier from Harvard Medical School and Beth Israel Deaconess Medical Center, said that the research asks several important questions, including how leucine selectively regulates mTOR. Flier also said it was critical to determine “whether normal physiological changes in amino acid concentration act in the hypothalamus to influence energy balance and metabolism.”

Seeley agreed, stressing that adding more leucine to the diet was not a recommendation: “We still have a lot to learn about how these nutrients would act if simply ingested with other nutrients, in what form they could be most effective, and even if they are effective at all when not administered directly to the brain,” he said.

Flier finished his editorial by saying: “The next challenge will be to determine whether novel therapies for metabolic disease will emerge from nutritional exploitation of these insights.”

The link between leucine and weight loss has already been reported in the literature. Indeed, Brazilian researchers recently reported that a 0.6 per cent L-leucine supplement led to a 23 per cent reduction in body fat for rats (Nutrition, Vol. 22, pp. 520-527).