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Dietary protein is essential for the growth of skeletal muscle, a process called hypertrophy. Circadian rhythms – the body’s 24-hour cycles of biological, hormonal, and behavioral patterns – modulate a wide array of nutritional and metabolic processes, including amino acid absorption and utilization. However, it is unclear how circadian rhythms affect muscle hypertrophy. A report published this month suggests that distributing dietary protein equally across meals is best for maintaining muscle mass.

Circadian clocks located in the brain and other organs are driven by changes in the expression of genes such as Circadian locomotor output cycles kaput, commonly referred to as “Clock.” Mice that do not express the Clock gene do not experience day-night variations in metabolism, disrupting amino acid absorption by skeletal muscle. Amino acids are required for activation of genes such as the mammalian target of rapamycin (mTOR), which promotes autophagy, a system of disassembly and recycling of unnecessary or dysfunctional cellular components that is essential for hypertrophy.

The investigators conducted a set of experiments in mice and an observational study in humans. They fed mice two meals per day containing either 11.5 percent or 8.5 percent protein for two weeks. Mice consumed these meals in three patterns of protein distribution: high protein at breakfast and low protein at dinner; equal protein at both meals; or low protein at breakfast and high protein at dinner. In a second experiment, the researchers fed mice branched chain amino acids, which are used in high concentrations by the body for building muscle, at breakfast or dinner. In both experiments, the researchers performed muscle overloading, which puts stress on muscles to encourage hypertrophy, similar to weight lifting in humans. They measured changes in muscle strength, muscle gene expression, plasma amino acid concentrations, plasma growth factor concentrations, and autophagy.

All mice gained muscle mass in response to muscle overload; however, mice that consumed a high-protein breakfast and low-protein dinner had greater gains in muscle mass and rate of hypertrophy than mice that consumed a low-protein breakfast and high-protein dinner. Likewise, mice that consumed a branched chain amino acid supplement in the morning gained more muscle mass and had a higher rate of hypertrophy than mice that consumed the supplement at night. Mice that do not express the Clock gene did not experience gains in muscle mass or hypertrophy with early protein or branched chain amino acid intake, suggesting these gains were circadian-related.

The researchers found that branched chain amino acid concentrations increased following a high-protein meal regardless of time, so these gains in hypertrophy were not due to circadian fluctuations in plasma amino acid concentrations. Likewise, insulin-like growth factor concentrations increased following a high-protein meal regardless of time and likely did not affect the rate of hypertrophy. However, the activation of autophagy in overloaded muscle was greater in mice that consumed a high-protein breakfast compared to a high-protein dinner, potentially contributing to a higher rate of hypertrophy.

Next, researchers recruited 60 women who completed a questionnaire about their consumption of protein foods at breakfast, lunch, and dinner. The researchers classified participants as early protein consumers or late protein consumers based on their answers and measured the participants' body composition, physical activity, and grip strength.

Muscle mass tended to be higher in participants who consumed protein earlier in the day, but this relationship was not statistically significant. Early protein consumers also had significantly greater grip strength and higher skeletal muscle index, which is the ratio of muscle mass in a person’s arms and legs to their height. These relationships remained significant even after taking diet and activity into account. Finally, skeletal muscle index increased as the percent of daily protein eaten at breakfast increased, meaning the more protein that was consumed in the morning, the greater the increase in skeletal muscle index.

These results indicate that circadian genes drive day-night variation in muscle metabolism and protein utilization. Early protein consumption is more beneficial for growing and maintaining muscle than late protein consumption.

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