Weight loss diet while physically inactive speeds up loss of muscle mass

A few months ago we wrote that a surplus of calories during a period of physical inactivity speeds up muscle breakdown. This finding tempted us to speculate that during a period of inactivity a mild caloric deficit might be the way to hold this in check. If that’s the case, then the caloric deficit would have to be very modest indeed, we now know after reading the study that Italian researchers at the University of Trieste published in 2007 in the American Journal of Clinical Nutrition. A caloric deficit of 20 percent at any rate is too big: that just stimulates muscle breakdown.

The Italians got 9 healthy participants to take 14 days of bed rest [Bed rest] on two separate occasions. On one occasion the participants were given the amount of calories they usually burn daily [Eucaloric]; on the other occasion they were given 20 percent fewer calories than they burn [Hypocaloric]. The researchers measured the effect of this treatment on the participants’ body composition.

On two other occasions the participants were allowed to walk around [Ambulatory]; during one of the 14-day periods they were given as many calories as they burned and on another occasion 20 percent less.

Bed rest cost the participants lean body mass – and the amount loss was four times as much when the participants were on a hypo-caloric diet.

From the figure above you can see that during inactivity the muscles absorb less leucine from the blood, and also convert less leucine into muscle proteins. Physical inactivity appears to reduce anabolic processes in the muscle cells. And a caloric deficit reinforces this reduction.

Calorie restriction accelerates the catabolism of lean body mass during 2 wk of bed rest


Muscle inactivity and low energy intake commonly occur in persons with acute or chronic disease, in astronauts during space flight, and during aging.

We used a crossover design to investigate the effects of the interactions of inactivity and calorie restriction on whole-body composition and protein kinetic regulation in 9 healthy volunteers.

Lean body mass (LBM) was measured by using dual-energy X-ray absorptionmetry before and at the end of 14-d periods of bed rest (B) and controlled ambulation (A) in patients receiving eucaloric (E) or hypocaloric (H) (≈80% of total energy expenditure) diets. Whole-body leucine kinetics were determined at the end of the 4 study periods by using a standard stable-isotope technique in the postabsorptive state and during a 3-h infusion of a 0.13 g · kg LBM−1 · h−1 amino acid mixture.

In the postabsorptive state, we found a significant (P = 0.04) bed rest × hypocaloric diet interaction for the rate of leucine oxidation, an index of net protein catabolism (A+E: 0.23 ± 0.01; B+E: 25 ± 0.01; A+H: 0.23 ± 0.01; B+H: 0.28 ± 0.01 μmol · min−1 · kg LBM−1). Bed rest significantly (P < 0.01) decreased amino acid–mediated stimulation of nonoxidative leucine disappearance, an index of protein synthesis (A+E: 35 ± 2%; B+E: 30 ± 2%; A+H: 41 ± 3%; B+H: 32 ± 2%). B+H decreased LBM by 1.10 ± 0.1 kg, which is significantly (P < 0.01) greater than the decrease seen with A+E, A+H, or B+E. Conclusion: Calorie restriction enhanced the catabolic response to inactivity by combining greater protein catabolism in the postabsorptive state with an impaired postprandial anabolic utilization of free amino acids. Source: http://ajcn.nutrition.org/content/86/2/366.short