Caloric restriction induces energy-sparing alterations in skeletal muscle contraction, fiber composition and local thyroid hormone metabolism that persist during catch-up fat upon refeeding

When a person eats significantly fewer calories for a long time, their body burns fewer calories at rest and loses muscle tissue; if they then return to normal eating without adjusting calorie intake downward, they gain fat.

Rats that underwent 2 weeks of severe calorie restriction followed by 1 week of normal eating showed slower muscle relaxation, a 4.75 millisecond increase in time to half-relaxation, a 13.4% increase in slow-twitch muscle fibers, and reduced thyroid hormone activity in muscle tissue, which was associated with lower energy expenditure and faster fat regain during recovery.

In rats subjected to periods of reduced food intake followed by normal eating, skeletal muscle fibers change permanently from fast-twitch to slow-twitch types, with a 31.5% increase in slow-twitch fibers after restriction and a 13.4% sustained elevation after refeeding, alongside changes in calcineurin and FoxO1 gene expression and decreased local T3 production.

In rats undergoing periods of reduced food intake followed by refeeding, skeletal muscle produces less active thyroid hormone T3 from its precursor T4, with measurable reductions of 18% after restriction and 14% after refeeding, corresponding to increased levels of the enzyme that inactivates T3 and decreased levels of the enzyme that activates it.

In rats, periods of reduced calorie intake followed by refeeding lead to a sustained increase in the DIO3-to-DIO2 enzyme ratio in skeletal muscle, which lowers local T3 levels, reduces energy expenditure in muscle, and promotes fat accumulation during weight regain.

In rats, periods of reduced food intake increase levels of two muscle-related proteins, calcineurin and FoxO1; after eating resumes, only FoxO1 stays elevated while calcineurin returns to normal, suggesting a persistent molecular change in muscle tissue.