Diminished Muscle Thyroid in Muscle Responses following Caloric Restriction

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calorie restriction, thyroid, infinite_labs

Diminished Muscle Thyroid in Muscle Responses following Caloric Restriction

By Robbie Durand

It is not uncommon for bodybuilders to lose muscle and they are getting ready for a competition. Results from some studies indicate a general endocrine response to hypocaloric diets that promotes increased hunger, reduced metabolic rate, and threatens lean muscle mass. No matter how much protein a bodybuilder consumes, it’s nearly impossible to keep all the muscle while dieting to get ripped up.

Severe calorie restricted diet is often associated with a reduced thermogenic response in muscle. The reduction in thermogenic response is due to some hormones such as thyroid, insulin, testosterone, and leptin. The hormones of the thyroid gland, particularly triiodothyronine (T3), are known to play an important and direct role in regulating metabolic rate. Increases in circulating thyroid hormones are associated with an increase in the metabolic rate, whereas lowered thyroid levels result in decreased thermogenesis and overall metabolic rate.

Some studies have reported that caloric restriction results in a reduction in thyroid function.  Studies involving calorie restriction report decreases in leptin, insulin, testosterone, and thyroid hormones.  Weight loss results in a loss of metabolically active tissue and, therefore, decreases basal metabolic rate. Researchers wanted to examine what happens in muscle tissue during a calorie restricted diet and also what happens after refeeding.

calorie restriction, thyroid, infinite_labs
This catch-up fat phenomenon is postulated to result partly from suppressed skeletal muscle thermogenesis that occurs with food refeeding, but the underlying mechanisms is not entirely understood.

Weight regain after caloric restriction results in accelerated fat storage in adipose tissue. If you have ever seen a competitor after 12 weeks of dieting, it does not take long for them to put back the weight they loss during calorie restriction.  This catch-up fat phenomenon is postulated to result partly from suppressed skeletal muscle thermogenesis that occurs with food refeeding, but the underlying mechanisms is not entirely understood. Researchers examined muscle genes responses during and after caloric restriction. Using a rat model of semistarvation-refeeding, In brief, rats were food-restricted at 50% of their spontaneous food intake for two weeks (semi-starved rats), after which they were refed the amount of chow corresponding to the spontaneous chow intake of control rats matched for weight at the onset of refeeding.  The researchers measure the animals muscle size and performance under both conditions of calorie restriction and refeeding.  The researchers also measured the muscle thyroid levels by taking muscle biopsies.

calorie restriction, thyroid, infinite_labs
The researchers found that the muscle fibers of the rats were altered from a fast twitch, explosive muscle fibers (i.e. more energetically costly) to a more fuel efficient slow twitch muscle fiber while dieting. The researchers speculated that the changes in muscle fiber types from a fast-twitch to a slow twitch muscle fiber resulted from the reduced calories and a need to make the muscle more fuel efficient.

At the end of the study, caloric restriction led to reduced muscle growth, whereas isocaloric refeeding resulted in weight gain at a slightly higher rate than in the control rats. This is a very common experience that many competitors have, they look better after the competition has ended with re-feeding.  The semistarvation-diet induced changes persisted during recovery and correlated with impaired expression of transcription factors involved in slow-twitch muscle development. The researchers found that the muscle fibers of the rats were altered from a fast twitch, explosive muscle fibers (i.e. more energetically costly) to a more fuel efficient slow twitch muscle fiber while dieting.  The researchers speculated that the changes in muscle fiber types from a fast-twitch to a slow twitch muscle fiber resulted from the reduced calories and a need to make the muscle more fuel efficient. 

There is evidence that slow-twitch muscles use less ATP  than fast-twitch muscles. The researchers found the calorie restricted diet resulted in a decrease in metabolic rate in conjunction with decreased availability of muscular thyroid T3 levels. The altered thyroid hormone metabolism, fiber type composition, and contractile properties constitute mechanisms by which diminished skeletal muscle thermogenesis could contribute to energy conservation during weight loss.The altered muscle function was likely induced by changes in thyroid hormone levels. This is strongly suggested by diminished T3 availability within skeletal muscle.

In sum, the researchers concluded that diminished muscle thermogenesis following caloric restriction results from reduced muscle T3 levels, alteration in muscle-specific transcription factors, and fast-to-slow fiber shift. These energy-sparing effects persist during weight recovery and contribute to catch-up fat.  This study reports that calorie restriction results in a reduced muscle thyroid levels that led to a shift in fiber types from the fast twitch muscle hypertrophy prone fibers to slow twitch aerobic muscle fibers.

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