Does Muscle Fibre Type Determine Dietary Weight Loss?

Skeletal muscle

Skeletal muscle

Skeletal muscle, even when it is not active, is an important determinant of energy expenditure, making up 15-20% of resting metabolic rate.

But not all muscle is created equal – it is made up of varying amounts of slow Type 1 and faster Type 2 fibres, the former being more dependent on oxygen consumption and capable of endurance exercise, the latter being more capable of non-oxidative activity as in short sprints or lifting weights.

People tend to differ in the make up of their muscle fibres, which explains why some people are better at running marathons while others excel at the 100 meter dash.

But does this difference in muscle composition also affect the response to dietary weight loss?

This question was addressed by Martin Gerrits and colleagues from the University of Ottawa, in a paper just released online in the Journal of Lipid Research.

The researchers studied 26 otherwise healthy obese women who were either in the top (diet-sensitive) or bottom (diet-resistant) quintile of weight loss during the first six weeks of a medically supervised 900 kcal/day low-calorie diet program. All participants were highly compliant with the diet (meaning that they only ate 900 kcal/day) and did not differ in levels of physical activity. Subjects were matched for both age and initial BMI.

While the diet-sensitive group lost 11.4 kg, the diet-resistant group lost only 7.5 kg during the first 6 weeks. This translates into an almost 25% or 350 kcal/day lower caloric deficit in the diet-resistant group.

Following program completion and weight stabilization, skeletal muscle biopsies showed a higher proportion of oxidative Type 1 fibres in the diet-sensitive compared to the diet-resistant women.

Gene expression analysis also showed upregulation of genes involved in oxidative phosphorylation, glucose and fatty acid metabolism in the diet-sensitive group.

Thus, these data support the notion that the rate of weight loss in response to dietary caloric restriction may very much depend on your muscle composition.

Clinically, this would mean that people who are better at aerobic (endurance) exercise may find it easier to lose weight than people who are better at anaerobic exercise (sprints or lifting weights).

Perhaps the reason that people who combine diet with endurance exercise tend to lose more weight than people who prefer resistance training has more to do with their muscle composition than with their choice of exercise (or the calories burnt doing it).

Edmonton, Alberta