The hypothesis was put forward in a new paper published in the journal Exercise and Sports Science Reviews. The work was done by a team of researchers from the UK and Lithuania.
Among their primary data points, the researchers gleaned information from six studies on high fat diets and muscle performance. All were done in mice. The review article also inlcudes as many as 50 citations.
The paper notes that increased body weight in the form of fat can actually increase muscle size and power compared to lean controls. But this is only true when the animal, whether it be a mouse or a human, is young. As people age and remain obese, muscles become fatty, too, and decline in performance. This continues to the point where the obese mice or humans have weaker muscles than their lean peers and yet still have to carry more weight.
Fat in muscles could account for decreased function
One of the theories developed in the paper as to why this is true is that when obese subjects take in too much food, they can overrun the ability of existing adipose tissue to store that excess energy fast enough. The body then starts to create fat reserves within the muscles themselves in the form of intramyocellular lipids (IMCL).
This is especially true in the case of a high fat diet (HFD). Fat is the most energy dense macronutrient and it is far easier to take in large amounts of calories as fat than in the form of carbohydrates or protein.
The researchers postulated that there may be compensatory mechanisms in younger muscle tissue that ameliorate this IMCL buildup.