A study from the University of California, Davis shows that a high-fat diet prevents exercising mice from enlarging their muscles (Journal of Physiology, December 2010). The mice received either a low fat, high carbohydrate diet or a high fat, low carbohydrate diet for 14 weeks. Each group was divided into those who performed progressive resistance exercises with their plantaris muscles or those that did not do this exercise. Those who exercised on the low fat, high-carbohydrate diet had substantially larger muscles than those who exercised on the high-fat diet. Chemical analysis of their muscles showed that the high fat diet group had lower levels of polysomes (Akt and S6K1) necessary for making protein.
If this study can be applied to humans, it will mean that not only does a high-saturated-fat diet make you fatter, it also keeps you from enlarging your muscles. We know that both full fat cells and eating large amounts of saturated fats (the dominant fat in meat) turns on your immunity to cause inflammation that can prevent the body from making protein necessary for enlarging muscles. (Journal of Nutrition, January 2009). A high saturated-fat diet also blocks insulin receptors and thus prevents your body from responding to insulin, which is necessary for muscles to heal from intense workouts. Insulin drives amino acids, the protein building blocks, into muscles to help them heal faster. Anything that blocks muscles' ability to respond to insulin will decrease amino acid entry into muscles and thus delay healing so you can't recover as fast for your next workout.
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