Take a look at these images:
The USDA assigns grades to cuts of meat based upon the degree of what’s known as marbling. The cut on the lower right is considered to have “slight” marbling, while the one on the upper left has “moderately abundant” marbling. And the higher the marbling, the higher the grade of meat.
The key point to note here is that marbling is simply intramuscular fat that has built up as a result of the accumulation of unexpended triacylglycerols. And the most effective way to produce marbling (intramuscular fat) in livestock is to feed them high amounts of cereal grains such as corn or barley, and to limit their total energy turnover (physical activity).
So if you’re consuming a grain-based diet with limited physical activity, you are essentially “marbling” your own muscle tissue.