People with pockets of fat hidden inside their muscles are at a higher risk of dying or being hospitalized from a heart attack or heart failure, regardless of their body mass index, according to new research. This 'intermuscular' fat is highly prized in beef steaks for cooking. However, little is known about this type of body fat in humans, and its impact on health. This is the first study to comprehensively investigate the effects of fatty muscles on heart disease. The new finding adds evidence that existing measures, such as body mass index or waist circumference, are not adequate to evaluate the risk of heart disease accurately for all people.