expended at rest. For example, 1 MET is the rate of energy expenditure while at rest. A 4 MET activity expends 4 times the energy used by the body at rest. If a person does a 4 MET activity for 30 minutes, he or she has done 4 x 30 = 120 MET-minutes (or 2.0 MET-hours) of physical activity. A person could also achieve 120 MET-minutes by doing an 8 MET activity for 15 minutes. In a systematic review of physical activity and major chronic diseases, a meta‐analysis of an 11.25 MET h/week increase in physical activity yielded: a 23% lower risk of cardiovascular disease mortality (0.77 relative risk (RR), 95% confidence interval (CI), 0.71-0.84), and 26% lower risk of type 2 diabetes (0.74 RR, 95% CI, 0.72-0.77). Exercise guidelines The American College of Sports Medicine and American Heart Association guidelines count periods of at least 10 minutes of moderate MET level activity towards their recommended daily amounts of exercise. For healthy adults aged 18 to 65, the guidelines recommend moderate exercise for 30 minutes five days a week, or vigorous aerobic exercise for 20 minutes three days a week. Activities Limitations The definition of MET is problematic when used for specific persons. By convention, 1 MET is considered equivalent to the consumption of 3.5 ml O2·kg−1·min−1 (or 3.5 ml of oxygen per kilogram of body mass per minute) and is roughly equivalent to the expenditure of 1 kcal per kilogram of body weight per hour. This value was first experimentally derived from the resting oxygen consumption of a particular subject (a healthy 40-year-old, 70 kg man) and must therefore be treated as a convention. Since the RMR of a person depends mainly on lean body mass (and not total weight) and other physiological factors such as health status, age, etc., actual RMR (and thus 1-MET energy equivalents) may vary significantly from the kcal/(kg·h) rule of thumb. RMR measurements by calorimetry in medical surveys have shown that the conventional 1-MET value overestimates the actual resting O2 consumption and energy expenditures by about 20% to 30% on the average, whereas body composition (ratio of body fat to lean body mass) accounted for most of the variance. Standardized definition for research The Compendium of Physical Activities was developed for use in epidemiologic studies to standardize the assignment of MET intensities in physical activity questionnaires. Dr. Bill Haskell from Stanford University conceptualized the