Metabolism represents the sum of energy produced and utilized in the human body. This is the process for how you take in, store, and expend calories 24 hours a day. Most of the energy expended (60-75%) is accounted for by resting metabolic rate (RMR) that is the energy used for basic needs as keeping body temperature and maintaining organ systems. Individual activity on an everyday basis accounts for 15-30% of the total energy used and represents the metabolism that we can influence by changing our behavior.
The main organ that increases its metabolism in response to activity and exercise in skeletal muscle. This is a large tissue that composes approximately 40% of the adult human body weight. The rate of metabolism in muscle is influenced by genetics, hormones, nutrition, stress, disease, and is estimated to be about 10-15 kcal/kg/day (4,5-7 kcal/Ibs/day). For example, for a human weighing 160 Ibs, there are about 60-70 Ibs of skeletal muscle consuming about 250-500 kcal per day accounting for 15-25% of the total energy use in 24 hours.
The main option to influence our metabolism is to exercise to increase energy use during and after training. Elevating your heart rate by doing cardio workouts is the most efficient way to spend energy during exercise. The second way to increase metabolism is by doing strength exercises. Loading muscles with weight starts a biological program to adapt the muscle to increased demand for strength. This program results in “hypertrophy” which means individual muscle fibers grow in size to form a larger muscle. More muscle weight means that you are expending more energy even when not exercising as the resting metabolic rate increases to maintain the muscles.
The changes that we can make to our metabolism rate is fairly small in comparison to the energy we spend to just maintain our bodies. But fairly small changes have a marked impact over time. If you are able to increase your metabolic rate and energy use with 20 kcal (1 % of a normal 2000 kcal diet) every day the yearly total is 7000 kcal. This equals 2 Ibs of fat without directly changing diet to restrict energy intake. In summary, the most recent science indicates that a combination of cardio and strength is the best option for most humans to increase their overall metabolic rate.