Basal Metabolic Rate (BMR) is the amount of caloric expenditure, or energy used, when at rest in a non-digestive state. Your BMR is the largest component of daily energy expenditure and is also highly variable depending on several individual physiological factors. Of these factors, it can be agreed that the most influential on BMR is the amount of fat-free mass or lean body mass that one carries. Additional factors that also play a role are age, sex, fat mass, hormonal secretions of leptin, as well as thyroid hormone T3 and T4.
What this means is that individuals with more muscle mass and lower body fat percentages will have a much higher BMR, so they will be able to consume more calories and will likely have increased insulin sensitivity, increased leptin sensitivity and higher productions of anabolic hormones. Individuals with lower lean body mass and higher fat mass will have lower BMRs and will likely have much more of a resistance to insulin and leptin which unfortunately will alter their ability to effectively transport glucose and feel satiety following a meal.
The implications of this suggest that people with higher body fat not only have lower caloric requirements due to a lowered BMR, but also find difficulty in feeling satisfied following a meal and also will not properly regulate hormones because of their high body fat percentages.
Calculating BMR has traditionally been done using several differing equations in the past, yet as new research comes out these equations must be altered or adjust to maintain relevance. Here are two of the popular equations:
Harris-Benedict (Lean Body Mass Unknown)
Katch-McArdle (Lean Body Mass Known)
Because of the distinctive correlation between lean body mass as well as hormones associated with musculoskeletal strength and hypertrophy training, it is extremely important that anyone who desires to increase their BMR, improve their body composition, or lose-weight must be engaging in regular strength training with a focus on increasing lean muscle tissue. http://www.calculatorpro.com/calculator/katch-mcardle-bmr-calculator/  http://www.ncbi.nlm.nih.gov/pubmed/16280423