Several factors determine your basal metabolic rate:

• Your body size and composition. If you weigh more or have more muscle mass, you will burn more calories, even at rest. So people who weigh more are more likely to have a faster metabolic rate — not a slower one — because a portion of excess weight is muscle tissue.
• Your sex. If you’re a man, you probably have less body fat and more muscle mass than does a woman of the same age, so you burn more calories.
• Your age. As you get older, your muscle mass decreases, which slows down the rate at which you burn calories.

Rather than slow metabolism, factors more likely to contribute to weight gain include:

• Eating too many calories
• Getting too little physical activity
• Genetics and family history
• Certain medications
• Unhealthy habits, such as routinely not getting enough sleep

Your complete DNA sequence will help shape the future of medicine

From:

http://onlinelibrary.wiley.com/doi/10.1038/oby.2009.162/abstract

Gender was a significant determinant of BMR (basal metabolic rate) in children and adolescents but not in adults. Our results support the hypothesis that the age-related decline in BMR is due to a reduction in FFM (fat free mass). Finally, anthropometric predictors of BMR are as accurate as body composition estimated by BIA (tetrapolar bioelectrical impedance analysis).

Pyschological Management of Food Stressors is more important

The totality of energy expenditure is comprised of three main parameters: physical activity, the thermic effect of food (TEF), and resting metabolic rate or RMR [12]. Physical activity itself is comprised of both formal and nonexercise activity thermogenesis, termed “NEAT” [13]. These parameters have been estimated to constitute 30% (physical activity), 10% (diet induced thermogenesis), and 60% (RMR) of total daily energy expenditure. RMR is largely derived from the influence of “active tissue mass”, itself related to tissue oxygen usage [14]. Basal metabolic rate (BMR) is defined as the energy requirement necessary to maintain the function of cellular processes and differs from (is lower than) the resting metabolic rate, which constitutes the energy requirement to quietly rest while awake [1]. Nevertheless, an adult of normal body weight has a BMR/kg exceeding that of an obese adult by 1 to 1.5 times [15].

Obese subjects who have been energy restricted to induce weight loss display a rapid reduction in such active tissue mass with less of a reduction in metabolically less active tissue compared with controls [16]. These values however are subject to environmental modulation. Diet composition for instance is known to directly impact the TEF, with protein consumption inducing increased caloric expenditure manifested as heat production when compared with the other macronutrients [17, 18]. Not only has the TEF been shown to be an important contributor to satiety, with protein also being the most satiating, but the TEF is known to be higher in lean subject as opposed to their obese counterparts [19]. Thus, through such means, diet composition may play an important role in weight loss. Indeed studies have confirmed this conclusion [20, 21].

As described, one of the largest contributors to daily energy expenditure is RMR. With such a large influence upon energy expenditure, diet therapy aimed at modulating these effects may prove salutary. Specific dietary alterations amenable to positive changes in RMR may be more effectual than changes in TEF.