Basal (or resting) metabolism refers to the total number of calories all the cells in the body need to stay alive and functioning. "Your resting metabolic rate is typically described as the total number of calories your body needs while at rest. This is made up of basic functions like supporting your vital organs, muscle and fat tissue and the energy that is required to break down food we eat," said Martin Binks, director of the Nutrition & Metabolic Health Initiative at Texas Tech University.
"Your total calorie requirement includes these plus what's needed to move around," he added. "Therefore, by nature of having more mass, a larger person burns more calories."
Melissa Majumdar, registered dietitian, personal trainer and senior bariatric dietitian at Brigham and Women's Center for Metabolic and Bariatric Surgery, put it this way: "If we kept two people still in a bed with different body weights, the heavier person will burn more."
This biological reality also explains why, after you lose an initial amount of weight -- say 10 or 20 pounds -- it's harder to continue losing weight. At a lower body weight, you burn fewer calories, and so the amount of energy, or calories, you once required decreases. That means you need to consume fewer and fewer calories to continue losing weight.
All of this relates to what is known as our basal or resting metabolism, which is directly related to our body weight. But other factors contribute as well, including our body composition, or the amount of lean muscle versus fat mass.
Although the exact number of calories burned by muscle versus fat is quite variable among individuals, fat is not very metabolically active -- meaning it takes very few calories to keep fat tissue alive: specifically one to two calories per pound per day, compared with muscle, which is often thought to burn between five and 13 calories per pound per day, according to Binks.
Why body composition matters
At a given body weight, someone with a higher amount of lean mass -- which includes connective tissue, muscle and bone -- will burn more calories than someone with less lean mass. Think of an athlete and a non-athlete with identical body weights; the athlete with a more muscular build will have a higher metabolism.
Similarly, a lean, muscular person who weighs 120 pounds may have a healthy amount of lean mass and a relatively "fast" metabolism. But another 120-pound person may have a lot less lean mass and more body fat and therefore a "slow" metabolism.
In fact, it is possible to have "normal-weight obesity" -- a term used when referring to a person who appears thin but who is not very active and therefore has very little muscle mass, Majumdar explained. "We know that someone's appearance doesn't tell the whole story and that nutrition and exercise are important for even the naturally thin person in preventing chronic disease," Majumdar said.
The role of physical activity
Our basal or resting metabolic needs, which are based on body weight, body composition and other factors, address only one variable involved in determining how many calories we burn.
Another important determiner of the total amount of calories we burn is the amount of physical activity we engage in. It's not metabolism per se, but it's significant, because it can vary greatly depending on how active we are and determines how many calories we need to eat each day in order to maintain our body weight. In addition to body composition, it's one of the factors we can change if we want to lose weight.
"While we may not have control over genetic factors that influence metabolism, physical activity -- both through exercise and everyday movements -- can account for 15% to 30% of metabolic needs," Majumdar said.
Here's something NEAT
Don't like going to the gym for a high-impact cycling class? You don't necessarily need to, as long as you don't sit still for periods of time.
NEAT, or non-exercise activity thermogenesis, is the energy expended for everything we do that is not sleeping, eating or sports-like exercise, and it includes walking and fidgeting.
"NEAT may have a significant effect on weight management and vary up to 2,000 calories per day for some individuals," Majumdar explained. In other words, even small movements throughout the day can add up and contribute to your total calorie burn.
Genes count, too
Interestingly, our genetic makeup may play more of a role than we think in terms of determining our body weight. One recent study found that healthy thin people are generally thin because, fortuitously, they have fewer genetic variants that are known to increase our chances of becoming overweight.
"These people may have no obvious metabolic rate difference, but they probably have a genetic makeup such that they may eat less or metabolize things in a different way," said Dr. Steven Heymsfield, a professor in the metabolism and body composition laboratory at the Pennington Biomedical Research Center, who was not involved in the study. "The genes are stacked in their favor."
The bottom line
If you feel that your genes aren't working in your favor, the good news is that there are things you can do to boost your metabolism and burn more calories. Start by building and maintaining lean body mass, which tends to decrease with age. This can be accomplished through resistance exercises like weightlifting, Binks explained.
And keep moving, even if it means getting out of your chair every 30 minutes and doing a yoga stretch or taking a walk around the office.
It all counts, which is pretty neat!
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