Chances are you have blamed some part of your body’s functioning on its metabolism. Common complaints, to name a few, include calling it "slow" and pegging it as the cause of weight gain. It has even been simplified to the old adage: Calories in, calories out. But it isn’t that easy. It's why WW asked experts to tackle the most common questions about metabolism to give you the big picture on what’s going on.
What is metabolism?
“Metabolism is the way we process foods and nutrients and convert them into energy. That’s what people think about when they think about metabolic rate—the rate at which we convert the foods we eat into energy,” says Scott Summers, PhD, chair of the department of nutrition & integrative physiology at the University of Utah School of Medicine.
When you eat, your body either uses the nutrients as fuel for activities, such as exercise, or stores them as heat—in the form of energy or fat.
What bodily functions are fueled by the metabolism?
While exercise certainly burns calories, there are other body processes that play a much bigger role in converting calories into energy. Here’s the breakdown:
About 70 percent of all the calories you burn are due to your basal metabolic rate. It’s the energy needed to keep your body functioning while at rest. “That’s just the energy it takes to be alive. That’s the energy it takes to run you—keep your heart beating, mechanical stuff like breathing in and out, digestion,” says Douglas White, PhD, associate professor in the department of nutrition, dietetics, & hospitality management at Auburn University.
The basal metabolic rate is theoretical, since measuring it requires complete inactivity, White says. Resting metabolic rate, or RMR, is the closest approximation to a basal metabolic rate and what researchers use to study what affects metabolism. (See below: Measure Your Resting Metabolic Rate.)
- Physical activity burns 20 to 25 percent of the total calories you expend. Your activity level and intensity affect that percentage. If you’re a professional athlete, that percentage may be even higher. But activity doesn’t just mean exercise, White says, but includes any movement. “Just walking upstairs, just standing up, that’s going to burn some calories,” he says.
- Food digestion accounts for approximately 10 percent of calories burned. Known as diet-induced thermogenesis, thermic effect of food, or TEF, White says: “This is basically the energy it takes to digest and absorb and transport carbohydrates, proteins and fats.”
- Finally, a tiny component of your metabolism is adaptive thermogenesis: And this only applies if you’re exposed to cold climates. “That’s your body’s reaction to cold,” White says. “So, for example, if you lived in Barrow, Alaska, you would produce a little bit of heat to keep your body warm.”
What determines my metabolism's speed?
Many people wonder what they can do to increase their metabolic rate. There are several things that determine our metabolism that we cannot change, such as:
- Genetics: Physiological characteristics we inherit from our parents—genetics—account for 80 percent of our metabolism, Summers says. “There’s a huge genetic component to how [our bodies react to] the food we eat.”
- Gender: Men tend to have a higher metabolism than women. “Probably because of the anabolic effects of testosterone, which tends to build more muscle. And more muscle tends to increase the basal metabolic rate,” explains White.
- Age: Metabolism decreases with age. Part of this is the loss of muscle mass as you age. “Even accounting for the loss in protein, there is an age-related drop in basal metabolic rate,” White says.
- Height: The taller you are, the higher your metabolism, White says.
Metabolic rate is also highly individual and ever changing, depending on what you do throughout the day. Even between individuals with the same gender, weight, and body composition, metabolic rates can vary. Researchers don’t yet know why, White says.
What health conditions affectmy metabolism?
Certain diseases can affect your metabolism. Your endocrine system regulates hormones throughout your body that affect metabolism, mood, sexual function, and growth and development. Diabetes and thyroid disorders are two common endocrine diseases that impact your metabolism, according to the National Institute of Diabetes and Digestive and Kidney Diseases.
Your primary doctor, or a specialist known as an endocrinologist, can check your hormone levels to make sure you don’t have a condition that is affecting your metabolism. In some cases, such as with thyroid disorders, hormone replacement medication can get your levels back to normal.
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How does my weight affect my metabolism?
It may seem counterintuitive, but the more you weigh, the higher your metabolism. That’s because the more you weigh, the more energy your body needs to keep it going, White says.
“It has real implications for when you do lose weight. You can never go back to eating the same amount you did before,” says Lawrence Cheskin, MD, director of clinical research at the Global Obesity Prevention Center at Johns Hopkins Bloomberg School of Public Health. “So that’s why people will often say, ‘This diet screwed up my metabolism.’ But it’s mostly proportional to the fact that now you’re a smaller person.”
Does what and how I eat affect my metabolic rate?
“What you eat doesn’t affect your metabolic rate very much,” Summers says. “I don’t know many dietary interventions that affect metabolic rate in a consistent way.”
Another thing to consider, your body has a set point and it wants to stay there, White says. That’s why crash dieting doesn’t work long term.
“If you eat too much, your metabolic rate slightly speeds up but not to the point that it offsets the number of calories coming in,” Summers says.
On the other extreme, severe calorie restriction might cause you to lose weight fast, but it slows down your metabolism and has other negative consequences. Your body tries to become more efficient as a life-preserving measure to keep you from starving, Cheskin says. “It adapts everything from your thyroid, to your degree of fatigue, to how much you sweat, and eventually, whether your hair starts to fall out because your body is trying to conserve energy.”
How does exercise affect my metabolism?
Activity is the largest component of metabolism that we can control, accounting for between 20 and 25 percent of our metabolism.
“Try and match your calorie intake with the amount of calories you’re burning safely,” Summers says. “If you don’t burn off the calories you take in, then you store them as fat. Any exercise, even the smallest amount of exercise has health benefits. So, something is better than nothing. And [more] is better than a little.”
What about exercise “afterburn”—doesn’t that help?!
Maybe you’ve heard of the metabolic benefits of exercise “afterburn”. The technical term for it is excess post-exercise oxygen consumption or EPOC. It’s the extended increase in oxygen uptake after a bout of exercise, but it doesn’t have a significant contribution to energy expenditure, says Christopher Scott, PhD, professor in the department of exercise, health and sport sciences at the University of Southern Maine.
“While any increase in metabolic rate helps ‘burn calories,’ the contribution of EPOC is certainly helpful but likely minimal,” Scott says, adding that athletes may have more meaningful increases in metabolic rates due to their training demands, but regular physically active people “not so much.”
“The best way to take advantage of the afterburn (EPOC) period is to perform a low-intensity active recovery after a bout of exercise,” Scott says, who studies human performance and wrote a textbook on the topic, A Primer for the Exercise and Nutrition Sciences: Thermodynamics, Bioenergetics, Metabolism.
“As strange as it may sound, EPOC periods may be better associated with the utilization of fat as a fuel as opposed to a meaningful caloric cost. Fat is a favored fuel of working muscle at a lower intensity,” Scott says. “So brief periods of high intensity exercise or resistance training followed by a longer period of walking may serve as the ideal exercise prescription for weight loss.”
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How to Measure Resting Metabolic Rate
Knowing your resting metabolic rate—the calories you need to consume to maintain your body’s daily functioning—can help you to lose or maintain weight. “Theoretically, since that’s 70 percent of your total metabolism, you can make adjustments to see what your total energy expenditure would be and then what would roughly be the number of calories you would need to consume to maintain your body weight,” White says. Here are three of the main ways it’s measured:
- Indirect calorimetry: How much energy you’re burning is directly proportional to how much carbon dioxide there is in your breath for your size. Indirect calorimetry uses a device to measure the amount of CO2 your body produces. Many doctor’s offices and health clubs have this equipment.
- Direct calorimetry: In direct calorimetry, you’re placed in an insulated water chamber to measure your body’s heat transfer. It’s a much more accurate measurement of energy expenditure, but it’s expensive and typically only found at weight loss clinics and medical centers.
- Harris-Benedict Equation: For the DIY approach, the Harris-Benedict Equation (HBE) uses age, gender, height, weight and activity level to estimate your resting metabolic rate. According to a 1984 study, HBE accurately predicted resting energy expenditure, another term for resting metabolic rate, in normally nourished individuals but not in those who were malnourished. There are several online Harris-Benedict calculators you can use.
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