Many people blame their metabolism as a reason beyond their control for why they are overweight, claiming those gifted with a “better” metabolism can eat anything they want and never gain pound. Metabolisms seem to be either fast or slow, high or low. But just what is your metabolism, then? What role does it play in body weight and body composition (muscle vs. fat tissue)? And how does exercise influence metabolism? Read on to learn more.
Metabolism: Energy Balance
Your body takes the food you consume and converts it into energy. What you weigh, and your body’s proportion of lean muscle and fat (what we call “body composition”) is ultimately determined by the number of calories you eat versus the number of calories your body uses. On any given day, if you take in the same number of calories that you use, your body is in “homeostasis” and everything stays just the way it is. But if you take in calories more calories than you expend, the extra calories are stored as fat. If you take in fewer calories than you use, the resulting caloric deficit will be made up for by tapping into the energy your body has already stored for later: stored sugar, in the form of glycogen (in your muscles and liver) first, and then fat.
Sounds pretty easy, right? Well, not quite so, unfortunately. More on this further down, but first, let’s understand the three ways your body expends energy: BMR, the thermic effect of food, and physical activity.
1 – Basal Metabolic Rate (BMR)
At the most basic level, your body needs energy for all the functions that keep you alive: breathing, blood circulation, blinking, swallowing, digesting, and growing or repairing cells. Energy is also needed to maintain “metabolically active” tissues – primarily muscle. The number of calories your unique body needs to continue to exist is what’s called your “basal metabolic rate” or BMR. The biggest factors influencing a person’s BMR are (1) body composition, meaning muscle vs. fat tissue; (2) gender; and (3) age. Gender, because men naturally have more muscle tissue than women; and age because as we get older, the amount of muscle tends to decrease and fat accounts for more of total body weight. Do you see a common theme here? The more muscle tissue you have, the more calories your body burns, even at rest, and therefore the higher your BMR.
BMR slows down with age, which makes it easier for fat to be stored. The biggest reason why is the loss of muscle and bone mass that occurs as we age. Between the ages of 30 and 70 our lean muscle mass declines by as much as 40%. Every pound of lean muscle burns about 35 to 50 kilocalories (kcals) per day. Losing just a half pound of muscle could theoretically lead to a weight gain of 2.6 pounds in a year, which snowballs into 26 pounds in ten years!
2 – Thermic Effect of Food (TEF)
The Thermic Effect of Food, or TEF is the calories burned during the digestion process. Remember, calories are energy – and every function of your body requires energy to occur. On average, TEF comprises about 10% of the calories you burn per day. There are three key factors that influence TEF: what you eat, how much you eat, and how often you eat.
What you eat
Some foods need more calories to digest than others. Protein has the highest TEF: For every 100 calories of protein you eat, it takes about 27 calories to digest them. Carbs are second highest, with 23 calories burned for every 100 eaten. Fat, unfortunately, uses only 3 calories (!) to digest 100 fat calories you eat. This is one reason to reduce your fat intake. Replacing fat in your diet with either carbs or protein will not only reduce your overall caloric intake (gram for gram, fat has more than twice as many calories as protein or carbs) but it will burn more calories just to digest.
Eating more fiber is another way to boost TEF. Because fiber-rich foods are harder to digest, more calories are used for digestion and so fewer calories are absorbed. According to a Tufts University study, if you were to increase your daily fiber intake to 24 grams, you’d absorb 90 fewer calories every day. This single change alone could potentially add up to a 9 pound weight loss in one year!
How much you eat
How much you eat at one meal plays a key role in TEF. Let’s say someone doesn’t eat a thing all day, and then gets home at 6PM and eats an entire day’s worth of calories at one sitting. Well, eating too much food at any one time has been shown to suppress TEF and cause more calories to be stored! The reason is that your digestive system only has so many receptor sites to absorb nutrients from food. When you eat too much at one time, your digestive system becomes overwhelmed – and so it stores more of the calories as fat. This is why multiple, smaller meals are overwhelmingly recommended if you’re trying to lose weight. Coincidentally, it’s also one of the stronger scientific arguments against the current trend of “intermittent fasting” where you eat just one meal per day.
When you eat
When your mom said breakfast is the most important meal of the day, she was right! Eating breakfast has been shown to result in a 5% boost in metabolic rate. On the other hand, waiting too long between meals causes a slowdown in metabolic rate – and a tendency to eat more at your next meal. Once again, eating smaller, more frequent meals allows TEF to kick in, and regulates your blood sugar.
On the other hand, too many meals may be counterproductive. Eating a snack or meal that’s less than 10% of your daily caloric intake does not result in an increase in calories burned from TEF.
Find a balance
The best way to optimize calorie burn through TEF is to find a plan that works best for you: that fits into your schedule, allows you to maintain even blood sugar levels, and avoids too much time between meals. Many published studies on weight loss recommend eating four meals, evenly spaced throughout the day – with each meal comprising about 25% of your total daily calories.
3 – Physical activity
The third component of the energy expenditure equation is physical activity. And this includes every level of activity – not just “exercise.” Physical activity can account for 15% to 30% of the calories your body burns each day; the more active you are, the more you burn. Regular activities of daily life, and even just fidgeting all expend calories. Think of taking the stairs instead of the elevator, parking further away from the store, mowing the lawn, and shoveling your driveway.
Ever heard the meme that “sitting is the new smoking?” There’s truth to that! Consider that at rest (sitting or lying down), your body uses 3.5 milliliters of oxygen per kilogram of body weight each minute, which translates into 5 calories burned. Just standing up doubles the oxygen cost of sitting, and along with it, the calories burned. So if you stand to talk on the phone instead of sit, you’re burning twice as many calories! Add movement to the mix, like walking slowly, and the calorie rate triples.
Adding Exercise
As you’d imagine, exercising increases total daily calorie expenditure. An added bonus: certain levels of exercise intensity create an increase in calorie burn for a period of time afterward. Known as Exercise Post Oxygen Consumption (EPOC), this is your muscles and liver rebuilding their glycogen (sugar) and muscle protein stores.
Exercise and Metabolism
There are many reasons to exercise regularly. One very important reason is to maintain and increase lean muscle tissue. You’ll raise your metabolic rate as well counteract the age-related decline in muscle and bone mass.
It’s a common misconception that only cardiovascular exercise (think very high intensity and long duration) is burns calories. Cardio workouts are an important aspect of anyone’s fitness program, but their best purpose is to increase the strength of the heart and lungs. To really increase metabolism, strength (a.k.a. resistance) training is the key. Strength training can burn just as many calories per minute as aerobic exercise, but you’re building lean muscle tissue – which is much more metabolically active than fat. The more muscle you have, the higher your metabolic rate. Each pound of lean muscle burns about 35 to 50 kilocalories per day! Learn more by reading Fat Burning Workouts – Help or Hype? and Your Body’s Fat Thermometer.
The Takeaway
Your body is amazingly good at maintaining the status quo. Moving it to a new “normal” takes effort and patience. Making small, progressive lifestyle changes that create a small daily calorie deficit; maximizing the thermic effect of the foods you eat and how; and incorporating more movement into your daily life – including strength training – you’ll get there.