When it comes to managing weight, the concept of “calories in” is clear, but what about "calories out" — also known as metabolism, or the rate the body burns calories?
Metabolism is one of the most important and most misunderstood factors in weight loss. Our resting metabolic rate and diet-induced thermogenesis — how fast our bodies burn calories after eating — are out of our control. These two factors account for about 85 percent of total metabolism.
Physical activity accounts for the rest of a person's metabolism. That’s where you CAN make a change. Exercise is the only way to produce a long-term impact on metabolism. The changes are small, but powerful, and can make a big difference in our weight.
Sadly, no foods can boost it beyond a tiny, short-term effect that's not enough to make any difference in your weight.
Understanding what's real about metabolism can be the difference between success or failure in losing and maintaining a healthy weight.
1. As you lose weight, your metabolism slows down.
Successful weight loss comes primarily from trimming calories from your daily caloric need. As you continue to lose weight, your daily caloric need decreases because there’s less body mass to sustain.
After losing weight, daily caloric intake needs to be adjusted downward to meet the caloric needs of the new body weight. And while the decline in metabolism varies with the individual, this deficit is more significant with a major weight loss, but still a problem with a weight loss of as little as 5-10 pounds.
Many people emotionally struggle with consuming fewer calories without the payoff of losing more weight. If cutting calories to stick to current weight is too challenging, many people opt to maintain at a slightly higher target weight.
Adding more physical activity can help offset the normal decline in caloric need.
2. Aging doesn’t lower metabolic rate.
It’s a myth that the process of aging alone causes metabolism to slow down.
While weight gain with aging is quite common, it’s a metabolic effect related to a decline in physical activity. Studies show a significant decrease in muscle mass — caused by a lack of physical activity —triggers the metabolic slowdown. It’s a widespread problem because most people tend to eat the same way, or more, as they age.
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When you’re not burning as many calories, but eating the same amount, you’ll gradually gain weight.
The good news: Increasing activity in all three areas of physical fitness — cardio, strength-training and flexibility — can help boost calorie burn and keep your metabolism stable.
No matter your age, think of 10,000 steps a day — about 4 miles — as a solid start for adding more activity, rather than as the ultimate goal. Try adding strength training, free weights or machines, and flexibility (yoga or pilates) if you can to round out the exercise triad.
3. Metabolism has three parts.
A threesome of metabolic inputs determines your final overall metabolism. Resting metabolic rate (RMR) is the number of calories needed to maintain basic, at-rest body functions. This accounts for about 75 percent of your daily calorie need.
Diet-induced thermogenesis (DIT: thermic effect of food) represents the calories needed to digest the food you eat. This is about 10 percent of your daily caloric requirement.
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Physical activity accounts for about the remaining 15 percent of your daily caloric needs.
While you can’t modify your RMR or DIT (these are genetically determined), you CAN boost this third component by being more physically active and contributing to great energy expenditure with regular exercise.
4. Exercise provides a temporary boost to your resting metabolism.
Studies show exercise, especially cardiovascular types like high intensity interval training (HIIT), provides a short-term increase in resting metabolic rate.
Look at this as a way to burn more calories and not a bonus to eat more food at a post-workout meal. With regular exercise, this small, short-term boost in metabolism can help make your weight loss or maintenance plan easier by a modest value-added calorie burn. This is particularly important during weight maintenance, when the total calories needed to maintain the new, lower weight is reduced from the start of a weight loss plan.
5. An increase in lean muscle mass can raise metabolism.
Every cell in the body burns calories to stay alive.
Some cells burn more calories than others: Muscle cells burn more calories than fat cells.
And no matter what your weight, if you continue to build your muscle mass, you will increase resting metabolism. This can be done using a two-part lifestyle challenge: eating more heart-healthy proteins — plant or animal — and stimulating your muscles through increased exercise.
The added protein provides the extra amino acids needed to support new protein growth, necessary for a stimulated muscle, where tiny tears normally occur with exercise, to rebuild and repair itself.
6. Some medications can slow resting metabolism rate.
Certain drugs, such as antidepressants and other mood-stabilizing medications, can lower metabolic rate over the long term and stimulate appetite. The amount of weight gain for any of these medicines can vary from person to person.
Drugs to treat diabetes can also cause weight gain, but via different metabolic mechanisms. Importantly, if you sense you are gaining weight from a new medication, don’t discontinue it, but check with your doctor for a different medication that doesn't have a metabolic impact.
If you do have medication-induced weight gain, studies show the added weight is not harder to lose than any other type of weight gain.
As always, remember to check with your doctor before making any changes in your eating, activity or prescribed medications.
Madelyn Fernstrom, Ph.D is NBC News Health and Nutrition Editor. Follow her on Twitter.