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How Metabolism Really Works

Spoiler: It's not the reason you have trouble losing weight.

Metabolism is a process. It's the way our cells change the food we eat into the energy we need to breathe, move, think and do . . . everything. It keeps us, as it keeps all living things, alive.

Instead of being grateful for metabolism, though, we tend to blame it. "Every day I see patients who say, 'I can't lose weight. My metabolism must be slow,' " says Mahtab Sohrevardi, MD, an endocrinologist at Rush University Medical Center.

It almost never is. Factors such as age, sex and body size cause natural variation in resting metabolic rate — the amount of calories, or units of energy, that we burn just by being alive. But those differences are normal, and they are not usually the root cause of obesity or inability to lose weight.

The real problem is almost always consuming too many calories and not getting enough exercise, which leaves us with excess energy that we store as fat. "The best ways to lose weight are staying active and eating a healthy diet — not trying to change the speed of your metabolism," Sohrevardi says.

But even though they can't usually increase the speed of our resting metabolic rate, our habits do affect our metabolism in five important ways:

1. How what you eat affects your metabolism

You might have heard that eating certain foods, or eating at a certain time of day, can make your metabolism faster.  In reality, the speed of your metabolism remains roughly the same no matter when or what you eat.

The few exceptions to this rule are not good weight loss strategies.

For example, studies have shown that caffeine slightly increases the rate at which you burn calories in the short term. But your body becomes used to caffeine over time, so the effect doesn't last if you're a regular coffee or tea drinker. Products promoted as metabolism boosters tend to be similarly ineffective, and some have dangerous side effects.

"Don't focus on how fast you metabolize food, but on the way you metabolize it," Sohrevardi recommends.

That means avoiding fried foods, such as potato chips; and refined sugars, such as those found in soft drinks, candy and many baked goods. These energy sources are the most likely to end up stored as fat.

Instead, choose whole grains, lean proteins, vegetables and fruit, which your body can more readily use to fuel its functions and activities.

2. How exercise affects your metabolism

You can't do much to affect your resting metabolism, which accounts for most of the calories most people burn each day. But building muscle can help. Muscle tissue uses more energy than fat tissue does, even when you're at rest.

This is a big reason that women, who tend to have less muscle tissue than men do, also burn fewer calories. And it's why older people tend to burn fewer calories than younger people do. Loss of muscle mass is a normal part of aging, but regularly working your muscles can help combat it.

Remember, though: While building strength can boost your resting metabolism, getting more aerobic activity is the most efficient way to burn more calories.

"Even walking 25 or 30 minutes a day, five days a week, is helpful," Sohrevardi says. And more intense exercise — running or aerobics, for example — burns even more calories.  

In addition to burning calories and building muscle, exercise has been linked to increased amounts of brown fat. The vast majority of our fat is white fat, the kind that stores energy. But we also have a few ounces of brown fat, often around our neck or shoulders, which actually uses energy to help keep us warm.

3. How your current weight affects your metabolism

The process of metabolism has two main parts. Anabolism helps you grow new cells, store energy and maintain your body tissues. Catabolism breaks down fat and carbohydrate molecules to release energy that fuels anabolism, keeps you warm and enables your muscles to contract.

Insulin is one of the many hormones that help regulate this cycle, by triggering anabolism after you eat. If you're significantly overweight, there's a high risk that your body will stop responding to insulin. As a result, sugar stays in your blood instead of being stored as energy.

This is the condition we call Type 2 diabetes. It can damage your organs and put you at risk for health problems such as heart disease, stroke and kidney disease. But Type 2 diabetes isn't always permanent. Many people can reverse Type 2 diabetes by losing weight through exercise, healthier eating habits or even bariatric surgery.

While building strength can boost your resting metabolism, getting more aerobic activity is the most efficient way to burn more calories.

4. How your former weight affects your metabolism

Having been overweight can continue to affect your metabolism even after you've lost the weight. That's one reason maintaining weight loss is much harder than keeping weight off in the first place.

"Take two people who weigh the same: one who's been at a normal weight all their life, and one who has struggled with obesity," Sohrevardi explains. "The first person can get an average amount of activity and eat an average amount of food, and nothing will happen to them. But often the second person, if they go from a restricted diet back to an average one, will have a high risk of putting a lot of the weight back on."

Researchers don't yet know exactly what causes this phenomenon. But studies have suggested that it has to do with hormonal changes after weight loss that both slow your metabolism and make you feel hungrier.

To help with this problem, Sohrevardi sometimes prescribes medications that suppress appetite. The FDA has approved a handful of appetite suppressants that have been shown to help people maintain weight loss when combined with exercise and healthy eating habits. There aren't yet medications designed to increase the speed of your metabolism.

5. How deprivation of food or sleep affects your metabolism

Regardless of your weight, eating too little can backfire by slowing the rate at which your body burns calories.

"I've seen people try not eating all day until dinner," Sohrevardi says. "That actually signals to your body that there's a shortage of food, so your metabolic rate goes very low. And as soon as you eat anything, your body is trying to store every single calorie in that food."

Even if you want and need to lose a lot of weight, you should aim to eat three or four small meals a day, comprising mostly vegetables, whole grains and lean proteins.

Finally, make sure you're getting seven to nine hours of sleep each night. Sleep deprivation can cause your body to produce too much insulin, which can lead to increased fat storage.

What if you really do have a slow metabolism?

While the state of our metabolism is usually linked to our weight and habits, underlying conditions do explain a small number of metabolic problems. Here are some of the conditions that cause unexplained weight gain:

Among these, hypothyroidism is the most common metabolism-slowing condition, particularly among women. Like all of the conditions listed above, it can cause many other symptoms, including fatigue, dizziness, skin problems and constipation.

"Whenever you have weight changes you can't explain, talk to your primary care doctor," Sohrevardi says. "Especially if you have other symptoms."

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