Why is it that one person can eat 4,000 calories a day and never gain a pound, while another can eat less than 1,200 and still experience weight gain?

Much of it has to do with metabolism. Metabolism is the way your body turns food into energy, and it’s very specific to each individual. For good health and weight management, it pays to learn more about metabolism in general, and about yours specifically.

Metabolism, in general

Metabolism is not the only cause of weight gain or loss. But since it does affect how quickly or slowly your body processes calories, understanding it is crucial to weight management.

Weight gain happens when there’s a calorie surplus — that’s when you take in more calories than your body uses. If your metabolism is slow, your body takes longer to use the calories that you take in, so you’re more likely to store the surplus. 

Weight loss requires a calorie deficit — that’s when you use more calories than you take in. If you have a fast metabolism, you burn more calories even at rest, so it’s easier to create that deficit.

It’s a common misunderstanding that you’re stuck — or blessed — with the metabolism that you have. But metabolism is not static. It fluctuates with age and hormone levels, and you can change it by making changes in your body composition, diet, activity level and other parts of your life.  

Your metabolism, specifically

To understand your own metabolism, it helps to focus on two measurable numbers:

BMR – your basal metabolic rate. This is the minimum number of calories your body needs to perform its basic, vital functions — such as breathing, circulating blood, maintaining brain functions and regulating your body temperature — when you are at rest. It does not include calories used during physical activity. A higher BMR indicates that your body uses more energy when you’re at rest. That’s generally a good thing for weight management and metabolic health. Calculate your BMR.

TDEE – your total daily energy expenditure. This number represents the total number of calories (energy) that your body needs in a day. Your TDEE includes your BMR plus the energy you need for physical activity. It also factors in the energy you use for digestion (about 10% of your daily total). Calculate your TDEE.

Some of the factors that affect metabolism — such as your age, gender, height and genetics — are beyond your control. But many can be modified. For example, slow metabolism is associated with low physical activity levels, low muscle mass/high body fat, poor diet quality, irregular eating patterns, excessive calorie restriction, high stress and hormone dysfunction. Fast metabolism is associated with the opposite — and all of those factors can be changed.

Nine ways to optimize your metabolism

1. Avoid extremely low-calorie diets. Although quick weight loss might sound appealing, diets that restrict calories to less than your BMR can slow your BMR — and that slowdown can persist long after the diet is over.
   
2. Eat higher-quality food. Whole, unprocessed foods require more energy to digest. So when you eat things like whole grains, fruits and vegetables, you burn more calories simply digesting them — nearly 50% more, according to one study — than when you eat processed foods.
   
3. Eat regularly. Skipping meals and fasting for prolonged periods force your body to conserve energy, which slows down your metabolism.
   
4. Build some muscles. Muscle tissue uses more energy than fat tissue all day long, even when you’re sitting still. Strength training — whether with weights, resistance bands, pushups or planks — can build your muscle mass, improve your overall body composition and raise your BMR. 
   
5. Stay active. Include some cardio (walking, shooting hoops) and flexibility training (yoga, tai chi) in your routine, as well. Activity of all kinds keeps your muscles strong and your metabolism humming along.
   
6. Sleep well. Sleeping poorly, or not enough, affects your metabolism and the hormones that regulate it. For healthy metabolism, make sleep a priority.
   
7. Keep stress under control. Stress releases a hormone called cortisol, which slows down metabolism. If your stress is high and continuous, find healthy ways to reclaim your calm.
   
8. Stay hydrated. Water bottles aren’t just a fad. Good hydration is a necessity for optimal metabolic function. 
   
9. Monitor your thyroid health. The thyroid gland produces hormones that keep your metabolism balanced. An over- or underactive thyroid throws that balance out of whack. Regular checkups can help you keep an eye on things.

When to talk to your health care team

Problems with metabolism can affect your health as well as your weight. Metabolic disorders and dysfunction are associated with obesity, diabetes, cardiovascular disease, fatty liver disease and other chronic diseases. A BMR that’s especially high or low also could indicate an underlying health issue, such as hypo/hyperthyroidism or cancer. It’s important to work with your health care team if you have any of the following issues:

• Difficulty with weight loss or gain, or unexplained weight loss or gain 
• Chronically low energy
• Sleep problems 
• Diabetes or prediabetes, elevated LDL cholesterol or triglycerides, or elevated liver enzymes 

Metabolism is the sum of all the processes in your body that keep you alive. It’s a big deal, so it’s worth focusing some energy on making sure it’s as healthy as possible.