Not so fun fact: Your metabolism slows down as you age. But at what age does your metabolism slow down? Why does it happen? And what can you do about it?

Research suggests that your metabolism stays relatively stable from age 20 until you hit 60, at which point it begins to slow down. But don’t throw in the towel once you blow out 60 candles — there are many factors that influence your metabolism, excluding age.

There’s a lot to unpack there, so let’s start with the basics.

What Exactly Is Your Metabolism?

Metabolism is how your body turns what you eat and drink into energy. This usually involves breaking down food into usable, constituent parts your body can use:

• Proteins break down into amino acids, like leucine, which is responsible for muscle growth.
• Fats break down into fatty acids that become the main components of cellular membranes, as well as an energy source.
• Carbohydrates break down into sugars, like glucose, which provide fuel for muscle contraction and other processes.

These processes occur along your digestive tract. Once broken down, these smaller elements find their way into your body’s cells, where they combine with enzymes to perform any number of your body’s functions, including digestion, temperature regulation, cell repair, and muscle growth.

Metabolic processes are either catabolic — in which bigger things break down to smaller ones — or anabolic — in which smaller things assemble into larger ones.

Fat oxidation is a catabolic process in which your fat cells are broken into fatty acids and metabolized for fuel. Muscle growth, in which amino acids combine to form proteins to help build and repair muscle tissue, is an anabolic process.

When it’s functioning well, your metabolism balances both anabolic and catabolic actions, known as homeostasis: plenty of things breaking down and building up — but the net change is zero.

Why Does Your Metabolism Slow Down?

The metabolism is often compared to an engine, humming along at a constant speed. We envy the 140-pound runner because we think she has a fast metabolism. We pity the 350-pound powerlifter, believing he has a slow one.

In truth, the metabolism is less like an engine and more like a thermostat, heating up and cooling down in response to choices you make and things you do. Four major factors determine its speed:

1. The most significant factor, by far, is your resting metabolic rate (RMR) — the many processes involved in respiration, circulation, digestion, and other functions that occur, unconsciously, 24 hours a day.
2. Next up is non-exercise activity thermogenesis (NEAT), or daily living activities: standing, sitting, typing, walking, cooking, talking.
3. Next is formal exercise, from gym workouts to weekend runs to sports to yoga classes.
4. Finally, the thermic effect of food (TEF) — the energy required to break food down into usable parts.

So if you go for a run, or have a big meal, or take a walk, your metabolism speeds up in response to your muscles’ and your digestive tract’s need for fuel. If you sit down to rest or skip a meal, it slows down.

So our assumptions about the metabolisms of our different-sized athletes, then, is backward: Assuming they’re both in homeostasis — neither losing nor gaining weight — the 350-pound strongman, who eats everything in sight, has a much faster metabolism than the 140-pound runner, who watches every bite. The bigger athlete churns through calories while the smaller one uses relatively few of them.

One final factor influencing the speed of your metabolism is age. Most people believe that older people gain weight because they have slower metabolisms than younger ones.

So is it true?

How Much Does Your Metabolism Slow With Age?

Some metabolic processes do, indeed, slow down with age. A study conducted on men showed that your sodium-potassium pumps, which play a role in muscle contraction, slow down about 20 percent, resulting in about 100 fewer calories burned per day.

Simultaneously, the mitochondria — your cells’ engines — lose some of their efficiency, resulting in a slower caloric burn than you had at 18 or 20.

But these processes play a far smaller role in metabolic rate than the factors listed above — no matter how old you are. Another study compared the resting metabolic rates of people of various ages. And while it found that the youngest people tested burned more calories per day than the oldest ones, it also found that if you controlled for differences in gender, muscle mass, and fat, the difference decreased.

Can You Change Your Metabolism?

The message is clear: If you want to keep your metabolism humming along as you age, keep moving! Maintain a formal exercise program, and, if possible, take up active hobbies as well. The more you move in the gym and out of it, the faster your metabolism hums along. So fall in love with moving early, and keep it up for life.

Perhaps the best way to stoke the metabolic fire is with strength training, which can help combat age-related muscle loss which can then be associated with metabolic slowdown. Next up in importance is cardiovascular activity, which helps burn fat and gives your metabolism a daily kick. Finally, if you can get to it, increase your NEAT with daily walks and physical hobbies like gardening.

You can also (minimally) increase the thermic effect of food by consuming protein-rich foods. Animal products and high-protein vegetarian sources like black beans require more energy to digest than carbohydrates and fats, which require much less. So emphasize protein in your diet for an additional metabolic boost.

While we are still exploring the factors that cause your body to burn slightly fewer calories as you age, you can potentially mitigate this slowdown with the right exercise program. A slow metabolism in your later years isn’t inevitable — but it’s up to you to prevent it.