We all had that friend in high school who could put away a mountain of pizza with a shrug and a declaration: “I have a really fast metabolism.” One of these days, even that friend will be singing a different tune. We all will — because of how age affects metabolism.

While research differs on the specifics, the general consensus is this:

“As your age goes up, your metabolism slows down,” says Hartford HealthCare bariatric specialist Joseph St. Pierre, DO.

Here’s what to do about it.

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Metabolism 101: What happens to calories

To understand how age affects metabolism, let’s start with metabolism itself.

Metabolism is a chemical reaction in your body that converts what you eat — aka calories — into energy you need for every bodily function. Anything your body doesn’t burn for energy is stored in fat cells.

This process runs at a different baseline rate for everyone, depending on your genetics.

“People with a higher metabolism naturally burn more calories, more quickly, than people with a lower metabolism. The opposite is true for people with a lower metabolism,” says Dr. St. Pierre.

Eventually, though, everyone’s metabolism trends in the same direction — down.

> Related: 4 Ways to Speed Up Your Metabolism

Mid-life changes: How age seems to affect metabolism

As our age creeps upward, particularly past the age of 60, most people are faced with an unwelcome mystery. They have not changed their eating habits. They have not changed their exercise habits. Yet slowly but surely, they are gaining weight.

What gives? While there can be many contributing factors — lifestyle changes, for one — metabolism is a major player.

“With age, your base metabolic rate slows,” explains Dr. St. Pierre. “Some research suggests that every decade, your metabolism drops 1 to 2 percent.”

Because body doesn’t burn through as much fuel, you tend to have more leftover calories. Which turn into fat.

One reason: Muscle matters

Muscles are a major consumer of your body’s calories, requiring energy even when you’re resting. So a healthy percentage of muscle mass keeps your metabolism churning.

But starting in middle age, the human body starts to become resistant to muscle growth. By some estimates, we start to lose about 1% of muscle mass each year. Then things snowball.

“As you lose muscle mass, you essentially require less energy,” says Dr. St. Pierre. “That leads to decreasing metabolic function.”

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What to try: 2 ways to boost your metabolism

As a reminder, clear any new exercise and diet changes with your doctor first.

  1. Work those muscles. Try resistance training, which uses bands, weights or your own body to build your muscles.
  2. Consume more protein. It’s crucial for muscle growth. Plus, proteins take a lot of energy to digest, which helps rev up metabolism.

If you commit to both strategies, you’ll see a different kind of snowball effect — a healthy one.

“As we become stronger, we find we have more energy. We become naturally more active, and burn more calories through that added physical activity,” says Dr. St. Pierre.

Who to call: Experts in metabolism at every age

Weight gain is complicated, and metabolism is only one piece of the puzzle. For advice on the bigger picture, talk to a weight management expert.

“We can support you with your goals for today, and set you up with good habits so you can maintain that healthy weight as you age,” says Dr. St. Pierre.

After all, with age comes wisdom — including the wisdom to ask for help.