Gaining weight with each passing year is something many people seem to take for granted. At least, that’s the sense I get when I talk to many of my older patients, who are reluctant to take care of ever-expanding waistlines. Many of them think that “everybody puts on a few pounds as they age,” or they’ll point out that “I’ve gotten this far eating the way that I do—I’m not about to change now.”

But aging and weight gain don’t necessarily go hand-in-hand. Here’s why weight tends to creep up as you get older, along with some steps you can take to keep your weight in check.

Why People Gain Weight as They Age

Gaining weight as you get older happens for the same reasons it happens at any stage of life: it’s a matter of a calorie imbalance. Whenever you eat more calories than your body needs—when your “calories in” are more than your “calories out”—the extra calories get stored away as body fat.

With aging, it’s the “calories out” side of the equation, the number of calories the body burns each day, that tends to tip the balance. And there are a few reasons for this.

You may have heard that as you age your resting metabolic rate, which is the number of calories your body burns through every day just to keep your basic processes going, tends to drop. And it does. With every passing decade, the average person’s daily burn drops by about 150 calories.

This is because your resting metabolic rate is determined, in large part, by how much muscle mass you have relative to the amount of fat you have. Pound-for-pound (or kilo-for-kilo), muscle tissue burns a lot more calories than fat tissue does. And we tend to lose muscle mass as we age. As the amount of muscle tissue drops, so does your resting metabolic rate.

This loss of muscle mass can be attributed to a few things. First, as people age, their activity levels tend to decrease. They may do less exercise overall, and the intensity level of the exercise might also decline.

As muscles are used and challenged less, it should come as no surprise that there may be some loss of this metabolically active tissue. To be fair, a small amount of muscle loss may be unavoidable. The body’s ability to repair muscle tissue wear and tear does seem to decline with age, even if exercise intensity remains constant.

So, when all is said and done, this shift in body composition with the loss of calorie-torching muscle tissue means that your daily calorie needs drop as a result. If there’s no change on the “calories in” side of the equation—if you continue to eat as you always have—those extra calories that you’re not using are going to get stored away. And you’ve got a case of slow and steady weight gain on your hands.

There’s something else at play here as well. As people get older, the quality of the calories they take in sometimes shifts, too, in a way that can work against them.

For example, taking in adequate protein is necessary to help maintain muscle mass. In fact, older adults may actually have slightly higher protein needs than younger adults. But as people age, calories that were once spent on lean protein might now be spent on carbohydrates or fats.

Several factors might contribute to this shift. Taste changes, financial concerns, or even the lack of desire or energy to prepare meals. All of these can drive choices more in the direction of starchy, sweet, fatty foods that are easy to eat and prepare.

Five Tips for Avoiding Age-Related Weight Gain

Now that you know why weight gain happens with age, it should be pretty clear what steps you need to take in order to keep your weight in check.

• Since muscle tissue burns calories at a much more rapid pace than fat does, maintaining muscle mass is clearly one of the most important things you can do. You are capable of building muscle throughout your lifetime, so engage in activities that challenge your muscles to you build and maintain them.
• In order to support muscle repair and growth, be sure to include plenty of protein in your diet. Plant sources of protein, such as peas, beans and especially soy, which is a complete protein, offer healthy and affordable options to meet your daily needs.
• If you’re not in the habit of including plenty of vegetables and fruits in your diet, now is the time to start working on it. Vegetables and whole fruits are nutrient-dense, which means that they pack a lot of nutrition for relatively few calories. Buy fruits and veggies in season whenever possible—they’re often fresher and more affordable. And don’t overlook the freezer section of the supermarket. Frozen fruits and vegetables are just as nutritious as fresh varieties; they’re super-convenient, and there’s no waste.
• Since every calorie counts, pay close attention to the extras in your diet. Calories from fats, sweets and alcohol add up quickly, so choose wisely and use them sparingly.
• Try to be in charge of your diet as much as you can. If cooking is a chore, seek out quick, healthy recipes, and always cook extra so you can put food away for another meal. Eating out is fun and convenient, but educate yourself as to the best choices so you don’t overdo it. Restaurant portions are often large and servers may push you to order extras you don’t want, so decide ahead of time what you’ll eat and commit to it.