How much does your metabolism really slow over the years?

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"My metabolism just isn't what it used to be." We've all said it before – usually as we lament the fact that if we even thought about eating like we did back in high school, our bellies would jiggle for a week.

But how much does your metabolism really slow over the years? And is it enough to explain why you're gaining weight, or having trouble dropping pounds?

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Your Metabolism, Decoded

Before we dive into exactly how your metabolism changes over time, it's important to know what your metabolism actually is – and everything that goes into setting it.

Simply put, your metabolism is the sum of every single process that occurs in your body. All of those processes require energy, or calories. The bulk of those calories – about 60 to 75 percent of your daily energy expenditure – goes toward just staying alive, explains certified strength and conditioning specialist Taylor Gainor, co-founder of LIT Method in Los Angeles. That number of calories burned is termed your basal metabolic rate, or BMR, and it's determined by your genetics, gender, height, weight, muscle mass and, of course, age, she says. (The other 25 to 40 percent of your daily caloric expenditure is due to talking, walking, chewing, digesting, exercising and recovering from that exercise.)

So when people talk about boosting their metabolism, what they are really talking about is increasing their basal metabolic rate: the base number of calories they burn per day.

Unfortunately, BMR does decrease through the years – starting at about age 20, explains Sarah Calamita, a registered dietitian and board-certified sports dietitian with the East Bank Club in Chicago. Yes, that early! At first, the decline is relatively slow and steady, at about 1 to 2 percent per decade. That means that at age 30, you'd have to eat about 150 fewer calories per day to weigh what you did at age 20, she says. Then, after around age 40 in men and 50 in women, metabolic rate often accelerates its decline, according to board-certified internist and endocrinologist Brunilda Nazario, associate medical director at WebMD.

Research from Johns Hopkins University shows that the average adult gains 1 to 2 pounds per year from early adulthood through middle age. Coincidence? Doubtful.

[See: 13 Fun Sports That Burn Calories.]

Click below for foods that boost your metabolism:

Foods that increase your metabolism
See Gallery
Foods that increase your metabolism
Green tea -- especially Matcha, is high in antioxidant EGCG which is known to speed up metabolism.
Grapefruit -- High in the compound naringin, this tangy fruit helps lower your blood pressure, leading to a slimmer waist.
Dairy -- especially nonfat and Greek yogurt, has high calcium and protein, which promotes weight loss.
Cold water -- Your body has to work to warm the water to body temperature, so it helps to speed your metabolism.
Spicy foods -- experts say spicy food can increase your heart rate, resulting in a metabolism boost.

Why So Slow?

As you age, your body just doesn't need as much energy to do its thing. It shifts from its "growing" stage into its "aging" one, with levels of the aptly named human growth hormone lowering right alongside your metabolism. Meanwhile, changing levels of other hormones – including thyroxin (produced by your thyroid gland), testosterone and estrogen – can directly affect BMR, Calamita says. All of this is natural – and actually a pretty small part of why metabolic rate declines with age.

The bigger reason? The older we get, the less muscle we tend to have. You can blame a combination of genetics, hormones and inactivity for that. In fact, physically inactive people (more on activity later) can lose as much as 3 to 5 percent of their muscle mass each decade after age 30, Nazario says.

And, like we said earlier, muscle mass is one of the primary determinants of your BMR. "Muscle is extremely metabolically active, meaning that it burns calories even when you're at rest, whereas fat is not," she says. So every decade that you lose muscle, you are burning fewer calories and, unless you are cutting calories, you are gaining weight.

[See: The 38 Easiest Diets to Follow: in Pictures.]

Increase Your Metabolism, No Matter Your Age

Explain it like that and the solution becomes pretty obvious: build muscle. After all, if a decline in muscle mass is the No. 1 reason metabolic rate drops and we gain weight as we age, building muscle is the No. 1 way to help prevent, or even reverse, the process, Gainor says.

And, yes, it's possible to build muscle during every stage of life. In fact, when Harvard researchers followed 10,500 healthy middle-aged and elderly men over the course of 12 years, those who participated in 20 minutes per day of strength training enjoyed smaller increases in age-related abdominal fat compared to those who spent the same amount of time involved in cardiovascular exercise. The guys who performed strength training had more muscle and, thus, faster metabolisms.

However, Calamita notes that, to get optimum muscle- and metabolism-boosting results from your strength routine, you need to supply your muscles with the building blocks necessary for them to recover after each workout and grow back bigger and stronger: protein. And while the current recommended daily allowance for protein is set at 0.8 grams of protein per kilogram of body weight (about 65 grams of protein per day for an 180-pound person), 2014 research published in the American Journal of Physiology – Endocrinology and Metabolism shows that consuming double that amount is ideal for older adults trying to prevent muscle wasting and promote muscle building.

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