Many people who struggle to lose weight peg their lack of progress on a “slow metabolism.” 

Often, these people lament that the reason they used to be lean and lithe was thanks to their “fast metabolism,” which has now slowed so much that it’s scotched their ability to lose fat.

Ironically, others (mostly young men) complain that their fast metabolisms prevent them from gaining muscle. By their lights, most or all of the calories they snarf are quickly incinerated by the roaring furnace that is their metabolism. 

In both cases, these folks chalk up their inability to make headway on either an underactive or overactive metabolism. It’s not their diet, their training program, or anything in their control that’s at fault, but that capricious metabolism that crosses them up.

Is this true, though? 

What the heck is your “metabolism,” and how does it work? 

And is your metabolism completely out of your control, or can you nudge it in the right direction with your behaviors?

Read on to find out. 

What Is Your Metabolism?

Technically, your metabolism is the collection of physical and chemical processes your body uses to produce, maintain, and destroy material substances, and to make energy available. 

When most people say “metabolism,” though, they’re typically referring to either their resting metabolic rate (RMR) or basal metabolic rate (BMR).

Resting metabolic rate refers to the number of calories your body burns at rest in 24 hours, whereas basal metabolic rate is an estimate of the number of calories your body requires to simply stay alive every day. 

As you can see, these terms aren’t semantically interchangeable, but as they come out to more or less the same number, you can think of them as synonymous for the purposes of this discussion. For the sake of simplicity, I’ll just use the term basal metabolic rate, or BMR, in this article. 

When someone says their metabolism is “slow,” they generally mean their BMR is lower than normal or than it was in the past, and if they say their metabolism is “fast,” they mean their BMR is higher than normal. 

That said, there are three other components of your metabolism worth considering: 

  1. The calories you burn digesting food, known as the thermic effect of food (TEF).
  2. The calories you burn during formal exercise, or what some scientists call exercise activity thermogenesis (EAT).
  3. The calories you burn from any activity that isn’t formal exercise, known as non-exercise activity thermogenesis (NEAT).

This is an important point to understand, because while many people speak of their “metabolism” like it’s some mysterious process entirely outside their control, you can positively influence every component of your metabolism (especially your levels of activity). 

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What Does It Mean to Have a “Slow” Metabolism?

In order to understand what qualifies as a “slow” metabolism (BMR), we first have to define what we mean by a “normal” metabolism. 

The most accurate way to measure someone’s metabolism is to have them breathe into a machine known as a metabolic cart, which then analyzes the gasses in their breath to estimate how many calories they’re burning. 

Scientists have been conducting studies using metabolic carts for decades, and have found that most people with average levels of muscle mass have a BMR of around 4.7 kilojoules of energy per minute, which translates into about 1,600 calories per day. As BMR scales with body size (it takes more calories to maintain and move a larger body), people who are smaller than average will burn less than this and people who are larger will burn more, but their BMRs will be in the ballpark of 1,300-to-1,800 calories per day.

How much do people’s BMRs vary from this average? For instance, is it possible for, say, a 200-pound man with a “slow” metabolism to have the same BMR as a 100-pound woman? 

While some people do have faster BMRs than others, the differences are too small to matter when it comes to weight loss. For example, a review study conducted by scientists at the University of Vermont found that about 68% of people’s metabolisms are within 5-to-8% of the population average, and 96% are within 10-to-16% of the population average.

To illustrate this more concretely, if we assume 1,600 calories as an average BMR, 68% of people would fall in the BMR range of about 1,500-to-1,700 calories per day and 96% in the range of about 1,400-to-1,800 calories per day. 

If you compare the people with the fastest and slowest BMRs, the absolute difference would only be about 400 calories per day, with most people falling somewhere in the middle. In other words, the vast majority of people have BMRs that are about what you’d expect based on their body weight.

So, what explains the outliers? Why do some people have slower or faster metabolisms? Are the (modest) differences due to genetics, hormones, or some other X-factor?

Nope. 

In reality, these differences can be almost entirely explained by differing levels of muscle mass—people with more muscle mass are going to have a higher BMR, and people with less muscle mass will have a lower one. The review study mentioned a moment ago didn’t measure people’s body composition, but other research shows that your level of muscularity is one of the prime determinants of your BMR. 

In other words, the reason some people have higher BMRs than others, despite weighing the same, is almost entirely because they have more muscle mass and less body fat.

While your level of muscle mass is partly determined by your genetics, anyone can gain muscle (and thus boost their BMR) by eating and training properly. Thus, your BMR is partly in your control.

And what of people who claim to be eating very few calories but can’t seem to lose weight? For example, some people claim to eat less than 1,200 calories per day without losing weight—is it possible they really do have a sluggish metabolism?

Also no.

Scientists at Columbia University College addressed this exact question in a study where they measured the metabolic rates of 10 obese men and women who claimed to have been eating less than 1,200 calories per day for 6 months without losing weight. 

The researchers carefully measured their BMRs using a metabolic cart and even calculated how many calories the people burned digesting their meals (TEF). Then, they ran the same tests on another group of obese people who’d been able to lose weight in the past.

The researchers found that the obese people who claimed to be eating less than 1,200 calories per day had perfectly healthy metabolisms—within 5% of what you’d expect based on their weight and body composition, and no different from the people who’d successfully lost weight. The real reason they’d been unable to lose weight was simple: they underestimated how much they ate by 47% on average, and overestimated their levels of physical activity by 51%. 

The researchers also found that these people generally, “perceived a genetic cause for their obesity, used thyroid medication at a high frequency, and described their eating behavior as relatively normal.”

In other words, they ate more and exercised less than they thought they did, felt certain that their failure to lose weight was due to genetics, a slow thyroid, or some outside factor, and didn’t accept that their own habits might be the real culprit, which is probably why they refused to change their behavior after 6 months of banging their heads against the wall. 

Ironically, other research shows that the people who are most likely to blame a lack of weight loss on a slow metabolism—overweight folks—actually have a higher BMR than their leaner counterparts.    

For instance, a study published in the journal Obesity found that contestants on the show The Biggest Loser—who weighed 330 pounds and were 50% body fat on average—had an average RMR of 2,600 calories per day, which is about 50% higher than that of a normal-weight person. 

After losing 130 pounds on average, their RMRs dropped to a more normal range—1,900 calories per day.  

While many media outlets claimed this was evidence that dieting had slowed their metabolisms (“metabolic damage” as it’s sometimes called), the truth is that these people’s metabolisms were simply returning to where they should be as their body weight dropped to a healthy number. 

Another example of this phenomenon comes from a study conducted by scientists at the University of Alabama with 24 overweight women. In this case, the women dieted to lose enough weight to reach a healthy body mass index (BMI), and the researchers measured their RMR before and after their weight loss.

Then, the scientists compared the women’s after-diet RMRs to the RMRs of women who weighed the same but who had never been overweight or dieted. They found that the RMRs of the previously-overweight women and the normal weight women were identical. No one had a “slow” metabolism.

The bottom line is this: most people have a perfectly healthy metabolism that’s burning just as many calories as you’d expected based on their body weight and composition. 

There are a few strange quirks that can depress your metabolism—severe zinc deficiency can reduce your metabolic rate by several hundred calories per day, as can some thyroid diseases—but these are fringe cases. Research shows that the vast majority of people don’t have a slow metabolism.

What Does It Mean to Have a “Fast” Metabolism?

Many people who struggle to build muscle—mostly young guys—are convinced that they have raging metabolisms that thwart their efforts to gain weight. 

These supposed “hardgainers” will swear up and down that they eat like a carthorse, devouring 4,000+ calories per day for months without seeing any change on the scale or in the mirror. (Browse some Internet message boards and you’ll even find people who claim to be eating 12,000 calories per day while losing weight). 

This idea seems to have some legs, too. After all, most of us have seen skinny people who regularly inhale pizza, ice cream, and other “junk” and never seem to gain a pound. 

Here’s the rub: the idea that some people have preternaturally “fast” metabolisms is based entirely on self-reported stories, observations, and anecdotes. 

And when you subject these tales to the acid test of scientific scrutiny, they turn out to be nonsense. 

First of all, as you learned a moment ago, most people have BMRs that are about what you’d expect based on their weight and body composition. Even when you compare the extreme low-end versus high-end of BMR (say, a very skinny 90-pound woman versus a 200-pound muscle-bound bodybuilder), you’re only looking at a difference of a few hundred calories per day.

Second, most people have no clue how many calories they’re really eating. Just as overweight people tend to underestimate their food intake, underweight people tend to overestimate theirs. Bar none, every single “hardgainer” I’ve encountered who claims to be eating copious amounts of food wasn’t meticulously tracking their food intake, and was overestimating portions, skipping meals, and simply not eating anywhere close to as much as they thought they were. 

When these people start weighing and measuring all of their meals and following a proper meal plan, they gain weight, and usually by eating fewer calories than they thought they needed. 

So, how is it that some people can eat so much more than others without losing weight? 

Simple: 

These people do have “fast” metabolisms, but not because of freakishly high BMRs. Instead, they have fast metabolisms because they burn a lot of calories through physical activity. For lack of a better term, they “earn” those calories.

Of course, this isn’t what most people mean when they sneer that it “must be nice that so-and-so gets to eat so much food thanks to their fast metabolism,” but them’s the breaks. 

Now, this doesn’t necessarily mean that these folks are burning these calories through formal exercise. While that often is the case, many people burn far more calories through miscellaneous movements throughout the day—the non-exercise activity thermogenesis (NEAT) that I mentioned earlier. 

This includes all of the calories you burn during random, low-intensity activities like standing, fidgeting, and walking from place-to-place during your waking hours. Any movement that isn’t formal exercise, basically.

While these little bouts of activity only burn a handful of calories at a time, they can add up to hundreds or even thousands of calories per day in extreme cases—many a mickle makes a muckle. 

For instance, a study conducted by scientists at the Mayo Clinic found that when people over-ate by 1,000 calories per day for 8 weeks, they subconsciously increased their levels of NEAT by an average of 531 calories per day—burning off about half of the excess calories they consumed.

That said, not everyone benefited from NEAT equally. Some people’s NEAT levels dropped by about 100 calories per day and others increased by almost 700 calories, and the former gained much more body fat than the latter.

You could say that the people who experience a large bump in NEAT when they overeat have fast metabolisms, but there are a few reasons this isn’t an accurate description of what’s going on: 

  1. Although NEAT is largely subconscious, you can boost your levels of NEAT if you adopt the right behaviors. That is, even if you aren’t genetically predisposed to do lots of NEAT, you can modify your lifestyle to include more movement throughout the day, thus reaping many of the benefits. 
  2. You have to overeat a lot to spark a big uptick in NEAT, and this still doesn’t completely wipe out the extra calories. Even the most genetically gifted . . . NEATers . . . still gain some body fat if they overeat. 

One last point worth mentioning is that you can also increase your metabolism by making better food choices. High-protein foods require far more calories to digest than high-carb or high-fat foods, and thus eating a high-protein diet is an effective way to boost your metabolism by a few hundred calories per day.

Whole foods that have undergone minimal mechanical processing, like chicken, potatoes, and broccoli, also require significantly more calories to digest than highly processed foods like protein powder, breakfast cereal, and juice. 

In the final analysis, it’s true that some people burn a lot more calories than others and thus can eat more without gaining weight, but this is mostly due to factors that are under your control. You can build muscle to increase your basal metabolic rate. You can exercise more. You can sneak more NEAT into your daily routine. You can eat more protein and whole foods.

And when you combine all of these factors, you can give yourself a fast metabolism.

You Probably Don’t Have a “Slow” or “Fast” Metabolism

Or, rather, not an innately slow or fast metabolism. 

While some people do have faster metabolisms than others, this is almost entirely due to the fact that they move more throughout the day, and to a lesser extent, because they have more muscle mass and make better food choices, not because they possess some alchemical ability to torch calories. 

This is good news, too, because it means you can take steps that will significantly increase your metabolism and make it easier to lose weight and keep it off. 

Here’s how: 

  1. If you aren’t in the habit of regular exercise, start. While cardio burns the most calories, strength training burns more than many people realize and has the added benefit of increasing your BMR, which cardio doesn’t (at least not to a significant extent). 
  2. Build muscle. As a corollary to the first point, the best way to boost your BMR is to build muscle, which requires you to follow a well-designed strength training program and diet plan for bulking up.
  3. Eat a high-protein, plant-centric, whole foods diet. Not only will this significantly increase the number of calories you burn digesting your meals, it will also ensure you aren’t suffering from nutrient deficiencies that can meddle with your metabolism. 
  4. Increase your levels of NEAT by sitting less and moving more. Take phone calls while pacing around your home or neighborhood, get up during commercial breaks to do chores, take up physically-intensive hobbies like gardening, woodworking, or outdoor sports, and so forth. 

The key takeaway from all of this is that you can give yourself a “fast” metabolism, but it’ll take a bit of work on your part.

+ Scientific References

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