Metabolism might be one of the more difficult physiological processes for people to get, which is probably why it gets tossed around in shady diet claims (boost your metabolism to burn fat!), dubious weight loss predications (Not eating breakfast will slow your metabolism!), and blatant lies (apple cider vinegar speeds up your metabolism!).

One of the best definitions of metabolism is one from The Mayo Clinic: “Metabolism is the process by which your body converts what you eat and drink into energy.”

Let’s learn what metabolism is before we answer all of your burning questions about how it affects your weight.

Your metabolism can be broken down into four things: BMR, or Basal Metabolic Rate; TEF, or Thermic Effect of Food; PA, Physical Activity Expenditure; and NEAT, which is Non Exercise Activity Thermogenesis. 

BMR + TEF + PA + NEAT = the calories you use in a day, or what most people equate with their ‘metabolism’.

When people believe that their metabolism is ‘slow’, this is another way of saying ‘My body doesn’t burn many calories a day’. The sort of people who can do things like eating a pint of Haagen Dazs a day without gaining an ounce are thought to have a ‘fast’ metabolism, although you’re about to learn that it’s not as simple as that.

But are some people blessed with a ‘fast’ metabolism while some aren’t? I’m going to answer that for you in a minute.

First, an easy science lesson about how those four parts of metabolism work.

The BMR is the number of calories that your body uses for its basic functions – heart beating, keeping your temperature stable, brain working, essentially – keeping you alive. BMR is the biggest consumer of calories that we’ve got – up to 60% (and, I’ve read, up to 80%) of our total calories are used to power our bodies at rest.

That means BMR is a big chunk of daily calorie needs: If you’ve eaten 1600 calories, almost 1000 of them are going to be used for BMR. BMR is thought to vary up to 15% from person to person though: We’ll get into why BMR can be so different between people.

The TEF is the number of calories your body uses to digest food. Each macronutrient – carbohydrate, protein, and fat – has a different thermic effect. Protein takes the most energy for your body to process (somewhere near 20% of its calories).

Fat takes the least (around 5%). This is one part of the reason why higher-protein diets can be effective for weight loss; you’re using more energy just to metabolize your meals. There are plenty of other factors involved though, so don’t run off and start eating only protein.

The thermic effect of food is generally thought to account for around 10% (and up to 15%) of your daily calorie intake. So out of your 1600 calories, 160 of them will go towards the TEF. These calculations aren’t a hard-and-fast rule, they’re just to loosely illustrate how this stuff works.

We have 440 calories left out of those 1600 that you’ve eaten, and surprisingly, most of those will go towards non-exercise activity thermogenesis. NEAT is anything that’s not purposeful exercise: taking a shower, standing in line at Starbucks, trying on clothes at Nordstrom (some of us more vigorously than others, ha ha)…you get the point.

NEAT can account for a significant number of calories per day – up to 2000 in some individuals.  Actual exercise for all of us non-athletes accounts for a small percentage of activity calories burned.

This chart says it all:

  J Int Soc Sports Nutr. 2014; 11: 7.

Published online 2014 Feb 27. doi:  10.1186/1550-2783-11-7

See how small the exercise (EAT) part is? And how large your BMR is? 

Now that you hopefully have a good idea of what metabolism is and what main factors influence it, it’s time to answer some of the most common questions I get on the subject.

There are plenty of bogus ‘metabolism boosters’ and ‘fat burners’ that supposedly ‘torch fat’. If only it was so easy!

(I think around 90% of the claims made about apple cider vinegar are totally false. Here’s my post on myths and truths around ACV

You’ve probably heard that the more muscle mass you have, the more calories you’ll burn at rest (aka increased BMR). A pound of muscle burns around 6 calories a day at rest; a pound of fat, just 2. This difference is very small, especially if you’re not planning on putting on 100 pounds of muscle.

That doesn’t mean you should stop exercising, but like I indicated above, it accounts for a very small amount of calories burned, and depending on muscle mass to burn more calories is probably not a great idea.

Weirdly enough, a very large meal can actually increase your TEF short-term. So when you eat a big feast, you’re actually burning more of those calories than you would after a normal-sized meal. Do it often enough though, and you’ll gain weight from the net gain in calories.

Why does BMR differ between people? 

For so many reasons, and these are the main ones:

Genetics. Of course – genetics pretty much plays a role in everything about you. Just as you can inherit your daddy’s eyes or your mom’s beautiful skin, BMR is influenced by genetics, too.

Age. As we age, our bodies naturally lose muscle, slowing our metabolism. However, recent research shows that this doesn’t occur in the majority of people, until after age 60. 

Sleep. Studies suggest that people who don’t get enough sleep may have a reduced metabolism.

Hormones. Thyroid hormone levels in particular can influence BMR. (hormones are a whole other future post)

Muscle mass. Men naturally have higher BMRs than women likely because they can naturally have up to 26lb more muscle mass. 

Men are also generally larger than women. This can make a difference: larger body = more calories needed. 

It’s part of the reason why many of my women clients complain when their husbands lose weight so much quicker (and seemingly with less effort) than they do. I know, it’s frustrating.

The size of your body. The bigger you are, the more calories you require to live.

We also suspect that gut bacteria plays a role in BMR, but we need more science to prove it.

metabolism myths

If protein burns so many calories being digested, why shouldn’t everyone eat mostly protein?

Because a mixed diet is so much more pleasurable, and other foods have nutrients that protein can’t provide. I’m a big advocate of a higher-protein diet, with around 40% carbs versus the often-recommended 60%.

(The Fast Metabolism diet is popular – read my review of it here)

Does going without breakfast slow my metabolism?

I’m not convinced.

Eating breakfast has been associated with weight loss in some individuals because breakfast-eaters may be more likely to be active, but as far as actual slowing of the metabolism, I haven’t seen convincing research done on humans to prove that. It appears as though the total amount of energy you take in over the day, and not the number of meals you eat, makes more of a difference.

Eat breakfast if you find that it helps you eat less later in the day, and of course, if you’re hungry when you get up. If you’re not a breakfast eater and you don’t feel that it impacts your food intake or your physical and mental activity through the day,  I’m inclined to tell you to continue on with what’s working for you. Everybody’s different.

Does spicy food speed up my metabolism and burn fat?

Nope. Spicy food and other things like green tea that are touted as ‘fat burners’ can spike metabolic rate VERY briefly, but don’t significantly impact calorie usage or the thermic effect of food. No food or supplement has ever been shown to raise metabolism enough, for long enough, to help anyone lose weight. Sorry, there are no magic bullets.

Does eating before bed cause weight gain?

It’s complicated. Recent research suggests that if you’re consuming the majority of your calories later in the day, you might want to rethink that. 

While our metabolism slows a bit through the night, it never stops (you’d be dead if that happened). But we’re learning more about chrononutrition, which is eating according to your circadian rhythm and how it affects our health. While I once thought that was all unproven, research is expanding our understanding of it.

 Also: never go to bed hungry, which can impact sleep quality. Have a small snack before bed if you want one, and don’t stress about it.

(Read more about chrononutrition and eating according to your circadian rhythm, here)

Is intermittent fasting a good way to speed up metabolism?

A recent review of studies (some of them rodent ones, however), showed promising metabolic effects of IF, although ‘speeding up metabolism’ wasn’t one of them. ‘Metabolic’ effects in this case have to do with hormones associated with hunger and fullness.

This 2018 study suggests that intermittent fasting appears to be preventative in terms of keeping metabolism from slowing, but I think we need more research to really determine this. What I can say is that it appears that intermittent fasting doesn’t offer any advantages over regular calorie-cutting in terms of weight lost. Hm. 

Overall, intermittent fasting appears to be one more tool in the toolbox for people who would like to lose weight, but it’s not for everyone. I’d personally rather you concentrate on the quality of your food than the number of calories you’re eating or not eating.

Can fidgeting and moving around a lot burn a significant amount of calories?

Definitely yes. NEAT actually accounts for the majority of our non resting energy needs. People who move more in general tend to be leaner, as are people who fidget. Except fidgeting is really distracting, so you might want to try to just move more – meaning, standing more, walking instead of driving to do your errands, and doing your own gardening.

How does dieting affect my metabolism? 

Our bodies are VERY smart, and they’re programmed to hold on to fat. This probably has something to do with the adaptations we’ve had to make in times of feast and famine. Whatever it’s from though, when we diet, our bodies make fat loss difficult for us.

When we do manage to lose fat through dieting, our bodies make several adaptations:

Hormones slow our metabolic rates to conserve energy (they also increase hunger to drive us to eat more)

If you’re also losing muscle from a low-calorie diet, this can further decrease your metabolism.

When you lose weight, you’re physically smaller, and therefore your body requires less energy. This is one of the biggest problems with dieting, as theoretically you’ll require less and less energy as you lose weight – until you’re at a diet calorie level that’s unsustainable.

If you’re a yo-yo dieter, that doesn’t mean your metabolism is ‘broken’. In fact, this study suggests that even yo-yo dieters can successfully lose weight despite their history of ‘weight cycling’.

Someone below asked me about the Biggest Loser Effect – basically, the phenomenon of participants in the Biggest Loser TV show who lost a lot of weight very fast, and have ended up with weight gain and slower metabolisms. These people were losing up to a pound of fat per day, which isn’t normal yo-yo dieting or even normal dieting by any stretch of the imagination.

Extreme weight loss in an extremely short time may certainly affect your metabolism in such a way, but again – this isn’t normal behavior for most people, and we need more science to explore this issue.

If you want to lose weight without downshifting your metabolism while you do it, try to build muscle mass through exercise, eat a diet that’s higher in protein, and don’t cut calories too much. Better yet, instead of focusing on cutting calories, try to focus on eating more whole or minimally processed foods and cutting down on refined, ultra-processed foods.

Can I ‘break’ my metabolism? Does metabolism ‘sleep?’

‘Broken’ or ‘sleeping’ metabolism are two terms that are used by the diet industry to sell you products that don’t work. Nobody who knows anything about how metabolism actually works, would ever say these things. 

If someone is trying to tell you that your metabolism is broken or sleeping, run away. Run away fast.

(Speaking of metabolism scams, read my review of the Metabolic Renewal program here)

Is my metabolism predetermined and am I stuck with it?

Absolutely not! Genetics is only a part of metabolism. Remember: moving more (increasing your NEAT), and eating more protein may help metabolism. Gaining muscle helps too, just not as much as you might think.

metabolism booster

Should I be ‘grazing’ to keep my metabolism high all day? 

This is a myth. Eating more frequently may actually cause some people to eat more. It’s understood that eating more or less often doesn’t make much of a difference in terms of weight loss; do whatever works best for you. But no: there’s no research suggesting that eating 6 or however many times a day runs your metabolism on high all day long.


Metabolism is complicated, but you can keep it all very simple by not believing the hype about it that the diet industry tosses around. Eat your protein and whole foods, move your body, and live your life.