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Everything Metabolism Explained : Definitive Guide
“Boosting your metabolism” has all the promise of a pot of gold at the end of the rainbow: by raising the magical metabolic rate, you can trick your body into effortlessly burning more calories while you lie on the couch enjoying your perfect abs. Some diets prescribe specific foods, like green tea or chili peppers; others advise you to eat six times a day to “keep the metabolic fires burning.” Your metabolism starts to seem like some incredible magical pill that leads to an automatic movie-star body if only you can get it zooming around at Nascar-worthy speeds with the right combination of pills and supplements.
A few of these “metabolism boosters” raise calorie burn by a few percentage points, but in the long run, these kind of shortcuts don’t deliver health or happiness. A good diet and a reasonable exercise plan will do much more for your body than stressing over the latest “metabolism superfoods” that magazine editors use for attention-grabbing cover stories.
A cautionary note: this information is for healthy people. Thyroid disorders or other diseases can cause much more complicated metabolic problems that may need various different therapies. This article is not advice for people with serious metabolic disorders; see your doctor for expert advice on your own particular situation.
What Is Metabolism?
Simply put, metabolism is the energy your body needs to burn to stay alive. To maintain itself as a healthy and functional system, the body needs to do two things: break down food and nutrients into their component parts (catabolism), and then use those parts to re-build itself (anabolism). Together, these two processes are known as your “metabolism,” which covers three main areas of energy expenditure:
Resting metabolic rate (RMR): the energy you need to keep your heart pumping, your lungs breathing, and the rest of your body up and running. RMR is by far the biggest component of your daily calorie requirements. It’s sometimes confused with BMR (basal metabolic rate). BMR essentially measures the same thing, but the standards for quantifying it are much stricter and only really applicable to lab research. RMR is slightly less precise measurement, but it’s more useful for real-life applications.
Activity-induced thermogenesis: the energy you need to do anything more demanding than lying flat on your back. This is the most variable of all the categories, and the easiest one for a person to change. In some elite athletes it can make up a large part of daily energy requirements, but in most people, it’s fairly minor.
Diet-induced thermogenesis (or Thermic Effect of Food): the energy you need to break down and absorb nutrients from the food you eat.
Broken down by the number of calories burned, these three metabolic processes look like:
Why do Some People Have a Faster Metabolism?
From the breakdown above, it’s clear that a “fast metabolism” isn’t just one thing. It could refer to a faster RMR, more calories burned during exercise, or more calories burned from breaking down food. So what can cause some people to need more energy in one or more of these three categories?
RMR, the biggest category, varies with age, gender, level of fitness, and even race. The most important factor contributing to all these differences is a person’s body composition, especially their amount of fat-free body mass (everything in your body that isn’t fat). Organs account for most of the calorie burn of fat-free body mass: muscles need around 15 calories/kilogram/day (around 7 calories/pound/day), but the heart and kidneys need 440 calories/kilogram/day (around 200 calories/pound/day), and the brain needs 240 (109/pound). Keeping your brain, liver, heart, and kidneys in good working order accounts for 60-70% of the RMR; muscle contributes 20-30%.
Most of the RMR differences between different ages, races, and sexes can be explained simply by looking at the amount of lean tissue and specifically the size of calorically expensive organs. African-Americans, for example, have a smaller organ mass than whites, so their BMRs tend to be lower relative to body size. Older people have smaller organs than younger people. Women have smaller organs than men. Just looking at fat-free body mass alone won’t explain these differences, but if you go a little deeper into the ratio of organs (which burn more calories) to muscles (which burn fewer), the effects of race and age become statistically insignificant.
A less important contributor to RMR is adipose tissue (fat). Fat burns only around 4.5 calories/kilogram/day (2 calories/pound), but this effect can add up quickly in people who have a lot of it. If a 250-pound woman loses 100 pounds, that’s 200 calories a day that she isn’t burning anymore and needs to account for in her diet.
Another factor affecting RMR is food intake. In the long term, an extreme starvation diet causes your metabolism to slow in response, because your body is conserving energy in an attempt to help you stay alive. On the other hand, overeating will raise your metabolic rate (although not enough to burn all the extra calories).
Genetics also play a role, although it’s not clear how important they really are. Estimates of the genetic factor in determining RMR range from 11% to 40%, depending on who you ask. Since some of these studies are older, and don’t take into account more recent research on organ size and fat-free body mass, it’s probably safest to err on the lower end of the estimates. However you look at it, though, genetics have some role, but they’re not the only factor. Nobody is doomed to obesity by a genetically slow metabolism.
As well as RMR, different people also vary with regards to the other two parts of metabolism, activity-induced thermogenesis and diet-induced thermogenesis. Activity-induced thermogenesis, the amount of energy needed for voluntary activities like exercise, is the part that’s easiest for human beings to change. It depends on how much physical activity you do, and what kind: in the same amount of time, higher-intensity movement like sprinting burns more calories than lower-intensity movement like walking.
The last of the three major components of the metabolic rate, and also the smallest, diet-induced thermogenesis accounts for the calories you need to burn just to get the nutrition out of your food. It’s affected by many different factors, including the macronutrient composition of the food (protein requires more energy to digest than carbohydrates or fat), the temperature of the food, meal size, and particular chemicals in the food itself.
Clearly, a “fast” metabolism is quite complicated! It’s not something you can just turn on and off by flipping a switch. But since everyone loves the idea of burning more calories every day without extra effort, a lot of weight-loss tips focus on increasing the metabolic rate, usually through either diet or exercise. A few of these techniques show some promise, but a lot of them are just silly, or at worst, downright dangerous.
“Boosting Metabolism:” Diet
There are so many crazy myths about food and metabolism that it’s hard to keep track of them all. First of all, there’s the bevy of “metabolism-boosting foods” that supposedly raise RMR by increasing your body temperature (thermogenesis). Since a higher body temperature requires more calories to maintain, these foods are supposed to force your body into using more energy. Green tea and chili peppers are the most commonly cited, but some enthusiasts will even recommend drinking ice water because you have to burn calories to warm the water to body temperature.
If only it were that simple! Unfortunately, the secret to effortless weight loss doesn’t lie in mainlining spicy foods and a waterfall of frozen green tea.
Stimulant supplements, like caffeine or ephedrine, do raise the metabolic rate. However, long-term use of caffeine leaves you insensitive to its effects, and the only documented metabolic benefits come with doses between 600 and 1,000mg/day (that’s 6-10 cups of coffee). Ephedra is so potentially dangerous that the FDA has actually banned it as an ingredient in supplements.
Capsaicin, the “metabolism-boosting” ingredient in hot peppers, does give you a temporary increase in calorie expenditure. Unfortunately, most of the studies used supplements instead of real foods: just adding hot peppers to your dinner won’t give you much in the short term, and there’s no evidence it does anything in the long term.
Cold water does burn calories by forcing your body to warm it up…to the tune of around 2 calories per glass. Unless you’re chugging liters of glacial runoff, this isn’t a particularly effective weight loss strategy.
One of the few foods that may actually be useful is green tea. Catechins are the main thermogenic component of green tea. One study gave several young men a green tea supplement containing catechins in reasonable amounts that an ordinary person might actually be able to get from drinking tea. The study found that subjects given the green tea extract burned about 4% more calories than subjects given a placebo or subjects given straight caffeine. Again, not a huge benefit, but there weren’t any side effects and there’s really nothing wrong with green tea if you like it.
Another attempt to “boost metabolism” using diet-induced thermogenesis involves manipulating macronutrient ratios. Protein requires the most energy to digest (about 20-30%); carbohydrates (5-10%) and fats (0-3%) require the fewest. Logically, it would seem that eating a diet very high in protein would be the best way to lose weight. There’s a little bit of truth to this: eating a protein-deficient diet isn’t doing anyone any favors, and is likely to be as bad for weight loss as it is for everything else. But there is such a thing as too much protein! Shoveling down endless meals of lean chicken breast with spinach will ultimately do you more damage than the weight loss is worth.
Yet another theory about raising metabolism through your diet is the idea that you need to eat constantly to stoke the “metabolic fires” and prevent “starvation mode.” This “metabolic fires” theory sounds completely plausible at first. It runs like this: when you eat a meal, your body needs to expend some energy to break down and absorb the nutrients in it. So eating raises your metabolic rate. The logical conclusion is that eating more times a day will boost your metabolism more, resulting in a higher overall calorie burn.
Unfortunately, proponents of this particular myth missed a crucial part of their premise. Eating does raise your metabolic rate, but it does so in proportion to the number of calories you eat. Eating six times a day will give you six mini-boosts; eating the same amount of calories in two meals will give you two larger boosts that are exactly equivalent to the six mini-boosts. For the curious, Martin Berkhan debunks this theory extensively on his blog.
You won’t even send your metabolism plunging with intermittent fasting. Short-term fasting actually increases metabolism, due to an increase in catacholmines (stress hormones). It’s long-term, chronic calorie restriction that causes a metabolic slowdown, not any particular meal pattern over the course of the day.
The upshot is that most dietary interventions won’t do much to increase or decrease your metabolic rate. It’s important not to send your metabolism running for the hills with prolonged periods of extremely low-calorie dieting, but other than that, specific foods and drinks won’t give you any kind of dramatic weight loss above what you’d see on an otherwise healthy diet.
“Boosting Metabolism:” Exercise
Instead of or in addition to diet changes, some people try to raise their metabolic rate by tweaking their exercise programs. Here, there are really three different factors to consider:
The number of calories burned by the exercise itself.
The number of calories needed to recover from the exercise (Excess Post-Exercise Oxygen Consumption, or EPOC). Whenever you exercise, you’re actually breaking down your muscles; you get stronger by recovering after the fact. Since this involves rebuilding muscles, it takes energy, and therefore burns calories.
The amount that a certain exercise raises RMR (if it does).
In general, aerobic exercise burns more calories during the actual workout, but not as many calories from EPOC. Anaerobic exercise (sprinting or weight training), on the other hand, burns fewer calories during the workout, but more after the fact. This is promising for time-crunched dieters, because it provides the same calorie burn with a smaller time investment. Weight training promises to set off a process of calorie burning that lasts throughout the rest of the day. It’s almost like your body is still at the gym, even while you’re doing other things.
So what kinds of exercise result in the greatest EPOC effect? In general, EPOC increases linearly with the duration of exercise, but exponentially with intensity. Short and sweet beats long and slow. The higher the intensity, the greater amount of energy burned afterwards. Sprinting is especially beneficial. However, the total effect of EPOC overall is fairly small, even for subjects exercising at a very high intensity for quite a long time. One study found that during an 80-minute session of quite high intensity (80% maximum heart rate), the subjects burned 700-800 calories from the exercise, but only 80 from the EPOC. In other words, the total energy expenditure from EPOC amounted to around 7% of the calories burned during the actual workout.
For very high intensity interval training, the rate of EPOC rose from around 7% to around 13% of the total calorie burn: still not terribly impressive. Even in strength training, the effects of EPOC appear to be minimal if the subjects perform a reasonable volume of work. One study on trained powerlifters found an impressively significant boost of 700 extra calories, but that experiment has never been replicated.
In other words, the effect of EPOC doesn’t make anaerobic training equivalent to aerobic exercise as far as calorie burn is concerned. If this is true, then how is anaerobic training (strength training) even effective for fat loss? Mostly through other factors that are interesting in and of themselves, but don’t have much to do with EPOC.
Then there’s the third question – does either type of exercise raise the baseline RMR, the amount of energy that your body needs just to keep its organs running? This is a topic that researchers have debated back and forth for years. Some claim that strength training raises RMR because increased muscle mass is calorically expensive. Muscles, the theory goes, burn calories “just by existing.” You’ll hear this one repeated all the time by trainers, coaches, and people trying to sell you supplements.
Unfortunately, muscle mass doesn’t actually burn all that many calories just by sitting there. At 7 calories/pound/day for muscle, putting on an additional 10 pounds of muscle would net you a whole 70 extra calories every day: that’s about one ounce of 90% lean ground beef, or about a cup and a half of strawberries. Not exactly a revolutionary fat-burning accomplishment!
The other theory about exercise and RMR is that the working out raises RMR independently of increased muscle mass: something about exercise just prompts your body to burn more calories even after accounting for the calorie burn of the workout itself and EPOC. This has been argued back and forth but a recent study found that neither resistance exercise (weight training) nor endurance exercise (jogging) increased the total daily energy expenditure of a group of young women apart from the calories burned during the exercise itself and the increase in muscle mass in the weight training group.
The only caveat to this is that it didn’t account for people on a calorie-restricted diet. It’s well known that exercise can prevent the metabolic slowdown of calorie restriction. This may simply be because exercise preserves fat-free body mass (muscle, which is more calorically expensive than fat), but may also be caused by some other factor; the jury is still out.
Unfortunately, the overall conclusion is that no kind of workout will give you a magical boost in RMR that lets you burn calories for hours afterwards just sitting on the couch. The best that exercise seems to do is to make up for the metabolism-lowering effect of a caloric deficit. Does this mean that exercise is useless? Not at all! Exercise does burn calories while you’re doing it, and it also has plenty of other benefits, like a greater ability to handle carbohydrates as fuel, a stronger body, and more self-confidence. Exercise is absolutely part of a healthy lifestyle; it just isn’t a metabolism-raising magic wand.
A slightly more promising avenue for increasing activity-induced thermogenesis is non-Exercise Activity Thermogenesis (NEAT), the energy you need for all kinds of random physical activity that isn’t exercise (cooking lunch, fidgeting in a boring meeting, scratching a mosquito bite…). Each tiny action doesn’t burn many calories, but you do it all day long. Nobody would jog on a treadmill for 16 hours straight, but all day we spend time fidgeting, shifting, tapping our feet, and otherwise getting very low-intensity movement via NEAT.
The cumulative effect can be quite significant. You don’t increase RMR by increasing NEAT; really, it’s just a way to get more physical activity into your day. But it’s free, easy, and consumes very little time: you can increase NEAT simply by setting a timer at your desk reminding you to walk around the room every 20 minutes. Or give up sitting altogether and work at a standing desk; you’ll naturally spend more time moving around.
Metabolism is a fickle creature. It’s much, much more complicated than some people genetically having a “fast metabolism” or a “slow metabolism.” But it’s also not entirely under our control, and a lot of the advice on how to raise your metabolic rate either produces such a small benefit that it barely matters, or doesn’t produce any benefit at all.
In the long run, there are no shortcuts to health. “Metabolism boosting” tricks that are actually beneficial, like incorporating more NEAT movement into your day, tend to be long-term lifestyle changes rather than quick fixes. The best way to maintain a healthy weight is not to focus on little tricks to increase metabolism, but instead to eat real food, exercise in whatever way leaves you feeling energized and strong, and manage your lifestyle to get enough sleep and avoid unnecessary stress. It’s not glamorous, and it doesn’t lend itself well to endless slideshows on pop culture websites, but it’s more likely to get you the results you want for good.