The words “slow metabolism” are incredibly overused. As in “I just can’t lose weight anymore; my metabolism has really slowed down.” Or “I’m not like you, I can’t just eat everything I want. Your metabolism’s faster than mine.”
Before blaming weight-loss failures on metabolic rates, let’s take a comprehensive look at how metabolic rates differ between people, what makes up your metabolic rate, and how all that changes when dieting or exercising.
To understand metabolism, you have to understand just a few basic terms. The energy your body burns in a day is called your total daily energy expenditure (TDEE), which has three components: resting metabolic rate, thermic effect of food, and activity energy expenditure.
1. Resting metabolic rate (RMR)
Technically, “metabolism” is the sum of all the chemical processes that keep your body alive. Colloquially, most people think of “metabolism” as the energy burn from the resting metabolic rate (RMR), which is the energy your body uses to stay alive, accounting for about about 70% of the TDEE of sedentary people.  And yes, there do exist people with a “fast metabolism” — a.k.a. high RMR — but their metabolic advantage isn’t as overwhelming as you’d think.
Most people (68%) stay within just 8% of the average RMR, and the vast majority (96%) within 16%. In other words, 96% of people stay within 200–300 kcal of the average RMR. 
Your RMR is mostly determined by your fat-free mass (FFM), even though most of it is water and thus not metabolically active. Your brain and visceral organs add up to only 12% of the active components of your FFM (even less if you’re muscular), yet they use a lot of energy: 200–440 kcal/kg/day (91–200 kcal/lb/day). By comparison, your skeletal muscle at rest requires only 13 kcal/kg/day (6 kcal/lb/day), which is still a lot more than the 4.5 kcal/kg/day (2 kcal/lb/day) you need to maintain your fat stores. 
It turns out that your natural RMR is a poor predictor of future weight gain,   which suggests that people with a high RMR compensate for burning more by eating more. Conversely, people with a low RMR may compensate for burning less by eating less — with one notable exception. One meta-analysis of 12 studies showed that high rates of weight regain were associated with a 3–5% lower RMR in formerly obese compared with control individuals.  But was the RMR of these formerly obese people their “natural” RMR? Probably not.…
Contrary to popular belief, obesity is associated with a high metabolic rate, as a heavier body needs more energy to do just about everything, from running down to breathing. However, people who slash down calories to lose a lot of weight very fast risk slowing their metabolism to below its “natural” level — maybe permanently. They feel hungry all the time, as their body fights to recover the energy stores (mostly, the fat) it lost too fast.
RMR can also slow as you age, since the older you get, the less FFM you have and the fewer energy-generating mitochondria dwell in your cells. It can also differ according to sex (not only do men normally have a higher FFM:BF ratio, but their organs consume more calories), genetics, and possibly gut bacteria. However, aside from your FFM, the factor that has the greatest impact on your RMR is temperature — both internal and external.
Even when both you and the weather are fine, you spend some ⅔ of your RMR to maintain a constant body temperature. To increase your core temperature by one degree celsius can require your RMR to go up by 10–13%.   That’s why cold exposure (through ice baths and other methods) has been promoted for weight loss.
Although people can have a fast or slow metabolism, extremes are pretty rare, often resulting from disease or prior obesity. Around 96% of people stay within 200–300 kcal of the average metabolism. Moreover, resting metabolic rates (RMRs) are not well correlated with weight gain.
2. Thermic effect of food (TEF)
Some of the calories in the food you ingest will be used to digest, absorb, and metabolize the rest of the food, and some will end burned off as heat. The overall process is known indifferently as thermic effect of food (TEF), dietary induced thermogenesis (DIT), or specific dynamic action (SDA).
TEF represents about 10% of the caloric intake of healthy adults eating a standard mixed diet,  but your actual number will depend on various factors, such as your lean body mass and the size and composition of your meal. The energy required to digest each macronutrient (its TEF) can be expressed as a percentage of the energy provided by this macronutrient: 
Fat provides 9 Calories per gram, and its TEF is 0–3%.
Carbohydrate provides 4 Calories per gram, and its TEF is 5–10%.
Protein provides 4 Calories per gram, and its TEF is 20–30%.
This is one of the reasons why increasing your protein intake can help you lose fat, in addition to preserving your muscle mass when you eat below maintenance.
Protein burns, by far, the most energy of any of the three macronutrients. For that reason, and because of its protective effect on muscle mass, protein plays a central role in many diet templates.
3. Activity energy expenditure (AEE)
Activity energy expenditure (AEE) is the most variable aspect of the TDEE, ranging from 15% in very sedentary individuals to 50% in highly active individuals.  It comprises both physical activity (PA) and spontaneous physical activity (SPA), also known as non-exercise activity thermogenesis.
PA covers significant physical effort, such as lifting weights, running, or riding a bike, whereas SPA covers minor, often subconscious acts of daily life,  such as fidgeting,  chewing, walking short distances, or maintaining one’s posture. Studies in respiratory chambers   and in free-living environments suggest that SPA accounts for 4–17% of the TDEE, or about 100–700 kcal/day.
Outside your resting metabolic rate and the energy that’s burned by eating food, somewhere between 15 and 50% of your energy burn comes from activity. This activity can be exercise or any of the dozens of activities/movements associated with living.
1. What can increase your RMR
Exercise! Yes, your resting metabolic rate can increase. The increase in metabolic activity from exercise can outlast your workout. In fact, excess post-exercise oxygen consumption (EPOC) can last for more than a day after you stop exercising, depending on the duration and intensity of your physical activity.
Aerobic activities and anaerobic activities affect your RMR differently. Aerobic activities tend to burn more calories during exercise, but not a lot afterward (low EPOC). Anaerobic activities, on the other hand, can stoke your furnace for hours after you stopped exercising (high EPOC).  In addition, anaerobic activities are better for building muscle, which requires additional energy. 
But what happens after you’ve reached a plateau and your muscle mass stops increasing? Do your muscles now require only maintenance levels of energy (13 kcal/kg/day, as we’ve seen), even if you exercise daily? No, they need more, because each time you exercise you damage them and must expend energy to repair them. This still counts as “muscle maintenance” (muscles get constantly destroyed and rebuilt even in sedentary people), but the caloric cost is much higher.
What other methods are there to increase your RMR? As we’ve seen, it’s been estimated that for every degree celsius increase in core temperature, your RMR rises by 10–13%. Therefore, some people wrap themselves in cold packs, turn down the heat in their houses, wear less clothing outside, or even bathe in ice. That would certainly increase their RMR, but these methods can be unsafe if improperly used or if you have certain medical conditions.
We probably need to mention meal frequency, but only to note that there’s no evidence to support the idea that eating more often will increase your RMR.     Rather, it’s the total number of calories that determines changes in body weight.
Of course, many are the supplements touted to boost your metabolic rate, but very few actually have evidence in even limited populations. We cover these in greater depth in our Fat Loss Stack Guide.
One of the most useful fat loss hacks doesn’t address RMR, but rather spontaneous physical activity (SPA). Standing for around half of your work day can burn almost 200 extra kilocalories, compared to sitting.  That’s a simple, reliable, sweat-free way to burn calories.
Regular exercise can increase your resting metabolic rate, as can other methods, such as cold exposure (with an eye to safety). Moreover, although taking the stairs rather than the elevator or standing while working won’t noticeably boost your RMR, this kind of small effort can be an easy way to burn more calories each day.
2. What can decrease your RMR
Unfortunately, losing weight translates into a decrease in RMR.  The very process of eating below maintenance will reduce your RMR by 5–15%, as your body becomes more sensitive to various hormones and neurotransmitters that regulate how energy is used and expended. 
This reaction is known as adaptive thermogenesis. During a hypocaloric diet, not only will your body clamor for more calories (you’ll be hungry), but it will also prevent your cells from spending as much energy (if you play sports, your performance will likely decline).
If you’ve dieted sensibly, your RMR should return to normal when you resume eating enough calories to equal your TDEE. You may also maintain your RMR through periodic “refeeding” days, when you refill your liver glycogen stores and try to convince your body that you’re not really dieting so hard.
Your RMR will decrease as you lose weight, since weighing less means having less body tissue to support (among other factors). Unless you diet too hard for too long, your RMR should normalize after the dieting period is over.
In popular lore, someone with “a fast metabolism” is someone whose high resting metabolic rate (RMR) allows him or her to eat a seemingly infinite amount of food without any fat gain. Past their growing years, such people don’t exist. While some adults do have a higher natural RMR than their peers, interindividual variations tend to stay within a range of 200–300 kcal/day. Moreover, people with higher caloric requirements tend to compensate by eating more, so that one’s natural RMR is a poor predictor of weight gain.
The good news is that if you want to rev up your metabolism, you can. Anaerobic activities can increase your RMR for more than a day; they can also increase your muscle mass, thus further heightening your caloric requirements. They’re a safer way to lose fat than cutting your caloric intake drastically, which can result in your body fighting back, your RMR taking a lasting hit, and endless yo-yo dieting. Yes, to shed fat, you do need to spend more energy than you ingest; but the more severe the deficit, the easier it’ll be to regain whatever fat you lost.
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