A Beginners Guide to Your Metabolism

by Trinh Le, MPH, RD
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A Beginners Guide to Your Metabolism

Contrary to popular belief, the majority of your daily calorie burn doesn’t come from puddles of sweat after a tough workout. Calorie burn is actually driven by your metabolism.

Metabolism is the entire process of converting calories into energy to power all your bodily processes. And it isn’t just about calorie burning! It’s also about calorie storing. Your metabolism determines the number of calories you need daily to maintain your weight. While there’s only one way calories can enter your body (nom nom!), there are many ways for calories to leave it. Here are the three major factors that affect your metabolism and overall calorie burn.


Accounts for 60–70% of daily calories you burn.

Basal metabolic rate is the number of calories your body needs to support the vital functions that keep you alive (breathing, digesting, filtering waste) while at rest. These functions eat up a whopping 60–70% of your daily calorie intake, making BMR the largest contributor to your metabolism. Your BMR doesn’t include the calories you burn for normal daily activities or exercise. Here are the key factors that play into BMR:

BODY SIZE A bigger individual requires more calories to sustain their body at rest and with any activity they do. Taller and heavier individuals have larger organs (muscles, brain, heart) that require more calories for upkeep.

BODY COMPOSITION Muscle is more metabolically active than fat, meaning more calories must be burned to maintain a pound of muscle compared to a pound of fat. Two healthy individuals of the same age, height and weight can have very different BMR if they have significantly different percentages of lean versus fat mass. Since most women have more fat mass compared to men, they have correspondingly lower BMR compared to men of the same height and weight.

AGE Your BMR is higher when you are younger, especially since calories are needed to supply your growing body. The trend is that as you age you slowly gain weight in the form of fat mass and lose weight in the form of muscle mass. Read 5 Ways to Cope with a Slowing Metabolism to learn more about how aging affects your metabolism.

GENETICS You knew this was coming! Some people are born with higher (or lower) BMR than others, and this is completely normal. Your genes are not something you can fix, but if you suspect you have a genetic condition that slows down your metabolism (such as familial hypothyroidism), this is something you should consult a medical professional about.

HORMONES They act like chemical dials allowing your body to turn your metabolism up or down depending on its needs. The two main hormones (thyroxine and triiodothyronine) directly responsible for turning up BMR come from your thyroid gland. Other hormones indirectly cause your thyroid gland to release more or less of these hormones, leading to a change in BMR.

HEALTH Generally, your BMR is higher when you are fighting off an infection or healing from a major wound. This is because your body requires more calories to accomplish both of these tasks.

Because these factors introduce so much variability, calculating someone’s exact BMR is hard to measure accurately without sinking serious cash into fancy equipment. Instead, BMR is generally approximated using an equation called the Mifflin–St. Jeor, which has been shown to be most accurate in predicting BMR for healthy adults compared to other existing equations. This equation approximates your BMR using your gender, body size and age when it calculates your daily calorie goal. You can even play around with this equation on  the MyFitnessPal BMR calculator.


Accounts for 10% of daily calories you burn.

Sure, it’s not an easy word to say, but at least the concept is somewhat simple. Food thermogenesis is the energy (calories) you need to digest and absorb food. Of all the macronutrients, protein requires the most work to digest followed by carbs and fat. About 10% of your daily calorie intake is used to digest and absorb a meal with mixed macronutrients, but here’s the breakdown in case you’re interested:

  • About 0–3% of the calories from the fat you eat are used to support its digestion.
  • About 5–10% of the calories from the carbs you eat are used to support its digestion.
  • About 20–30% of the calories from the protein you eat are used to support its digestion.

A high-protein, low-carbohydrate diet tries to leverage this phenomenon to burn calories, as it takes more energy to burn protein than carbs or fat. Since food thermogenesis only accounts for 10% of daily calorie burn, eating more protein will only have a small effect on your metabolic rate. While protein is still helpful for weight loss, you need to consider the cons of eating too much, including the wear and tear on your kidneys.


Accounts for 20% of daily calories you burn.

For most people physical activity accounts for 20% of daily calories burned, but this percentage can be higher on tough workout days. Keep in mind that it’s not just about the calories burned while working out; it’s also about the calories burned while working on the job (think: typing, carrying heavy loads, standing, fidgeting) and having fun (think: shopping, playing, singing). MyFitnessPal takes this into account by asking you to identify your usual activity level (sedentary, lightly active, active, very active) to more accurately predict your total daily calorie burn.



Our body is designed to extract energy from all three types of macronutrient fuel (carbohydrates, fat and protein), but that doesn’t mean a calorie is just a calorie. What our body is going through while it’s burning those calories determines which fuel it turns to the most. Here are some common conditions:

  • Exercise above 70% maximal heart rate: At this intensity, it’s difficult to talk because your body focuses on breathing to deliver oxygen to your exercising muscles. Under lower oxygen conditions, your body turns to carbs for fuel. Why? Because carbs can be burned quickly and it can be used in oxygen-deprived conditions.
  • Exercise below 70% maximal heart rate: At this intensity, you’re able to freely chat with your jogging partner and still get enough oxygen. In this scenario, the body prefers fat as fuel for these activities because this slow-burning fuel is perfect—especially when there’s plenty around.
  • Overnight fast: Your metabolism slows down during sleep, but calories are needed to repair cells and maintain normal bodily functions like breathing. Since you won’t be eating for the next 8–12 hours, your body will use fat to fuel most tissues and carbohydrates to fuel your brain. The carb that fuels your brain is glucose, and it’s usually stored as glycogen (stored carbs) in your cells. If there’s not enough, your blood sugar will drop and protein is pulled in for fuel, because you can convert protein in glucose (a sugar) to bring your blood sugar back up to normal levels.
  • Starvation: Your metabolism slows way down, making you feel tired and edgy. Protein and fat become the dominant sources of fuel. After 48 hours without food, your body runs out of glycogen to power the two organs that prefer it the most: blood cells and your brain. While glucose is the only fuel blood cells can run on, the brain will start learning to power itself with fat in the form of ketone bodies. Your body ramps up breakdown of muscles and organs (for protein fuel) and fat pads (for fat fuel).

Additionally, certain diseases and conditions will affect the fuel your body uses the most. For example, if you’re suffering from a third-degree burn, you’ll need much more protein fuel to heal and rebuild tissues.

You can also change the fuel type your body prefers during exercise by training. As you train, your body becomes more efficient at using oxygen during exercise. This allows you to burn more fat during higher-intensity exercises rather than mostly carbs. For this reason, you can perform at a higher intensity (e.g., run, cycle and swim farther and faster) without feeling tired.

Did-You-Know_v2 (1)


  1. The “fat-burn” zone on your exercise machine is the best setting for weight loss. Not necessarily. The “fat-burn” zone on your exercise machine usually operates at a slower pace to keep you at less than 70% of your maximal heart rate. This allows you to burn a larger percentage of the calories from fat, but the number of calories you burn will be less than if you challenged yourself. Upping the intensity of your workout allows you to burn more calories overall, which is helpful for weight loss.
  2. Eating six small meals daily will boost your metabolism. If the quality and quantity of calories you eat stays constant, eating six small meals instead of three square meals won’t boost your metabolism. The pro of this strategy is that it may help you stave off the hunger pangs if you’re cutting calories. The con is that it presents you with additional opportunities to overeat.
  3. Thinner individuals have a higher metabolism. False! Thinner individuals generally have a slower metabolism than heavier individuals. Heavier individuals require more calories to maintain their larger organs.
  4. Late-night eating will mess with your metabolism and cause weight gain. Your metabolism does ramp down at night, but your body will still handle the food you eat the same way. Again, focusing on calorie quality and quantity is most important.
  5. Drinking green tea will make you burn more calories. Green tea contains catechins, which supposedly help with fat burning. Several small clinical trials suggest that drinking green tea may benefit weight loss, but the effect is small and the study results were inconsistent. If you like green tea go ahead and brew yourself a batch, but don’t count on it for any sizeable metabolic spur.


  1. Squeeze strength training into your exercise routine. Adding muscle mass increases your BMR, the biggest contributor to your overall metabolism. This will allow you to burn more calories even when you aren’t exercising. Check out So You Want to Start…Strength Training to learn more.
  2. Ramp up your workout intensity. During an aerobic exercise (think: running, swimming, biking), go at a pace fast enough that you can benefit from “after burn,” a phenomenon where you burn extra calories after exercise.
  3. Meet your daily protein goal. High-quality protein sources will give you all the amino acids needed postexercise to help muscles repair and grow. To learn more about how to determine protein needs, check out our Beginner’s Guide to Protein.


  • C.W.

    This was an awesome article, thank you.

    Thought-out, consolidated research from many sources, and well-written overall. This is a topic I don’t know much about, and struggle to understand so any simplification like this is appreciated 😀

  • frgough

    Thinner individuals can and often DO have a higher metabolism (I guess you forgot your whole point on basal metabolic rate you made earlier). We all know people who can eat without worry and remain thin. If these folks are eating more calories than you, and are thinner than you, their metabolic rate has to be higher than yours.

    • excpired

      It could also just be confirmation bias that makes you think they are eating more than you – or it could be that you are insulin resistant / have metabolic syndrome and they don’t.

    • Jennifer

      Mmm, not really. My husband is thin as a rail. He doesn’t eat a lot (Alas! His diet does revolve around things that he shouldn’t eat). But he is very active in a busy sort of way. He can’t sit still except for meal times. I think that most thin people, from what I’ve observed, aren’t really big eaters, they’re just more active– whether by being busy or by keeping fit.

      • Traci

        I think in that way everyone is different Jennifer. I have a friend who eats a lot. She is thin and very hyper. I told her I was going to cut back on food portions, etc. once and she said “what are you cutting, you don’t eat very much.” Compared to her I don’t eat very much, that is for sure. I have Hoshimotos thyroid and I can lose weight with considerable effort and exercise, but mostly lower carb eating. Still have healthy carbs but more protein and veggies.

      • Guest

        Tell that to my brother-in-law who’s tiny and eats a lot. When we do potlucks, we joke that we have to count him as 3 people. He has a bottomless pit of a stomach.

  • Hyok Lee

    OMG, can the many small meals speeding up metabolism meme die already??? There have been multiple controlled studies showing that it makes no difference whatsoever whether you eat 3 meals a day or 6 meals a day, provided the overall calories and macronutrients were the same.

    • Janet

      That’s what the article stated.

  • Kelli

    I laughed out loud while reading the myth about green tea, because as I was reading I was drinking green tea for “fat burning” reasons. Thank you for this article! It was extremely enlightening! 🙂

  • Christin Seegers

    ” A bigger individual requires more calories to sustain their body at rest and with any activity they do. Taller and heavier individuals have larger organs (muscles, brain, heart) that require more calories for upkeep.”
    Thank you! I’m five foot seven and due to my “larger frame” I’m supposed to be 160 lbs. Not friggin 100 lbs like all these girls want at a size 0-2. At 119 lbs, I was wearing a size 9. Yeah, I’m a big woman.
    I got so tired of hearing about starving yourself, drink more water and eat more salads. And my job consists me of lifting about 2,000-3,000 lbs four days a week, then a couple hundred pounds the remaining two days. I work six days a week.
    I don’t need to eat less. My main issue for weight gaina lower metabolism was sodas, junk food, stress and lack of sleep.
    I cut back on my sodas to one diet soda a day, rarely any junk food now, lower stress and more sleep. Toss in some protein, lil bit more fruits and ‘some’ veggies (I hate veggies), casualroutine exercise and most of all, I kept eating.
    Then a miracle happened, I started losing weight.

  • Juan K

    It’s been a while since I was in high school but does anyone know if PE has evolved? A nutrition class seems like it’d be way more applicable for kids/adults than running laps and separating the “strong from the weak”.

    For the record I loved PE, but nutritionally still trying to figure it out.

    • Will

      -Zero Hour PE and The New PE
      -Naperville Central High School (Phil Lawler)
      -“Spark” by John J. Ratey, MD

      Youll be AMAZED at the difference an evolved PE curriculum has on its students!

  • Redshirt58

    Good article, in general, about nutrition. While I realize this was meant to be a short article, sources of some of your statements would have been helpful for the avid (investigational) reader.

    One thing particularly caught my eye (BUT ONLY because I’ve recently learned about the Atkin’s Diet in depth after my wife’s LIVER specialist ordered her on it to lose 20% of her body weight in (hopefully) FOUR months). I would be keenly interested in ANY research or studies that truly document (not merely “suggest”) your caution about possible kidney damage from a diet too HIGH in protein (and LOW in carbs).

    PLEASE, note this VERY RELEVANT quote from Chapter 9 of his book, DR. ATKINS’ NEW DIET REVOLUTION (© 2002):

    Fallacy #6: You eat too much protein when doing Atkins, which is bad for the kidneys.

    “Fact: Too many people believe this untruth simply because it has been repeated so often that even intelligent health professionals assume it must have been reported somewhere. But the fact is that it has never been reported anywhere. I have yet to see someone produce a study for me to review, or even cite a specific case in which a protein-containing diet causes any form of kidney disorder.

    The only remotely related phenomenon is the fact that when someone is already suffering from far-advanced kidney disease, it is difficult for that person’s body to handle protein. But protein has nothing to do with the cause of the kidney problem.”


    By the way, I had heard of Atkins a lot previously but not so much in recent times. That’s apparently because of his death in 2003 due to medical complications from an accidental fall off a ladder in the New York snow & ice. Dr. Atkins and The Atkins Diet are each perhaps the epitome of the “poster child” for the High-Protein, Low-Carb diets

    After his death, many other similar diets emerged, with very glitzy names & books & P.R. campaigns. Though ordered on The Atkins Diet by a very qualified liver specialist, when my wife went to her PCP, she got the standard “concern” lecture about possible kidney damage, reflecting the widespread basic ignorance of the four phases of the lifetime Atkins’ nutritional approach!

    Atkins was one of the pioneers of the High-Protein, Low-Carb dietary approach, his research dating back the 1970s. He had a REAL passion for health, the obesity/diabetes epidemic, the SCIENCE of metabolism and, secondarily, the effects of diet on blood glucose, insulin resistance, and one’s comprehensive metabolic panel (CMP) labs. While his organization still lives on, it now lacks the media attention that Dr. Atkins so well provided. He was the obvious face of the whole approach.

    • Amanda

      One of my classmates in college (in his mid 20s) ended up on dialysis from following The Atkins Diet. He told his story in a nutrition class and it was more than enough to deter me from that diet.

      • Redshirt58

        Amanda, sorry for the late reply. I thought Discus sent a notification of comments; I’ll have to check my settings.

        Please see my detailed response to Rebecca!


    • Rebecca Nixon

      Both Harvard and The Mayo Clinic have published studies that link high protein intake with kidney damage. You’re quoting “Fallacy” and “Fact” from a book written almost 15 years ago. Calling it very relevant is a bit of a stretch.

      • Redshirt58

        Hi Rebecca,

        Thank you for replying to my comment! I’m very interested in learning more about this. Could you possibly post links to the articles/studies you reference?

        I know, when speaking about the Atkins diet with nearly all medical professionals, including even dietitians, that few seem to actually KNOW the details of the program; hence, I presume they’ve never really read the book. The “misinformation” they quote, whether PRO or CON, clearly demonstrates that to me – to the extent that I’m suspicious that Dr. Atkins may be getting the tertiary blackball in med school curricula.

        For example, my wife’s doc, a nurse for 20+ years before becoming a doctor, is 60+ pounds overweight, suffers so much with migraine headaches that she would have been forced to take a medical disability retirement, is NOT exactly the epitome of good health. Yet, she told my wife she tried Atkins & it worked to help her lose weight . . . BUT, when she returned to “a normal diet”, she gained it all back! Since Atkins is a program “for life”, with 4 distinct phases (the last – Phrase 4 – being the “for life” very healthy & balanced phase), she apparently only read about Phase 1, the quick weight loss phase – NEVER intended to be the diet that, admittedly, might have the long-term side-effects you reference. Interestingly, she seems puzzled that my wife’s A1C (same doc diagnosed my wife as diabetic) plus ALL her other labs, have gone from “out-of-range” to well within normal ranges, including NO abnormal kidney or liver function tests!

        Just because the book is 15 years old doesn’t necessarily make the facts wrong, per sé, though more recent studies may have added to our collective knowledge. My concern, and my interest in researching the studies you mention, is the possibility that these studies, like my experiences with medical providers, may also have been based on partial truths.

        One of the main cornerstones I’d Atkins is the extreme harm refined sugar does to our bodies. While Atkins boldly requires that one give up refined sugar (and, in Phase 1, even most all natural sugars, too), most other diets waltz around America’s obsession with sugar (Weight Watchers, for example), encouraging cutting back but rarely eliminating it from our diets.

        Another, contrary to current medical guidelines, is that one must NOT cut out fats from the Atkins diet. To do so drastically reduces its effectiveness and can then even make it dangerous. My wife’s doc was frustrated with Atkins because she admitted it was so difficult to eliminate the fats from the Atkins program (that was a clear “tell” she didn’t understand the diet).

        If you’ve read Atkins, you know that his dietary program wasn’t a quick whim of Dr. Atkins; rather, it was based on 25+ years of research & clinical practice in his clinic (with other docs) on thousands of patients and, in his Q & As, he addresses many of the health concerns cited – kidney damage being one. He clearly states that, for those having kidney disease or lacking normal kidney function, to use extreme caution or avoid certain phases, when implementing his program.

        Anyway, my comment wasn’t intended to argue your posting but, as it turned out, I guess I vented my frustration with the seemingly endless array of misinformation – yea, even disinformation – about the Atkins diet.

        Having said that, PLEASE help me by providing the specifics references to the studies you cite.


  • Sweets

    This is a VERY comprehensive easy to understand article…quite impressive. Thank you so much! I’m sharing with the whole world right now!

  • Rebecca Nixon

    Regarding protein causing kidney damage:
    Let’s take a look at your “actual facts” and their “cited references” shall we?

    Your article cites two sources regarding this issue, however both are the same; an organisation called “International Society of Sports Nutrition.”

    Seeing as the contrary information I have is from the Harvard Medical School and since I’ve never heard of this particular “society” I decided to do some digging!

    Their messy, thrown-together, unprofessional website lists a gmail email address (how very authoritative!) as well as a list of sponsors, including many PROTEIN SUPPLEMENT MANUFACTURERS, for example, “MuscleTech”.

    Their board of directors only has THREE people on it! Let’s check these guys out, yes?
    Jose Antonio: His only claim to fame seems to be an ebook on Amazon promoting caffeine as a health supplement (zero reviews).
    Douglas Kalman: Has one website, which has an online store “coming soon” where he will be SELLING PROTEIN POWDER over the internet.
    Tim Ziegenfuss: published works promote the consumption of caffeine, hormones, and of course, protein. A favourite on “T Nation” where they sell testosterone over the internet.

    Looks like these guys really have your best interests front and centre, Christine!

    I don’t know if you work for one of these fake medical organisations or if you are only guilty of being misled by them, but I think you need to drop the snarky attitude and eat some humble pie, here.

    • Bre

      I actually suffered from this. My kidneys began to fail because I was taking in to much protein. I began to retain water because my kidneys couldn’t process the large amount of protein. I became very sick. It took almost 3 months to recover from this. now I am very careful about how much protein I take in.

  • asmcriminal

    If you burn 2500 calories a day and eat 2,000 calories a day what is your rate of weight loss? It does not matter if you eat the 2,000 right before bed or in 1 meal or whatever. 2,000 calories is 2,000 calories it does not magically cause weight gain if you eat late or if you do not eat it in multiple times a day. 2,000 calories IS 2,000 calories. If you burn 2,500 calories a day this included through a 24hr period which includes MORNING AND NIGHT.

    • GeniusUnleashed

      Read about intermittent fasting (IF). There’s a reason not eating late is more beneficial for weight loss.

  • Jo

    Great article!

  • Truthnolies

    Cells are organs now? And yes the carb that your brain uses is glucose…because all carbs are glucose.

    Good rules of thumb in this article but I’d be weary of the details.

  • I like that MFP allows editing of the Calorie Intake Goal, which I calculate with a different software using my measured Resting Metabolic Rate (easier to measure, but with greater variation than BMR due to measurement conditions — see Dietary Reference Intakes for Energy, … from USA’s National Academies, for definitions that differentiate between BMR and RMR).

    In speaking with an UnderArmour research member at QS’15 in SanFrancisco this past summer, he suggested that MFP & UA Record would continue to be satisfied using Mifflin-St Jeor (1990) as an BMR predictive tool, even though he agreed that this was *negligent use of a population average display tool* (ie, should not be used as a Calories Burned prediction tool, but instead to show the normal range).

    I recommend that MFP make the BMR & RMR editable (like Enquos App, e.g.), to lead the way by setting an example that *measuring* MR is the only method for determining an individual’s caloric burn.

  • Vangel WLE

    1 fix, don’t be guided by an exercise machine on how long or hard to exercise.
    2 No configuration of eating boosts your metabolism, only exercise does.
    3 thinner people don’t eat as much collectively over a 7 day week. ( eat less exercise more 60 – 90 minutes + if overweight or obese that’s the medical world health guidelines for obese and overweight, the guidelines most medical or nurses no not know of )
    4 depends, some need a glass of milk at night to calm the stomach before bed, to help them sleep.
    5 this is the Asian coming out, green tea, rubbish. Asians are the thinnest people in the world so we must listen to them, right? they are obsessed with their weight, and its the total Asian lifestyle of eating a hand full of bland food a day and walking heaps till they stay thin or they wont get married.
    3 tips – exercise yes, but you are not an exercise expert, so leave it. and you cant raise metabolism with food that would make any significant difference.

  • Strangerinastangeland

    1-2-3. 1/6 cals from fat, 2/6 cals from protein, 3/6 cals from carbs.

  • Dee

    I’m always interested in these blogs as my metabolism is under active due to hyperthyroidism compounded with menopause. I workout 4 days per week incorporating cardio and weight training and watch my diet for the most part and still, no weight loss. I’m stuck! Does anyone have any suggestions? Thanks

    • If you can count your Calorie Intake over the next few days (with MFP), then you’ll have a good rough estimate of your “Maintenance Calories” for the current workout routine. I would aim to keep the workout routine (for long-term health/wellness benefits), and chip away at the calorie’s IN — say, reduce by 10%.
      Strategy of when to eat, how often, and what combinations of foods is something you’ll have to practice and improve on (so that you can stick to the 10% deficit).
      Meanwhile, it would be wise to have a specialist measure & track your (serum) T3 and (breath-based) RMR in another month (assuming you have done so recently, as a baseline to start from). Hope you can find a good specialist that empowers you “to put hir out of business”, so to speak. Best!

    • Elaine M Woodward

      Don’t you mean hypothyroidism

  • Haydi

    Love that these articles are backed by studies! I am a nurse so I can appreciate that you guys write your articles using facts and not opinion.


  • Scarlet

    An informative article saying facts about metabolism! You need to know what eat for it is the source of your bodies energy. Great!

  • Daniel Bradford

    Can someone please confirm if I should be exercising above or below the 70% MHR? I have read all over the place to be below 70-80 but this blog seems to contradict even itself. Saying if you up your intensity you will burn more install calories. Does this mean that more calories would then be burnt from carbs rather than fats which is what is desired to lose weight?