Most of us have heard about, talked about, or complained about our metabolism at some point in our lives but not many of us really know what it is and how it works. Magazine articles promising to “boost your metabolism” give us a false sense of what metabolism is as well as our ability to control it (we all know someone who has chugged green tea, started running sprints, or bought a certain supplement in the hopes of increasing their metabolism). Spoiler alert: metabolism isn’t some thing that we can manipulate, like a bicep or our hair color. It is a lot more complex, and interesting, than that. It’s a natural process for making energy, and your body is a pretty sophisticated system with a lot in place to regulate your metabolism and ensure your body has everything it needs to function.
So What Exactly is Metabolism?
Metabolism is the process - actually a series of chemical processes in your cells - your body goes through to turn the energy from your food into the energy your body uses. Your body needs and uses energy for four main processes:
Basic bodily functions - like breathing, circulating blood, regulating body temperature, and creating cells. Your major organs - like brain, heart, kidneys, and liver - also need energy to function, as does your nervous system. The amount of energy needed for basic bodily functions is called your basal metabolic rate (or BMR). On average, this accounts for most of your energy needs - about 70%.
Digestion - it may sound crazy but your body actually needs energy from food to break down and digest the energy from food. It’s called the thermic effect of food (TEF) and it requires energy to process nutrients so that our body can use them. The simple act of eating actually increases metabolism. It’s also important to note that digestion is controlled by our autonomic nervous system (ANS) so the energy we need for basic bodily functions is also important for digestion. On average, this accounts for about 10% of energy needs.
Exercise activity - things like going for a run or strength training require energy in order to do these activities. The higher the intensity of the exercise, the more energy your body needs to perform and recover from these exercises (i.e., you may need more or less energy depending on the type of exercise). Depending on your activity level, this can account for 10-30% of your energy needs (for sedentary people this typically accounts for about 10%, while for highly active individuals this can be up to 30%).
Non-exercise activity - like cleaning your house, playing with your kids, carrying groceries, moving around your office, or tapping your feet to music. This is just your daily life movement - not intentional exercise - that requires energy to do and it’s called non-exercise activity thermogenesis (NEAT). This typically accounts for the least amount of your energy but it can actually be an important factor in weight management.
All of the energy needed by your body to do these things - to function, digest your food, exercise, and just move around through life - is measured in calories. This is a good place to pause and highlight this point because quite often people think about (and demonize) calories incorrectly - a calorie is just a measurement of energy. Calories are often deemed as the enemy - something to be scared of, limit, or avoid; yet this is literally the energy your body needs to function. Restricting calories means restricting the energy your body has to work properly. That’s why highly restrictive, calorie-counting diets don’t typically work (or last). More on that here.
Basal Metabolic Rate
While metabolic rate doesn’t exactly represent whether you have a healthy metabolism, it does provide information on how your body burns energy. Everybody has a minimum number of calories needed for basic bodily functioning, and your BMR represents this. A good way to think about this is to think of your body like a car - it needs fuel to run, whether that is driving slowly in a parking lot, driving fast on a freeway, or idling in a driveway.
No matter what the car is doing, fuel is being burned just to keep the engine on. Your body is the same. It needs fuel (calories from food and drinks) to keep it functioning - whether it is sleeping, walking between rooms in a house, or running a marathon. In fact, your BMR accounts for most of the calories you burn in a day. On average, about 70% of the calories you burn daily come from doing nothing except your body just trying to keep itself alive. For this reason, your BMR is pretty consistent and won’t change day-to-day. After all, your body knows what it needs to function so it doesn’t need to vary its energy needs much. This is a pretty good reminder of how amazing your body is and how it is a highly sophisticated machine.
While BMR is consistent among each person, it does vary from person to person depending on several factors like age, sex, body size, and body composition. And even two people who are the same age and sex, with the same body size, might have different BMRs. While this can be frustrating to some people, it’s also a good reminder of how unique each of us is - our bodies need different amounts of energy to function.
Measuring Your Metabolic rate
So if BMR varies from person-to-person but is pretty constant in each person (and pretty important since it accounts for most of the energy your body needs) how do you know what your rate is? In other words, how do you know the minimum amount of calories your body needs to function properly? Unfortunately, it’s really hard to accurately measure this but there are three options:
Direct calorimetry: this is the most accurate way but is also the most expensive and inconvenient. Using an air-tight metabolic chamber, energy burned is measured by the rate your body produces heat. This is usually done over a 24-hour period - monitored by professionals - with absolutely no activity - including eating or drinking - so the body is at complete rest.
RMR calculators: Similar to BMR, your resting metabolic rate (RMR), is how much energy your body needs at rest but is less restrictive than BMR. Internal processes like digestion and external factors like environment aren’t excluded. The internet has a plethora of RMR (or BMR since many people use those interchangeably) calculators. These use basic information like your age, sex, height, and weight to calculate your RMR using an equation. There are several like the Harris-Benedict equation, which is probably the most commonly used one, and the Mifflin-St. Jeor equation, which, when compared to indirect calorimetry, has predicted better than the other equations. However, use any of these of with caution; they are just approximations, and results can vary among different calculators, sometimes by several hundred calories. Even the best equation can only get within about 10% of the actual RMR value measured in a lab, which, if you are trying to manage your weight based on this, 10% is a big variance when it comes to energy balance. And energy balance is the ultimate equation to pay attention to when trying to manage weight loss or gain (i.e. taking in more or less energy than our body needs will result in more body weight or less body weight, respectively).
Indirect calorimetry: this uses your breathing to determine metabolic rate based on the ratio of your oxygen consumption to your carbon dioxide production and nitrogen waste, known as your respiratory exchange rate. It usually involves a mouthpiece, nose clip, and computer that records oxygen consumption in real-time. It’s definitely not as accurate as the metabolic chamber but it’s better than the RMR equations and usually more accessible - some gyms even have them.
Speeding Up Your Metabolism
While there are numerous magazine articles claiming ways to increase your metabolism - including several myths - there is a lot of debate in the medical community about whether you can increase your metabolism. There is, however, increasing research suggesting the link between the gut and metabolism as well as how diet influences both. So there is some evidence to suggest that our fuel type (aka our food choices) can play a role in gut and metabolic health. This means choosing nutrient-dense fruits, vegetables, legumes, nuts, and lean proteins that our body needs to effectively convert energy. While some foods - like coffee, chilli, and green tea - have been noted to temporarily speed up your metabolism, this effect is so short-lived that it doesn’t have any lasting effects on your metabolism or weight. If you like these foods, continue to eat/drink them but don’t expect them to suddenly boost your metabolism.
Additionally, while physical activity won’t have much impact on how effectively your body converts energy, it can have an impact on your daily energy needs (and balance). Intentional physical exercise can increase calories burned that day. If your body is working extra hard during a workout, it will need some extra energy to replenish and continue functioning. That said, not all exercise demands the same energy needs - a gentle, restorative yoga class will not demand the same energy requirements as hill sprints - so it’s important to be mindful of that. Also making small changes in daily movement - like taking the stairs or parking farther away - will help your body use up a little more energy. The key here is little - this isn’t an excuse to reach for a donut because you took the stairs. This isn’t about burning calories - this is about slowly changing the default from being sedentary to having movement, which can have a big impact on your health and weight.
Finally, one notable way to help impact your metabolism - and overall health - is to build lean mass (aka lean muscles). The more muscle you have, the less fat you have, and lean mass takes more energy to maintain. This means that lean muscle can actually impact your metabolic rate because it takes more calories to maintain that muscle. The effect can be minimal so, again, this isn’t an excuse to reach for that highly processed, sugary food. It’s also important to note that more muscle may mean increased hunger, so it’s important to refuel your body with healthy foods (remember two paragraphs up where we talk about nutrition’s impact on the gut and metabolism). Honestly, that is one of the biggest treats you can do for your body - feed it minimally processed, whole, nutritious foods so it can function properly.
Slowing Down Your Metabolism
So this is the sucky part - while it’s pretty hard to speed up your metabolism, it’s much easier to slow it down. There are two big factors to this - age and restrictive dieting.
Even if you have the same body composition - fat vs. muscle - your energy needs change as you age. Researchers aren’t entirely sure why your metabolism slows down as you age, but it does - starting as early as age 20! There are theories that it is related to a decrease in physical activity and reduced muscle mass. For instance, after the age of 30, adults lose 3-8% of their muscle mass per decade. While that might sound like it sucks - and while aging is out of our control (but way better than the alternative) - the factors that seem to contribute to this decline in metabolism are actually somewhat in our control. After all, we can control our physical activity and muscle mass. One Harvard study found that as little as 20 minutes a day of strength training - compared to aerobic activity - in healthy men aged 40 and older had significant impacts on belly fat and weight gain. This is one of the reasons that exercise - and a variety of exercise - is so important for health.
Restrictive dieting, as well as constant stress, can lower your metabolic rate drastically - by as much as 15% (and some studies suggest up to 23%). Your body is designed to protect itself from too little calories (aka starvation), so if it senses it’s not getting enough fuel it will slow down your metabolism to protect against starvation. This is exactly the opposite of what someone on a diet wants and this is often why restrictive dieting doesn’t work or why the overwhelming majority of dieters gain weight back. It’s also incredibly hard to maintain a restrictive way of eating ALL the time. In addition, restrictive dieting can contribute to nutritional deficiencies, impact menstrual cycles, fertility, reproductive hormones, and thyroid function.
Supporting a Healthy Weight
When people talk about, worry about, or try to manipulate their metabolism, it’s almost always in the name of weight loss. But, as mentioned previously, metabolism is really about transferring energy from our food to our body so it can function properly. And weight management is really about energy balance. Your body is constantly trying to balance its energy needs and is looking for that “just right” amount. Our bodies are designed to store excess calories in fat cells so if you consume more energy (aka calories) than your body burns, your body will store it (gain weight). On the flip side, if you consume less energy (aka fewer calories) than your body expends, you will lose weight. It is also important to remember that not all calories are created equal so simply reducing or increasing calories may not get the results you desire - nor will any quick fix, magic bullet promising everything. Our fast world demands we have quick solutions - when it comes to a healthy weight or body, going slow is really the way to get there. It’s about creating a healthy lifestyle that takes time, intention, and small changes. It’s a daily practice that includes eating nutritionally sound food your body needs; engaging in consistent, healthy physical movement/activity; the ability to manage daily stresses; and allowing your body time to repair with adequate sleep. While this might not be the answer you were hoping for, it’s the one that will work.