Metabolism and Weight Loss: What You Need to Know
When people face difficulties trying to change their body composition, the metabolism is one of the most common scapegoats. Is this blame warranted?
Maybe a “slow metabolism” is scrutinized for challenges with weight loss or maybe a “fast metabolism” is blamed when it seems impossible to gain weight. It all starts with how we define the word “metabolism.”
These claims are often misguided, and I’ll explain why in this article. Then we’ll explore what you can actually control to change your body composition to reach your goals.
What is the human metabolism?
The word “metabolism” is often poorly understood and misused by the general population. It’s frequently used interchangeably with energy expenditure (how many calories you burn throughout the day) or resting/basal metabolic rate (how many calories you burn at rest).
The human metabolism is far more complex than these general statements. Your metabolism is a multidimensional and tightly regulated system of chemical reactions and processes within dedicated pathways in the body to maintain cellular function.
Some of these components include glycan biosynthesis, biosynthesis of secondary metabolites, biosignaling pathways, biodegradation of xenobiotics, and the metabolism of lipids, carbohydrates, nucleotides, amino acids, cofactors & vitamins, and other energy systems. Yeah… that’s a lot!
Typically, when we’re referring to the metabolism in the health and fitness space, we’re only referring to the overall energy metabolism, or basal metabolic rate (BMR).
It’s also important to note that total daily energy expenditure (TDEE) is vastly different than basal metabolic rate. TDEE is known as the “calories out” side of the energy balance equation, which is where the real impact on weight loss or weight gain occurs. TDEE is an all-encompassing term that includes BMR, thermic effect of food, and activity-induced thermogenesis (both from actively and passively exercising).
For sedentary people, the BMR accounts for close to 70% of the overall energy expenditure, which is obviously no small amount.
There are individual outliers, but the vast majority (96%) are within 200-300 calories of the average BMR. This even accounts for people with different bodyweights, which plays a large role in metabolic rate (more on this in the next section).
Can you “slow” or “worsen” your metabolism?
The most influential factor that determines your basal metabolic rate is your body mass, which includes both your fat-free mass (FFM) and fat mass.
Muscle mass is “expensive”, meaning that it requires a lot of energy just to have on on your body. At rest, it requires about 6 calories/lb, which excels in comparison to the “inexpensive” fat mass that only requires 2 calories/lb to maintain.
The more muscle tissue and fat mass that you have, the more energy that you expend at rest. Plain and simple.
However, where people run into issues with their metabolism “slowing down” is when they lose large amounts of muscle mass while dieting.
There is distinct difference between weight loss and fat loss. Creating an energy deficit will absolutely lead to weight loss, but the type of tissue that is lost (muscle vs. fat) depends on a variety of factors (exercise, protein intake, rate of weight loss, etc.).
If large amounts of muscle mass are lost in the process, weight loss may stagnate because the tissue that remains on your body is less “expensive” and is burning fewer calories per pound. This situation is distinctly exemplified on the Biggest Loser TV show when contestants rebound after rapid weight loss and often regain even more weight than what they started with.
There’s no way to avoid lowering your BMR when you lose weight. Since there’s less mass to maintain, less energy will be required to normalize your system.
Strength training consistently and intensely, keeping protein intake high, and keeping the rate of weight loss at a moderate pace can improve your body’s ability to preserve and retain muscle mass while in a caloric deficit.
Can you “boost” or “improve” your metabolism?
Now that we’ve covered how not to detrimentally lower your metabolic rate, you may be wondering if there are any ways to speed it up. Let’s take a look!
Gain Weight
The first one is fairly obvious and follows our general equation that was defined earlier: gain weight, but preferably muscle mass! This will increase your overall body mass, so there will therefore be more energy required to maintain it.
Nutrition
What about detox diets or juice cleanses? These don’t do anything magical! They usually reduce the overall caloric intake, which results in weight loss, but that’s about it.
If we break down a diet into any isolated foods or supplements, are there any metabolic boosters? Not really! There are nutrients and compounds that increase energy expenditure (caffeine being a prime example), but these do so acutely and are pretty insignificant in the grand scheme of things.
Protein and fiber are relatively thermogenic and therefore expend a fair amount of energy during the process of digestion, so they resultantly have a lower net caloric effect on your energy balance. However, in terms of long-term effects to your BMR, there isn’t much that dietary interventions can do.
Exercise
Exercise not only acutely increases your TDEE, but also has a lasting effect on your BMR. When you engage in intense exercise, excess post-exercise oxygen consumption (EPOC) can last upwards of 24 hours after the workout itself.
Aerobic exercise, like traditional steady-state cardio, has a lower EPOC compared to anaerobic exercise, like resistance training. This means the post-training caloric expenditure (even at rest) is higher after an anaerobic training session, assuming duration and intensity are comparable.
Intense bouts of exercise will not only increase TDEE, but can increase your BMR long-term if the stimulus (and other relevant conditions) is conducive to muscle growth. As discussed earlier, packing on muscle mass directly increases your BMR!
Even if net muscle growth isn’t obtained from intense exercise, the muscle fiber reparation process is energy “expensive” as well. Simply damaging muscle from training and then repairing it to the baseline requires energy to do so.
Your BMR only considerably changes as an adaptive response to long-term stress over time. Nothing acute or temporary will notably affect your BMR long-term.
If my metabolism isn’t to blame, what can I control?
Most people significantly underestimate how much they’re consuming (calories in) and eat beyond their maintenance needs to sustain an energy balance. This creates a caloric surplus, which over time, can really add up.
The most impactful change you can do to “boost your metabolism” isn’t to try to increase your BMR (that takes care of itself), but to move more and increase your TDEE. There’s plenty of ways to manipulate the energy balance through dietary, activity, and lifestyle changes (like standing vs. sitting) that I explain in this article.
Take Home Messages
Your basal metabolic rate adjusts as you gain or lose weight (due to a gain/loss of body tissue). When you’re in an energy surplus, your BMR speeds up. When you’re in an energy deficit, your BMR slows down. However, once maintenance has been reached, your BMR will renormalize.
In the general population, people with extremely fast or slow metabolisms don’t exist. There are people with more body mass and people that move more, but lifestyle factors contribute to the majority of fluctuations in energy balance.
The best way to “speed up” your metabolism is by exercising consistently and intensely. Aerobic exercise is great for temporarily increasing total daily energy expenditure and anaerobic exercise is best for it’s lasting BMR effects.
As always, the fundamentals of an energy balance can’t be avoided. Ingesting more energy that you expend will lead to weight gain and vice versa. Focus on what you can control and let your body take care of the rest!