We all have that one friend. That person who seems to have fat simply fall off with little perceived effort, or that seems to be able to eat anything and everything without putting on weight. Or, maybe you are that person…not that I’m jealous or anything. These folks are commonly referred to as having a “fast metabolism.”

So what sorcery is this? Is it a lucky roll of genetics? A combination of some tricks that is some big secret? Well…kind of. In this article, we’ll discuss what metabolism actually is, the main factors determining metabolic rate, and why there can be such varying rates between two people.

Is It True You Can Have a Fast Metabolism?

What is “Metabolism?”

Metabolism is a term commonly used for several different, related factors. In fitness, it’s usually referred to in the context of fat loss. While this is the application most are familiar with, it’s a little broader than that. Metabolism refers to the chemical processes in your body that keep you going. It primarily consists of three main factors:

• Taking energy from food, and using it for cellular processes
• Converting food into building blocks for the body
• Elimination of waste products from the body

You can think of it kind of like maintenance on a house. The three parts of the construction can be seen as the work/labor, the building materials, and continual cleanup of the work site.  So why can some people build faster than others?

Factors Affecting Metabolic Rate

1. Caloric Intake

What Do We Use Calories For?

Calories are the unit of measurement for energy transfer in all of the body’s reactions. When you eat something, your body breaks the chemical bonds in the food, releasing energy (calories) to your system. Your body then uses that energy in a few basic ways, e.g. Addressing your body’s upkeep needs (metabolic processes) and fueling muscles (movement).

This is the basis of the “calories in = calories out” model. When you take in more caloric energy than your body needs, it will use what’s necessary then save the rest via bodyfat. Conversely, if your caloric intake is insufficient for your body’s processes, it will pull the needed extra energy from your bodyfat stores.

A common analogy is viewing calories as bodily currency. You must pay for bills, groceries, and other mandatory costs of living. If you have leftover money, you send it to the bank for storage. If you’re running low on cash, you withdraw it from your savings to use. This is essentially how calorie balance works in the body.

Your body’s energy needs can be summed up by the following:

Calorie needs = BMR + TEF + EAT + NEAT

Don’t freak out over the alphabet soup of terms. We’ll break each of these down soon enough.

Calories and Metabolic Adaptation

Not only are calories the basis of metabolism, but their abundance also affects metabolic rate. If your body senses it is in a calorie deficit, it will slow metabolic processes in the body to lessen its energetic needs. Conversely, if in a caloric surplus metabolic rates increase.This metabolic slowdown is likely the basis of the “starvation mode” myth. This concept is a greatly over exaggerated effect.

The metabolic adaptation measured in most studies is relatively modest. Even when examining varying amounts of weight loss, metabolic changes hover at around -8.5% on average. This indicates that while metabolic adaptation is a significant factor in weight change, it’s far from the deciding factor.

2. Basal Metabolic Rate (BMR)

Your basal metabolic rate (BMR) is a measure of how much energy your body consumes at rest. It represents the energy needs for your body’s basic processes and upkeep. This value is independent of other common factors such as food intake or activity levels. Think you are lying in bed, not snacking, not moving much at all. On average, BMR is responsible for 60-75% of the body’s caloric needs.

BMR is affected by several factors. Body size and body composition are the most significant, encompassing other factors such as age and sex. On average, humans tend to maintain a stable BMR between ages 20 and 60, with ~0.7% drops each year past 60. Finally, individual differences in genetics and hormone profiles account for at least some variation from person to person, through this is minor compared to the above factors.

Body Size

All tissues have an energy requirement to maintain. Therefore, the more body tissue someone has, the greater their energy requirements are. Men tend to have more body mass on average than women, and we also tend to lose body mass as we age due to inactivity and decreased appetite.

Body Composition

Not all tissues are made the same. While they all require energy to maintain, lean body mass (muscle, organs, bone, etc.) requires more energy than fat mass. This is a significant factor in aging, as we tend to lose lean body mass and gain fat mass as we age.

This also helps explain the difference between the sexes in terms of BMR. Men tend to hold more lean mass on average than women, and women have more essential body fat than men. Additionally, higher levels of androgenic hormones like testosterone are generally more conducive to muscle gain and maintenance.

Thermic Effect of Food (TEF)

We normally think of food as a calorie source, as consuming it provides the bulk of our body’s available energy. However, there is actually an energy requirement for it as well. Breaking down the chemical bonds in food requires energy, as does nutrient digestion and absorption. This requirement is called the thermic effect of food (TEF).

Different foods have different TEF values, based primarily on their macronutrient content. Protein tends to require the most energy to process, taxing about 20-30% of incoming energy. Carbohydrates require 5-10%, and fat has a minimal requirement of about 0-3%.

The average contribution of TEF to metabolic rate is subtle at around 10%, but moving from a low to a high protein diet has the greatest effect overall. The more protein in your diet, the more your body has to work to metabolize your food.

External Factors: Activity

So far, it doesn’t sound like there’s a lot that varies from person to person. Enough factors to partially explain why some folks seem to naturally burn calories faster than others, but it still feels like we’re missing a big piece of the puzzle. So, let’s examine the last big factor: Activity levels.

1. Exercise Activity Thermogenesis (EAT)

n short, exercise activity thermogenesis (EAT) is the calorie burn from dedicated exercise. Workout frequency, intensity, training type, etc. vary greatly, as does the calorie expenditure. On average, though, dedicated exercise has a surprisingly small effect on overall energy expenditure, averaging at about 5%.

This seems strange at first, given how you can truly do some gut wrenching workouts. However, remember that there are 24 hours in a day, and for most folks, they’re only spending a couple of hours exercising every day, often less frequently. So, while there are distinct physiological benefits to lifting and cardio, the calorie burn over the long term tends to be modest.

As for those other 21-22 hours in the day…

2. Non-Exercise Activity Thermogenesis (NEAT)

Non exercise activity thermogenesis (NEAT) refers to any activity you do that isn’t necessarily associated with exercise. All the walking, recreation, and even restless fidgeting contribute to your body’s energy needs. If you’ve noticed a recent fascination in fitness with tracking step counts, this is the reason why.

When it comes to caloric needs, NEAT tends to be the biggest factor that varies among individuals. There is a myriad of different influences on NEAT, including lifestyle, profession, and personal habits. Cumulatively, these all can lead to startling differences in calorie expenditure from person to person, ranging anywhere from 15 to 50% of total energy expenditure. That’s a LOT.

For example, a farmer working a field is going to naturally expend much more energy than an office worker sitting at a desk. Watching TV is far less energy taxing than something like hiking. Someone constantly fidgeting and pacing will have all of that add up over time. So on, so forth.

So, Back to That Friend…

I obviously don’t know your friends like you do. However, with all that’s said above, there are three things I would likely wager as the big reasons for their “fast metabolism”:

First, there is definitely a possibility that their metabolic rate is naturally elevated over the norm. This is what most immediately jump to as the culprit. While this alone wouldn’t account for thousands of calories’ worth of extra calorie needs, but elevated BMR and TEF certainly can contribute. 

Next, we have NEAT. This tends to be a huge factor in these sorts of circumstances. While you may have a general idea of your friend’s habits, it’s unlikely that you can parse out their day minute by minute. A lot of the overlooked, unaccounted for extra movement can increase daily expenditure by hundreds or even thousands of calories in some situations. 

The last factor wasn’t really addressed in this article, but it fits into the idea of not being able to see your friends 24/7. Just because you see someone eating huge meals doesn’t necessarily mean that’s how they eat every meal, day in and day out. Remember that the body’s energy stores don’t necessarily work on a 24-hour cycle. It’s all about the net calorie intake over weeks and even months, not how much one can pound down in a single meal.

Conclusion

As with most fitness generalizations, the idea of a fast metabolism is rooted in some truth but tends to be misunderstood. For boosting your calorie expenditure, the main standards are still your best bet: Eat a high-protein diet, exercise regularly, and keep yourself active outside of the gym. Coupled with a calorie intake in line with your goals, you’re good to go!