Metabolism: Separating the Facts from the Falsehoods

by | Updated: March 28th, 2019 | Read time: 4 minutes

For a word that doesn’t easily roll off the tongue, “metabolism” sure does get tossed around a lot. You hear people say they have a slow metabolism or fast metabolism, which affects their weight-loss efforts. You may even watch workout videos or hear fitness instructors touting to “rev metabolism.” But to determine whether there’s any substance behind these claims, you need to dive deeper into the term itself.

Woman Eating Yogurt Breakfast with Coffee Wonders if This Lead to Fast Metabolism |



Let’s first ask the question: What is metabolism? Metabolism refers to the thousands of chemical reactions that convert the food you eat into fuel. These reactions are constantly occurring in your cells at all times, every day to provide your body with energy.

The sources of energy your body utilizes are macronutrients. These consist of carbohydrates, protein and fat. Macronutrients are metabolized by the body through catabolic and anabolic reactions. Catabolic means breaking down substances; anabolic means building up or creating substances. Now look closer at how this works for each macronutrient.

Metabolizing carbohydrates

Carbohydrates, including glucose, fructose and galactose, are broken down into a molecule called glycogen. You’ve probably heard of glycogen, but the process doesn’t end there. Glycogen is further broken down into pyruvate, before finally breaking down into acetyl CoA. Acetyl CoA is the molecule that starts the citric acid cycle. This cycle creates high-energy carriers called adenosine triphosphate, or ATP. ATP is the currency of energy in the human body and the final product of metabolism.

Metabolizing protein

Protein metabolism starts with proteins breaking down into smaller segments called amino acids. These amino acids are then broken down further to be utilized by the urea cycle or the citric acid cycle, resulting in more ATP. The goal is always ATP.

Metabolizing fats

Everyone wishes they could metabolize more fat, right? Here’s how it works: Fats are broken down by hydrolysis, a process that forms glycerol and fatty acids. Glycerol is oxidized to form pyruvate and then acetyl CoA. Meanwhile, fatty acids are converted into acetyl CoA, but through a series of different chemical reactions. If fats are consumed in excess and carbohydrate energy is limited, fats can be metabolized into ketone bodies – another form of energy you may have heard about through the ketogenic diet.

Metabolism slows down with age

It’s a fact that body composition plays a role in metabolism. A body with greater muscle mass has a higher rate of metabolism. Why? Because muscle mass increases your basal metabolic rate (BMR), or the rate you burn calories at rest. Unfortunately, you tend to lose that coveted muscle mass as you age. That means the best thing you can do to protect your metabolism is to build a lean, muscular body. You don’t have to be a bodybuilder, but focus on strength training to improve body composition – less fat, more muscle!


Certain foods can boost metabolism

Metabolism-boosting foods do NOT exist. Toss that myth out the window. There is no magical food that can speed up metabolism. Spicy foods might rev your heart rate or make you sweat, but they do not impact metabolic reactions in the slightest. Green tea is another common claim to fame when it comes to increasing your metabolic rate. Again, there is no evidence to support this health benefit of green tea, which is better known for its anti-inflammatory properties. Stick to a balanced diet overall, and you will have no problems fueling your metabolism.

Eating after a certain time of night slows metabolism

No. The reactions of metabolism are constant and occur regardless of the timing of meals. It doesn’t even matter if you’re awake or asleep. Your metabolism is always running. Having a later dinner or bedtime snack won’t hurt you, as long as you’re making healthy choices and remaining within your caloric recommendations.

Eating breakfast jumpstarts metabolism

Though breakfast is the most important meal of the day, it does not necessarily boost metabolism. Its importance is derived from research that shows higher test scores and higher cognitive ability when consuming a meal within two hours of waking up. As mentioned previously, metabolism occurs when you’re sleeping and when you’re awake. While you may not jolt metabolism by eating first thing in the morning, you do support healthy blood sugar levels, healthy weight and better cognitive function. So eat breakfast!

What’s the big takeaway? The only evidence-based way to support a healthy metabolism is to exercise. Physical activity increases your BMR, so you can torch more calories when you’re not working out. Feel the burn!