Published 8th September 2022

Metabolism, metabolic rate, and what impacts them

You may have heard people talking about how a fast or slow metabolism affects weight loss. Or maybe you’ve seen claims about how certain foods can “boost your metabolism.”

But what is metabolism?

First, it’s important to know the difference between metabolism and metabolic rate.

Metabolism refers to the chemical reactions within our bodies that keep us alive and functioning.

Metabolism has three primary purposes:

  1. to convert food into energy

  2. to convert food into the building blocks of essential compounds, like proteins

  3. to get rid of waste in your body

Metabolic rate is different. It refers to the amount of energy you use in a given time frame. Typically, we measure this as the number of calories that your body uses in a day.

In this article, we'll explore what affects your metabolic rate and whether you can change it. We'll also look at the metabolism of different macronutrients.

Metabolic rate and calories

The amount of energy, or calories, you burn over a day depends on several factors. 

Basal metabolic rate 

About 75% of your daily energy expenditure goes to keeping you alive and maintaining essential functions. This is called your basal metabolic rate (BMR). It includes vital functions like breathing, sleeping, and blood circulation. 

Genetics, sex, age, body composition, and certain health conditions can all influence your BMR. 

Thermogenesis

Thermogenesis is a metabolic process that uses energy and causes your body to produce heat. There are different types of thermogenesis:

  • Thermic effect of food: Your body uses this energy to digest, absorb, metabolize, transport, and store food.

  • Exercise-related activity thermogenesis: This is the energy you use for exercise, such as running, lifting weights, or doing yoga. 

  • Non-exercise activity thermogenesis: This is the energy you use for other types of physical activity, such as standing up, keeping your posture, or walking from room to room.

What affects metabolic rate?

Several factors can influence your metabolic rate, and they do this in different ways.

Age 

Your metabolism is at its highest around the age of 12 months. It then slows until you reach your 20s, and after that it remains pretty stable.

While many people believe that your metabolism gets slower as you reach middle age, recent research shows that it doesn't start to slow until you're around 60.

Sex 

Sex can affect metabolism due to differences in body composition, fat storage, and hormones. Because of these factors, males tend to have a higher metabolic rate than females. 

Keep in mind that this is a general trend and isn't necessarily true for each person. 

Genetics

Your genes also play a role in your metabolic rate. 

This is a growing field of interest, and there’s plenty more research to be done, but certain genes seem to be associated with differences in metabolism.

For example, researchers think a gene called the “fat mass and obesity-associated gene” may affect how we metabolize food and store energy. 

Body composition 

Body composition, especially lean body mass, impacts your metabolic rate. 

Muscle uses more energy than fat when we're at rest. So, a body with more lean muscle tends to have a higher BMR than one with less lean muscle.

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Infection or illness 

When you're unwell or fighting an infection, your immune response forces your body to work harder. This extra work requires your body to use more energy. 

In fact, your energy expenditure increases by around 7% for each degree Fahrenheit your body temperature rises.

Menopause status 

During menopause, your metabolic rate decreases. As estrogen levels decrease, there’s an increase in levels of body fat. Menopause is also associated with reduced muscle mass. These both affect your BMR. 

Menopause is also associated with changes in fat and sugar metabolism, the gut microbiome, and lifestyle habits, which may also influence your metabolic rate.

Can I change my metabolic rate?

With so many factors at play, is there anything you can do to increase your metabolic rate? 

Unfortunately, the short answer is no — you can't significantly change the rate of your metabolism. 

You can use more energy by exercising and building lean muscle mass. Gaining lean muscle mass can increase how many calories you burn, even when resting. 

But you can't change the things that affect your BMR, like genetics or age. 

Food metabolism

Now that we’ve discussed metabolic rate, we’ll move on to food metabolism.

While your body can store energy to use later, the primary source of energy is food. However, your body metabolizes different nutrients in different ways.

Fats 

Per gram, fats contain more energy than carbohydrates or protein. However, the process of metabolizing fats is much slower, so fats stay in your bloodstream longer than sugars.

In fact, it can take 6–8 hours for your body to process the fat in your blood after a meal. Because it takes so long to get fat out of the bloodstream, levels can continuously rise through the day with each meal. 

Carbohydrates

On average, carbs provide just under half the amount of energy, per gram, as fat.

Your body digests the various types of carbohydrates differently, so each kind of carb impacts your metabolism differently. 

For example, simple sugars, like those in candy, are easier to digest. So, you burn fewer calories during digestion. 

On the other hand, complex carbohydrates, found in whole grains and vegetables, take longer to break down and require more energy to process. 

Fiber is another type of carbohydrate. It doesn’t directly provide energy, but it can help slow digestion, benefit blood sugar responses, and encourage a healthy gut microbiome. These effects all promote a healthy metabolism.

Protein

Protein offers the same amount of energy per gram as carbs. 

Based on research dating back to the 1990s, around 20–30% of the energy in protein is used during digestion, compared with 5–10% of the energy in carbs and 0–3% of the energy in fat. 

But it's important to remember that you don't metabolize macronutrients in isolation.

Some small studies suggest that a high protein intake could increase your energy expenditure, but it won’t impact your metabolic rate. Still, high-quality protein is an integral part of a healthy diet. 

Although we generally know how the body metabolizes different types of foods, your personal responses are unique. With the ZOE at-home test, we can analyze your blood sugar and blood fat responses, as well as your gut microbiome, to give you personalized food recommendations.

How can I keep a healthy metabolism? 

Although you can’t significantly change your BMR, there are ways to look after your overall health and help your metabolism work at its best. 

Some tips include:

  1. Making sure you have a healthy, balanced diet. Eating plenty of unprocessed plant foods, like vegetables, fruits, legumes, and nuts and seeds, as well as healthy fats, is great for your overall health.

  2. Nourishing your gut microbiome. ZOE scientists have found a link between the gut microbiome and how you respond to food. They found 15 “good” bugs that are linked with better metabolic control. Promoting these bugs may benefit your overall health.

  3. Cutting down on ultra-processed foods. Ultra-processed foods don't take a lot of energy to digest, so you burn fewer calories when you eat them. They also negatively impact your gut microbiome and are linked to a higher risk of weight gain and obesity.

  4. Getting a good night's sleep. ZOE research found that going to bed earlier helps prevent unhealthy blood sugar spikes after eating the following morning, which is good news for your metabolic health. 

Summary

Metabolism includes all the cellular processes in your body that keep you alive and functioning. 

Metabolic rate is the amount of energy your body uses to maintain these processes.

No single factor can determine your metabolic rate. Your body composition, age, sex, genetics, food intake, exercise levels, and sleep all affect it. And, as we know from ZOE research, every person's response to food is different. 

Eating a healthy, balanced diet, looking after your gut microbiome, getting a good night's sleep, and cutting down on ultra-processed foods can all benefit your metabolic and overall health. 

If you want to understand more about your personal metabolic control, ZOE can help. 

Our at-home testing kit analyzes your blood sugar and blood fat responses, as well as your gut microbiome. Using this information, your personalized ZOE program helps you identify the best way to eat for your body.

Sources

A high-protein total diet replacement increases energy expenditure and leads to negative fat balance in healthy, normal-weight adults. The American Journal of Clinical Nutrition. (2020). https://academic.oup.com/ajcn/article/113/2/476/5986961

Comparison of methods to assess energy expenditure and physical activity in people with spinal cord injury. Journal of Spinal Cord Medicine. (2012). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3240915/?report=reader

Daily energy expenditure through the human life course. Science. (2021). https://www.science.org/doi/10.1126/science.abe5017?rss=1

Diet and the menopause. (2016). https://www.bda.uk.com/resource/diet-and-the-menopause.html

Energy metabolism. (2022). https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Basics_of_General_Organic_and_Biological_Chemistry_(Ball_et_al.)/20%3A_Energy_Metabolism

Fat mass and obesity associated (FTO) gene and hepatic glucose and lipid metabolism. Nutrients. (2018). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6266206

Gender differences in metabolism. An Ecological Approach to Obesity and Eating Disorders. (n.d.). https://btugman.pressbooks.com/chapter/gender-differences-in-metabolism/

Measurement of energy expenditure. Public Health Nutrition. (2005). https://pubmed.ncbi.nlm.nih.gov/16277824/ 

Starchy foods and carbohydrates. (2020). https://www.nhs.uk/live-well/eat-well/food-types/starchy-foods-and-carbohydrates/ 

Temperature. Clinical Methods: The History, Physical and Laboratory Examinations. (1990). https://www.ncbi.nlm.nih.gov/books/NBK331/

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