Metabolism: explained

Updated: Jul 11, 2019

All of life is a chemical reaction.

Metabolism refers to the sequence of chemical reactions that happen in your body that keep you alive. These reactions rely on nutrients that are broken down to produce energy. This energy is in turn used to make new proteins and DNA. Your body’s main energy sources are carbohydrates, fats, and proteins; it prefers to burn carbohydrates, then fat, and finally protein if all else fails.


When you eat carbs your body metabolises it into its simpler form, glucose. Glucose is used as an initial energy supply, stored as glycogen for future energy and any excess is stored as fat. Glycogen stores are limited, the excess is stored as fat. This is why you get fat if you eat more carbs than your body needs for energy/storage.

Carbohydrates can be simple (“junk food”) or complex (wholegrains). Simple carbohydrates are rapidly absorbed by the gut and enter the bloodstream very quickly. So if you need a quick boost of energy: eat chocolate. The problem is that since simple carbs enter the bloodstream so fast they are metabolised quickly. This causes you to rapidly lose that energy boost, which is why you often feel lethargic after eating junk food. On the other hand, complex carbs are degraded by the gut at a much slower rate, and therefore slowly enter the bloodstream. This provides a more sustained, but less obvious energy boost.

Carbs are used first for energy, so individuals will often eat high carb foods before a workout - “carbo-loading”. During exercise, the body will use the sugar molecules in the carbohydrates to provide energy for your muscles and brain. Once you run out of sugar (or the form it’s stored in, glycogen) your body turns to the other fuel sources, fat and protein.


When you eat fat, your body metabolises it into fatty acids/glycerol for energy or it is stored in the form of triglycerides for future use. Triglyceride storage is essentially body fat. If there are insufficient carbs available, your body will burn fat stores to provide energy that your muscles and other body tissues use.

Fat isn’t burned first as it doesn’t provide energy as efficiently as sugar. This explains why athletes hit “the wall” a few hours into a workout. The wall is the point where all carbs have been burned up, and they are now running on fat reserves. Fat doesn’t provide as much energy as carbs which results in the feeling of fatigue. People have difficulty losing weight when they exercise vigorously for only half an hour. The fast vigorous exercise burns mostly carbohydrate stores (glycogen), the body doesn’t touch its fat reserves.

At night your body has been burning carbohydrates to keep all your organs functioning, so in the morning your body has less carbs available to burn. Exercising early in the morning before having breakfast, causes your body to tap into its fat stores earlier than it normally would.


Protein is broken down into amino acids which are building blocks for muscles, hormones and enzymes. Your body rarely burns protein as its sole fuel source, and when it does it is usually when you’re starving. When no carbohydrates are present in the diet, the body can convert amino acids into glucose to supply the brain with adequate energy. If insufficient protein is provided by the diet, the body starts chewing on muscle cells.

Speed of metabolism

Body size, age, gender and genes all play a role in determining your metabolic rate. Muscle cells need more energy to maintain than fat cells, so people with a higher muscle to fat ratio tend to have a faster metabolism. As we age, we tend to gain fat and lose muscle so metabolism decreases with age. Typically, men have a faster metabolism as they have more muscle mass, heavier bones and less body fat than women, which is why their daily calorie allowance is higher. Your metabolism may be partly determined by your genes. Genes play a role in muscle size and your ability to grow muscles, both of which affect your metabolism.

People who struggle to lose weight often blame a slow metabolism. However, the opposite may be true, overweight people may actually have a higher metabolism than their leaner counterparts, reflecting the energy requirements of maintaining a larger body size. More often than not, the reason you’re putting on weight is not because of a slow metabolism, it’s because you’re consuming more calories than you're burning.

Foods that may boost your metabolism

Lean meat, green tea, spinach, oatmeal, peppers, eggs, avocados, apple cider vinegar, and full-fat yogurt.

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BANT Registered Nutritionist,  

Registered Nutritional Therapist CNHC & Health Coaches Academy Certified Health Coach

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© 2016 by Michelle Boehm