Metabolism refers to all the life-sustaining chemical processes in your body, including converting the food that you eat into energy for movement and basic life functions like keeping your organs functioning. A number of factors can influence how effectively your body can convert that fuel from food into energy. That’s why two people eating the same amount of calories can see different outcomes with regard to their weight.
Energy in is pretty straight forward, it’s the calories that you consume from the carbohydrates, fat, and protein in your meals. These are the main sources of fuel that your body can break down and convert to ATP, which is the usable form of energy in your body.
How much we eat is influenced by appetite & hunger, which is impacted by a number of factors.
- Your levels of leptin – the hormone that promotes a feeling of fullness and satiety.
- Your level of grehlin – the hormone that makes you feel hungry.
- How well-balanced your meals are – a well-balanced meal that includes protein, fat, and fiber will keep you feeling satiated much longer than a meal or snack of mostly simple carbohydrates and sugar, which will cause your blood sugar to spike and then crash, making you feel hungry again.
- Physical activity – depending on the person, lots of physical activity can either increase or suppress your appetite.
One of three things happen after you consume energy from your food and drink:
- It’s transformed into ATP and utilized to keep your body functioning and give you the energy for movement.
- Unused energy is stored as fat or as glycogen, which is glucose that’s stored in the liver and muscles to fuel your muscles.
- Some energy is released as heat. That’s why chronically cold hands and feet can be a sign of a slower metabolism.
There are four main components of energy expenditure.
Resting Metabolic Rate (RMR): RMR accounts for 60-75% of calories used. It’s the number of calories burned when the body is completely at rest. RMR is the energy needed for breathing, blood circulation, organ function, and basic neurological functions.
Lots of lean body mass (i.e., muscle) will increase your RMR. High thyroid production is another factor that can increase RMR. That’s why people with hypothyroidism, meaning they are not producing enough thyroid hormone, often gain weight that is difficult to lose. RMR also declines with age, about 2-3% per decade due to a loss in lean body mass. That’s why strength training is so important to maintain a high metabolism as you get older. Also, very-low-calorie diets will temporarily decrease your RMR. You want a small calorie deficit when you’re trying to lose weight, but drastically reducing your calories can slow your metabolism because your body will go into starvation mode and try to conserve the energy from the calories you are eating.
Thermic Effect of Feeding (TEF): TEF makes up about 10% of calories used per day. It’s the energy used for digestion after consuming a meal. Carbohydrates and fat have a lower TEF compared to protein. Therefore, eating a higher protein meal utilizes more calories to digest it than a high-carbohydrate meal.
Thermic Effect of Activity (TEA): TEA accounts for 15-30% of calories used. TEA is all your movement during the day including walking, standing, structured exercise, cleaning the house, etc. It’s the type of energy expenditure that has the most variability day to day and that you have the most control over. You can slowly change your RMR over time, but not as much as you can change your TEA in a single day. However, while you want to be active, you don’t want to over-exercise because that is going to become a stressor on your body that spikes your stress hormones and can actually slow your RMR. Strength training is the form of activity that can have the biggest impact on your overall metabolism because the more lean body mass you have, the higher your RMR, which accounts for the greatest amount of energy expended per day.
Non-Exercise Activity Thermogenesis (NEAT): NEAT is a component of TEA. It’s the energy expended for everything we do that is not sleeping, eating, or structured exercising. It’s the energy we consume during daily activities like walking to your car, folding laundry, typing, fidgeting, etc. When you hear advice to do things like take the stairs instead of the elevator or use a standing desk, this refers to incorporating more NEAT into your day. You can only do so many structured workouts during the week, therefore doing more NEAT activities is an easy way to increase your energy expenditure without the risk of over-exercising.
Other Factors that Impact Metabolism
Sleep – Sufficient sleep is very important for weight loss. Inadequate sleep may lead to a lower RMR as well as an increase in the number of calories you consume. Studies have found a link between short sleep duration (generally less than 6 hours per night) and an increased risk for obesity. The link between not enough sleep and weight gain may be due to changes in appetite (i.e., decreased leptin or increased ghrelin), higher cortisol levels, decreased insulin sensitivity, and eating more simple carbohydrates (mostly sugar) and fat.
Stress – Stress produces cortisol that increases the amount of glucose in your blood while inhibiting insulin release. Insulin works to move sugar out of your blood and into your cells, but cortisol decreases the release of insulin so your body has enough glucose in your blood to quickly react or run away from a dangerous situation. Your body reacts to mental stress the same as if you were in a physical situation that you needed to run from, like standing in front of a bear. When insulin can’t move glucose out of the blood and into your cells, your cells become starved for energy, which signals feelings of hunger so you eat more to get your cells some fuel. Those excess calories that you eat may end up being stored as fat.
What Does a Healthy Metabolism Look Like
Now that you know the basic components that make up your metabolism, what does it look like to have a healthy well-functioning metabolism? Metabolism impacts more than just your weight. While it’s nice to be able to easily lose weight, a healthy metabolism also leads to:
- Being able to easily maintain a healthy weight
- Good digestion – no constipation, bloating, stomach pain, etc.
- Having steady energy throughout the day
- Healthy skin, hair, and nails
- Fewer PMS symptoms
- Ability to sleep through the night and get quality sleep
- Strong libido
- Strong immune system
- Mental clarity and focus
How Protein Boosts Your Metabolism
How Your Digestive System Works
Nutrition 101: What are Macronutrients?