Being overweight or obese is more likely a result of interactions among genetics, diet, physical activity and other factors rather than a slow metabolism.
Metabolism is the process by which your body converts what you eat and drink into energy for life. When you’re at rest your body still needs energy for breathing, circulating blood and repairing cells. The energy used when your body is at rest is your basal metabolic rate (BMR).
The body burns calories:
Through the energy necessary to keep the body functioning at rest – the BMR. This is partly determined by inherited genes
Through daily activities
Your BMR is influenced by:
Body size and composition. If you weigh more or have more muscle mass, you’ll burn more calories even at rest. People that are heavier are more likely to have a faster BMR rather than a slower one.
Gender. Men tend to have less body fat and more muscle mass than women of the same age and weight. Having a higher muscle mass means burning more calories.
Age. Muscle mass decreases with age, slowing down the rate at which you burn calories.
The body is like a car engine that is always running. When at rest, the car is idling. A small amount of energy is being burned to keep the engine running. The fuel source for humans are the calories found in foods and beverages consumed — energy that may be used right away or stored (as glycogen or fat) for use later.
How fast the “engine” runs on average, over time, determines how many calories are burnt. If the metabolism is fast, the body will burn more calories at rest and during activity, the person will need to ingest more calories to maintain weight. This is one reason why some people can eat more than others without gaining weight. A person with a slow metabolism burns fewer calories at rest and during activity and will therefore have to consume less to avoid becoming overweight.
However, metabolism isn’t purely the key to weight. The obesity epidemic cannot solely be blamed on an inherited tendency to have a slow metabolism. Environment plays a significant role, such as changes in diet and lack of exercise.
Changes in weight over a lifetime are determined by calories in and calories out. Regardless of whether metabolism is fast or slow, the body stores excess energy in fat cells. If a person drinks and eats more calories (energy "input") than the body is using through daily activities, like exercise, rest and sleep (energy "output"), they will gain weight and vice versa. The body also treats a lack of food as starvation, this slows down the BMR and fewer calories burned over time. This is a reason why losing weight is often difficult.
However, all calories are not created equal. 100 calories of fat, protein and carbohydrates are the same in a thermodynamic sense, they release the same amount of energy when exposed to heat, but these foods affect the body in very different ways. They influence satiety, metabolic rate, brain activity, blood sugar and the hormones that store fat.
Calories from different foods are not absorbed the same. If a person eats high-fiber foods, such as nuts and certain vegetables, around three-quarters of the calories they contain are absorbed, the rest is excreted from the body unused. Therefore, the calories listed on labels are not what the body is actually receiving.
Sugary, refined and processed foods that spike blood sugar levels are associated with weight gain as they are rapidly digested and easy to consume in large quantities, not merely because they bypass the energy balance.
When it comes to weight, metabolism is important and does have a genetic component. It is still uncertain whether the metabolic rate can be changed. Changing how one balances the calories taken in against calories burned up through activity can be controlled to some extent. However, calories taken in through healthier food options are more beneficial to the body and weight.
These factors are more likely to contribute to weight gain than a slow metabolism:
Eating too many calories
Lack of physical activity
Genetics and family history
Unhealthy habits, such as insufficient sleep or stress