For decades scientists have suspected that obese people might have a “slow metabolism,” burning fewer calories than normal-weight individuals.
However, this hypothesis is controversial. A new review was published recently looking at the available evidence.
This is what they discovered.
This was a scientific review on the association of obesity with changes in calorie expenditure.
What is Calorie Expenditure?
Calorie expenditure is the rate at which you burn calories. Along with oxygen, calories are the fuel that keeps your metabolism running.
Calorie expenditure is divided into the following categories:
- Basal energy expenditure (BEE): The amount of calories used to maintain vital body functions.
- Resting energy expenditure (REE): The amount of calories burned by the body during rest. REE accounts for 50–75% of total energy expenditure (1).
- Activity energy expenditure (AEE): Physical activity increases the amount of calories burned.
- Thermic effect of food (TEF): Eating causes a temporary rise in the amount of calories burned, known as diet-induced thermogenesis.
Bottom Line: Calorie expenditure is divided into several subcategories, such as resting and activity energy expenditure. Together, they comprise total calorie expenditure.
Can Low Calorie Expenditure Cause Obesity?
Weight gain is caused by an imbalance of calories eaten and burned (2).
Calorie intake and expenditure depend, to a large extent, on your behavior.
- Calorie intake: You can reduce calorie intake by eating less and choosing foods that are low in calories. However, for many people, this is difficult because of a lack of knowledge, low motivation, eating disorders or food addiction.
- Calorie expenditure: You can burn more calories by using your muscles, exercising, gaining muscle and eating protein. For many people, this is difficult due to lack of motivation, a desk job or a physical disability.
Changing either calorie intake or expenditure can shift the balance, leading to either weight gain or loss over time (3).
However, calorie expenditure is affected by various factors and not completely adjustable. Most notably, it is determined by age, gender and genetics.
Bottom Line: Along with calorie intake, calorie expenditure strongly affects your body’s energy balance. Relative to calorie intake, low calorie expenditure leads to weight gain over time.
Do Obese Individuals Burn Fewer Calories?
Obese individuals have traditionally been thought to have a “slow metabolism” (6).
Specifically, the difference was, on average, 360 kcal/day. The difference ranged from 49 kcal/day to 826 kcal/day, depending on the level of obesity, and was highest when severely obese individuals were compared to normal-weight people.
This may be because obese individuals tend to have higher amounts of metabolically active fat-free mass — mainly muscles. When people gain large amounts of fat, they also tend to gain muscle mass to support the additional weight (8, 11, 12).
In contrast, a few studies suggest that the rise in calorie expenditure after eating (thermic effect of food) may be lower in obese people, but the evidence is mixed and inconclusive (14, 15, 16, 17).
Overall, obese people may burn more calories than normal-weight people, suggesting that slow metabolism is not to blame.
Nevertheless, some individuals may burn fewer calories while dieting or fasting, making it harder for them to lose weight (18).
Bottom Line: Obese individuals tend to burn more calories at rest, compared to normal-weight individuals. Most studies suggest that differences in fat-free mass may explain these findings.
Summary and Real-Life Application
In short, this review showed that obese individuals burn more calories at rest, compared to normal-weight people.
This is partly because they have more muscle mass to support the excess weight.
Taken together, these findings suggest that a slow metabolism is generally not an underlying cause of obesity, at least not in the average person.