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Why do we gain weight? This complex question touches on various aspects of our daily lives and our genetic makeup. Weight gain is often the result of an imbalance between the calories in and calories out.
But that’s not the only reason! Other causes can also explain the shifts in our body mass, such as a poor diet, often rich in sugars and fats, a sedentary lifestyle, stress, lack of sleep, and even our genetics. Understanding the science of weight gain is essential for shifting towards a healthier and more balanced lifestyle.
Read our complete section on weight gain to find out all the causes and explanations, as well as getting important tips and advice for getting back into shape.
Want to lose weight? Get a health check first!
Why do we gain weight? Understanding the energy balance
At the heart of weight management lies the notion of energy balance, a key concept that governs the relationship between the calories we consume and those we burn…
Energy expenditure
Our bodies need energy to live and function. We use energy to :
- Engage in physical activity or sport, as well as any movement such as walking or climbing stairs.
- Maintain our vital functions (breathing, digestion, body temperature, cardiovascular and cerebral systems). This is known as the basal metabolic rate. It represents between 60 and 70% of total energy expenditure and varies from one individual to another depending on age, sex, weight, etc
To calculate your basal metabolic rate, you can access our Daily Calorie Needs Calculator.
And of course, to meet this energy expenditure, we need energy intake.
Energy intake
This energy intake comes solely from the food we eat, via macronutrients (proteins, lipids and carbohydrates). It is measured in calories. For example, 1g of protein provides 4 calories (the same for carbohydrates) and 1g of fat contains 9 calories.
This intake is not constant, and the body works 24 hours a day. Our body has therefore found a way to store calories during periods when there is no intake, and this reserve is called fat.
The science of calories: What happens when the energy balance is out of balance
An imbalance in energy balance can lead to a change in weight:
You lose weight when your calorie intake is lower than your energy expenditure.
You gain weight when your calorie intake is higher than your energy expenditure.
When our energy intake exceeds our body’s needs, this excess often results in an accumulation of body fat, leading to weight gain and potentially obesity. This condition increases the risk of developing various chronic diseases, such as cardiovascular disease, type 2 diabetes, hypertension and certain types of cancer.
On the other hand, an energy deficit, where energy intake is insufficient in relation to expenditure, can lead to excessive weight loss, malnutrition and reduced muscle mass. This imbalance can affect the immune system, reduce bone density and impair cognitive and mental functions.
So, as you can see, an imbalanced energy balance can have significant consequences for our health and well-being.
Weight gain is generally the result of taking in too many calories in relation to energy expenditure, but there are other factors that can explain it.
Causes of weight gain: What are the reasons behind weight gain?
Other factors that influence weight gain
The reasons for weight gain can be many and varied. Knowing yourself well enables you toidentify the factors behind weight gain and adapt your eating habits and physical activity to rebalance your energy balance.
Bad eating habits
Bad eating habits, such as overeating or eating badly, obviously lead to weight gain.
This is reflected in :
- Eating too much fat and sugar. For example, excessive consumption of ultra-processed foods (certain breakfast cereals, biscuits, chocolate sweets, frozen meals, sugary drinks, etc.) or fats (oil, butter).
- Eating outside of mealtimes, such as nibbling and excessive snacking
- Unstructured eating (skipping meals, swallowing meals quickly, eating at different times, etc.)
- Poor perception of hunger and satiety, leading to overeating.
A sedentary lifestyle
A sedentary lifestyle is a major factor in weight gain. Unfortunately, our changing lifestyles mean that we are increasingly sedentary, which encourages excess weight. And the less we move, the fewer calories we burn!
Age
As the years go by, weight gain seems inevitable. Weight increases progressively throughout life due to hormonal changes (pregnancy, menopause, andropause), lower basal metabolic rate and lack of activity.
Find out more about :
Why do we put on weight as we get older?
Why do you get fat at the menopause?
Quitting smoking
Giving up smoking can lead to weight gain. Because smoking affects the hunger hormone and intestinal flora, it’s not uncommon to gain weight when you stop smoking. What’s more, ex-smokers often tend to snack to cope with anxiety and compensate for the act of having a cigarette.
Find out more: Why do you put on weight when you stopsmoking?
Stress, anxiety and depression
Psychological or social factors can also play a role in weight gain. Triggering events such as unemployment, the death of a close relative, divorce, etc. can cause stress or even depression, leading to impulsive eating or poor nutrition.
Find out more: Why does stress make you fat?
Medications
Some medicines (antidepressants, cortisone, sleeping pills, the contraceptive pill, etc.) cause weight gain. To prevent the side effects of these drugs, the best solution is to watch what you eat and make sure you eat a balanced diet.
Find out more: Medication and weight gain
Genetics
We are not all equal when it comes to weight gain. According to Inserm, a genetic predisposition can influence our body mass index. In fact, a person is 2 to 8 times more likely to become obese if family members are obese themselves, with this figure varying according to gene mutation. Being overweight can therefore be hereditary.
When you stop exercising
Stopping sport reduces the basal metabolic rate and consequently energy expenditure. If no dietary adjustments are made, i.e. if the ex-sportsman continues to eat as before, his energy balance will tip the wrong way. Their body will store fat.
Working late night shifts
Working nights or shifts often leads to poor eating habits and weight gain. It’s difficult to eat at regular, healthy times. Meals are usually eaten on the run, leading to unbalanced eating and snacking.
Sleep deprivation
According to an American study, sleep-deprived people eat the highest-calorie foods. In a sleep-deprived person, the study revealed that the stomach secreted an abundance of ghrelin (the hormone that stimulates the sensation of hunger) and reduced the secretion of leptin (the satiety hormone). This hormonal imbalance leads to a constant desire to eat sweet or fatty foods and a tendency to bulimia.
Find out more: Why do we put on weight when we’re tired?
Unexplained weight gain
Do you pay attention to what you eat, do you exercise, and yet you continue to put on weight? If so, it could be due to a hormonal deficiency linked to your thyroid, medication or emotional kilos. Unexplained weight gain should not be taken lightly. It’s best to consult your GP.
Find out more: Why do people put on weight for no apparent reason?
