There are several causes of obesity but the main reason why you become obese boils down to one principal factor. And that factor is energy (calorie) intake greatly exceeds energy (calories) spent or calories burnt.

In other words, there is an energy imbalance, which results in weight gain. Over time, this consistent lack of energy balance results in obesity.

Obesity is classified as a disease and has attained epidemic proportions all over the world.

This has happened due to wrong eating habits, reduced physical activity, and an increase in medical conditions for which obesity-causing medications are taken.

Though more and more people are becoming overweight and obese, it is a paradox that nutritional deficiency is seen on a large scale.

Eating foods and beverages that store you with empty calories makes you fat but does not give any nutritional benefits.

Being overweight with a BMI of 25 to 30 kg/m² exposes you to almost the same health risks as being obese with a BMI of 30 plus.

If you eat food that provides more calories to the body than it requires, then these extra calories are converted into fat and stored as such in the adipose tissue.

As a result of this extra fat that is stored, the fat cells increase in size. When they can no longer increase in size, the fat cells start increasing in number. When you start losing weight, the fat cells start losing their size but not their number.

There are various causes of obesity (adiposity). These causes and the risk factors can be grouped under three heads as shown below.

  1. Medical causes
  2. Genetic causes
  3. Lifestyle causes

Medical and health causes of obesity

There could be some health disorders or certain medicines given for the treatment of certain ailments, which cause obesity on prolonged administration.

If you are associated with any of these conditions or are on the medication mentioned below and are gaining weight, you must consult your doctor.

1. Hypothyroidism

Hypothyroidism (underactive thyroid) results in a lack of thyroid hormones in the body. These are the hormones, which are responsible for metabolism in the body. Lack of thyroid hormones slows down the metabolism of calories and sugar resulting in fat deposits and weight gain/obesity.

2. Cushing’s syndrome

This is a condition in which the adrenal glands secrete an excess of their hormone called cortisol. Excess cortisol stimulates the liver to produce more glucose. This excess glucose, which does not get metabolized fully is deposited as fat, especially around the midriff.

People who develop Cushing’s syndrome have fat deposits on the face giving it a round appearance and fat around the neck and abdomen. Their arms and legs, however, remain thin.

3. Polycystic Ovarian Syndrome (PCOS)

PCOS is an endocrinal disorder affecting about 5% to 10% of women of childbearing age and in which small cysts develop on the ovaries.

In PCOS, there is a hormonal imbalance, which results in insulin resistance.

As a result, the body does not utilize insulin properly, which results in a rise in blood sugar levels. This excess blood sugar gets deposited as fat and over time causes obesity.

4. Psychological conditions

The eating habits of some people are greatly influenced by emotions. Factors such as boredom, stress, and sadness make them turn to eating for relief and as a way to pass time.

Depression and low self-esteem due to the social impact of obesity make a person look for comfort in some activity and he or she finds this comfort in eating. This makes a person spend more time eating, resulting in consuming more food.

5. Medicines that cause obesity

Certain medicines increase the fat deposits in the body on prolonged administration and cause weight gain. They are:-

  • Antidepressants used in the treatment of depression such as amitriptyline, paroxetine, and sertraline
  • Antipsychotic drugs such as haloperidol, clozapine, and lithium
  • Anticonvulsants used to treat seizures such as Tegretol, Equetro, Carbatrol
  • Corticosteroids such as prednisolone
  • Medicines used in the treatment of diabetes type 1 such as insulin, and those used to treat diabetes type 2 such as sulfonylureas, thiazolidinediones
  • Oral contraceptives
  • Certain medicines used to treat high blood pressure such as beta blockers and angiotensin-receptor blockers
  • Antihistamines

These medicines cause weight gain and obesity by increasing your appetite, slowing down the rate at which the body burns calories, and causing water retention in the body.

Genetic causes of obesity

Many people have a genetic predisposition that can lead to obesity. However, it is possible to become obese even if you don’t have a genetic predisposition.

The human obesity genes involved are genes that interact with the environmental factors associated with calorie intake and calorie expenditure and which increase the obesity risk.

Obese parents, very often, are an obesity cause.  Being overweight and obese tends to run in families.

If one or both parents are obese, the chances of their children being obese are three times higher than children whose parents are not obese.

These genetic causes of obesity are due to the obesity-promoting environment in the household, and also due to the fact that families do share food habits and physical activity habits, which are passed onto the children.

1. Obesity Genes

Studies have found that 80% of children of two obese parents were obese as compared to only 10% of obese children of parents of normal weight.

High-calorie foods and physical inactivity may be the custom of the households where the children are also exposed to those foods and a sedentary lifestyle.

The obesity gene also determines what type of fat build-up will be there in the children – whether it will be abdominal fat or subcutaneous fat (cellulite).

However, there are ways to prevent obesity and stay at a healthy weight. Switching over to a healthy lifestyle, eating healthy foods, and encouraging physical activity by the family reduces the chances of childhood obesity in children.

Genes also determine your appetite and fat-storing tendency. Obesity genes can make you a voracious eater and can also make your body store more fat than somebody else.

2. Leptin deficiency and obesity

Certain hormones involved in fat regulation are also controlled by our genes. Leptin is a hormone present in the fat cells, which controls our appetite, curbing it when the fat stores are high.

Genetic variations can cause leptin deficiency leading to loss of control on the appetite, which causes you to overeat and become obese. Leptin and obesity, therefore, are inversely proportionate to each other – meaning if leptin decreases, appetite and therefore, obesity increases.

However, when the genetic cause of obesity is not in effect, obesity is linked to high levels of leptin.

Leptin is produced in the fat cells. Due to more and bigger fat cells in obese people, more leptin is produced. And the reason for overeating in spite of high leptin levels is believed to be due to the development of leptin resistance in obese people.

3. Ethnicity

Certain ethnic groups are more prone to obesity due to their genes. For example, obesity is more common among Hispanic women and non-Hispanic black women than among non-Hispanic white women Asians. The term “Hispanic” here refers to Mexican Americans.

Life Style causes of obesity

1. Dietary causes 

  • Consuming more calories in food (foods high in fats) causes weight gain, which over time results in obesity.
  • Obesity and overeating are very strongly connected and almost always coexist. This is an important cause, which is often identified by the evergreen scene of an obese person frequently chewing on food.
  • Portions of food served at eating places are so big that a person consumes at one time what otherwise can be sufficient for the day.
  • One of the important reasons for becoming obese is eating fast food or processed foods, which are high in trans fats.
  • Not eating high-fiber foods such as whole grains, fruits, and vegetables.
  • A diet high in carbohydrates especially simple carbohydrates (bad carbs) causes weight gain. Simple carbohydrates are found in sugars, fructose, soft drinks, beer, wine, etc. These are rapidly absorbed into the blood and cause a spike in blood sugar levels. The excess blood sugar is converted into fat and stored as such. Complex carbohydrates, the good carbs, on the other hand, are gradually absorbed and the sugar from these foods is absorbed gradually and used as energy. Complex carbohydrates are found in bananas, beans, chickpeas, lentils, nuts, oats, sweet corn, whole grain cereals, and yams.

2. Alcohol and obesity

Alcohol causes obesity especially when consumed daily and in large amounts because alcohol contains empty calories, which get stored as fat. Secondly, people do eat a lot of fried snacks with alcohol, which again becomes a contributing factor.

3. Lack of exercise and obesity

Lack of exercise and obesity are very strongly linked together. Those of us with a desk job spend most of our time sitting.

We travel in cars and at home, we watch television or read. There is no physical activity.

As a result, calories consumed do not get utilized and get stored as fat. Over time you gain weight and ultimately you become obese.

A normal adult must exercise or walk briskly for 30 minutes every day while a young adult could increase this to 45 to 60 minutes.

Television has been responsible for the lack of that evening physical activity and children and adults have likewise been affected. Computer activity has increased immensely and this has reduced physical activity for most people, especially the young.

4. Lack of sleep

According to a recent study, sleeping less, besides other adverse effects, does cause weight gain. This study indicates that people who slept for 5 hours every night were 73% more likely to become obese while people who slept for 6 hours every night were 27% more likely to become obese as compared to those who slept 7 to 9 hours daily.

The explanation for this sleep and obesity link is yet to be found, but what is said is that lack of sleep could trigger an increased intake of food, which gets stored as body fat.

Secondly, it is during sleep that certain hormones, which control appetite and use of energy, are released.

An example would be insulin, which controls blood sugar. Lack of sleep causes less insulin secretion and therefore, increased blood sugar.

This excess of sugar gets deposited as fat in the body. Another example would be the hormone leptin, which controls your appetite.

A lack of leptin does not curb your appetite when your fat stores are high resulting in excess food intake.

5. Stopping smoking causes weight gain

Some people tend to gain weight after they stop smoking. This is because the food starts to taste better due to the better functioning of the taste buds and sense of smell.

Another reason cited is that nicotine in tobacco increases the metabolic rate so that more calories are burnt as compared to when you stop smoking. However, this should not be construed as a plus point for smoking as the effects of smoking are very dangerous to health.

6. Age and obesity

As you age, you lose muscle mass, which can slow down the burning of calories. If you do not control your calorie intake, extra calories that do not get burnt are stored as fat.

7. Menopause

In women, menopause is an additional cause of weight gain and obesity. There are two factors responsible for this.

One is the advanced age, which is about 50 years. Your metabolic rate is not what it used to be and fewer calories are burnt due to less physical activity, which is common at this age.

Gradually, with age, the rate at which you tend to use energy declines, and your calories build up. Your metabolism rate slows down because you lose muscle mass.

Secondly, due to the menopausal-related reduced estrogen, your metabolism rate is lowered. Therefore, the conversion of stored energy into physical-work energy is hampered.

Lack of estrogen also reduces the use of starches and blood sugar by the body. This causes increased fat storage.