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Causes of Weight Gain and Obesity

Dr Roger Henderson
Reviewed by Roger HendersonReviewed on 29.04.2024 | 7 minutes read

Obesity is a growing problem, currently affecting one in four adults in the UK. It can have serious implications for your health, including increased risk of breast and bowel cancer, Type 2 diabetes, heart disease, stroke, and disability from pressure on your bones and joints. Doctors have a scale to classify whether your weight is healthy, underweight or overweight, and it’s based on your body mass index (BMI), which takes into account your height and weight (kilograms per metres squared). 

You have a healthy BMI if it’s between 18.5 to 24, and underweight if less than 18.5. A BMI of 25 to 29 is termed overweight, 30 to 39 is obese, and over 40 is classed as very obese.

BMI gives an indication of obesity, but a diagnosis is made based on other factors such as waist circumference. Men with a waist size of 94 cm or women with a waist of more than 80cm put their health at risk. This article looks at the reasons why weight gain and obesity may occur, and gives you practical advice on how to help maintain to a healthy weight.

The health risks of obesity

Obesity is a complex medical condition characterised by an excessive accumulation of body fat. It is associated with numerous health risks and can significantly impact an individual's overall well-being. 

Some of the key health risks linked to weight gain and obesity include:

  • Cardiovascular Disease: Obesity increases the risk of heart disease and related conditions, including hypertension (high blood pressure), coronary artery disease, and stroke. Excess body fat can build up plaque in the arteries, narrowing them and restricting blood flow.

  • Type 2 Diabetes: Obesity is a major risk factor for developing type 2 diabetes. It can lead to insulin resistance, where the body's cells do not respond effectively to insulin, resulting in high blood sugar levels.

  • Respiratory Issues: Obesity is linked to respiratory problems, such as sleep apnoea, which can cause interrupted breathing during sleep. It can also lead to reduced lung function and increased risk of conditions like asthma.

  • Joint Problems: The excess weight stresses joints, increasing the risk of osteoarthritis and other musculoskeletal issues.

  • Cancer: Obesity is associated with an increased risk of certain cancers, including breast, colon, kidney, and endometrial cancer. 

  • Fatty Liver Disease: Non-alcoholic fatty liver disease (NAFLD) is more common in obese individuals and can progress to liver inflammation and cirrhosis. 

  • Mental Health: Obesity can have psychological effects, contributing to depression, anxiety, and low self-esteem. 

Genetic factors related to obesity

There is a great deal of research currently underway looking at whether weight gain and obesity may be caused by our genes, and over 400 different genes have been identified as possibly having some involvement in weight gain. However, only a tiny number of these appear to be significant and for most people diet and activity remains the key factor accounting for weight gain.

Genes may play some role in your obesity if:

  • You have been overweight all your life

  • You are unable to lose weight on a low-calorie diet

  • One or both parents are obese - if both parents are obese your likelihood of being obese yourself can be as high as 80%

Genes are probably a lower contributor for you being overweight if you have most or all of the following:

  • You are very influenced by food being available to you.

  • You are moderately overweight, but are able to lose weight relatively easily when you follow a healthy diet and regular exercise program.

  • You lose weight after improving your eating or exercise habits. 

You can assume that any genetic tendency towards obesity is very low if your weight is normal and doesn't increase even if you eat too much or rarely exercise.

Dietary factors

There are many dietary factors that contribute to the development of obesity, one of which is the availability of relatively cheap, highly processed food that is high in fat, sugar and calories. Others include:

  • drinking too much alcohol; alcohol contains a lot of ‘hidden’ calories that can be easily forgotten about

  • eating in restaurants where food may be high in fat and sugar

  • eating more than you need

  • drinking high-sugar drinks such as cola and fruit juice

  • comfort eating – eating when you’re not hungry due to other factors in your life such as low self-esteem or unhappiness

Lack of physical activity

Not doing enough physical activity is a very important factor in the development of obesity - many of us now sit at a desk all day or use transport to get to work rather than walking or cycling. In our leisure time we may watch television or play computer games rather than exercising or being active.

If you do not use up energy because of a sedentary lifestyle (not getting enough exercise), your body stores the excess calories as fat, and obesity may result.

In the UK it is recommended that adults do at least 150 minutes of moderately intense aerobic activity (where you are out of breath but can still have a conversation) such as cycling or fast walking. The good news is that this does not need to be done all at once! You can therefore do 30 minutes a day for 5 days a week, or any combination to fit round your diary. Unfortunately, two out of three adults in the UK do not do this amount of activity each week.

If you are obese and want to do more exercise, you should try to do a little more than this each week but start slowly and gradually build up your activity each week, along with eating more healthily.


Psychological factors

Weight gain and obesity can also occur due to psychological factors, such as emotional eating. Emotional eating is where we eat in response to negative emotions (such as stress, anxiety or low mood) even when we may not be hungry and studies have shown this can contribute to weight gain more than factors such as the price of food and its availability. It can be linked to both positive and negative emotions but binge-eating is specifically associated with negative ones - particularly depression.

Negative emotional eating is usually associated with consuming high-fat, high-sugar foods rather than healthier options, which in turn increases the likelihood of weight gain occurring. Some obesity specialists now also believe that the constant eating of high-fat foods can lead to a negative emotional state, which in turn leads to more comfort eating and so a vicious circle of unhealthy eating and weight gain develops.

Environmental factors

The environment we grow up in or live in as adults can also play a considerable role in the development of obesity. Obstacles to eating healthily are often greater for people with low incomes, who are poorly educated, or who have language barriers and healthy diet campaigns can often fail to reach this group. Socioeconomic factors also influence this such as poor access to healthcare and poverty.

Some specialists now talk about ‘obesogenic’ food environments where factors such as poor access to healthy food, aggressive marketing of unhealthy convenience foods and poor availability of areas for walking or cycling can all make it more difficult to maintain a healthy weight.

Medical factors

Some medical conditions can contribute to weight gain and obesity if not treated properly - examples of these include hypothyroidism (an underactive thyroid gland), polycystic ovary syndrome (PCOS) and Cushing’s syndrome. Some prescription drugs can also contribute to weight gain with some common examples of these being:

You may also find that you can put on weight after stopping smoking. This is usually due to substituting cigarettes for things like sweets or biscuits rather than from smoking preventing weight gain.


Unfortunately, there is no magic pill, single food group or radical smoothie to shortcut weight loss – it requires hard work and planning from you. Weight loss programmes can help to guide your choices and provide a supportive group to cheer you on.

Some areas have exercise on prescription available, where your doctor can refer you to the local gym or swimming pool for a number of sessions. If you have a high BMI you may be a candidate for a new medication called Wegovy that can help you lose weight - you can learn more about this here.

Weight loss surgery (bariatric surgery) may be offered under certain criteria – usually with a BMI of over 40 – and it requires a great deal of commitment before and after surgery.

Sometimes it can feel an uphill struggle to lose weight, and add to feelings of low self-esteem and confidence. If you are already feeling depressed, it can be hard to find the motivation to address your weight, and easy to comfort eat, which adds to the pounds. If your mood is low, see your doctor to discuss how you can best tackle this – set one goal at a time to put yourself in a position to succeed.

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Dr Roger Henderson
Reviewed by Roger Henderson
Reviewed on 29.04.2024
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