Obesity - Causes 

Causes of obesity 

5 A DAY: natural history in the supermarket

Get tips on shopping for your fruit and vegetables. This video is part of the 5 A DAY campaign.

Media last reviewed: 11/01/2012

Next review due: 11/01/2014

Most cases of obesity are caused by eating too much and moving too little. If you consume high amounts of energy from your diet but do not burn off the energy through exercise and physical activity, the surplus energy is turned into fat.


The energy value of food is measured in units called calories.

The average physically active man needs 2,500 calories to maintain a healthy weight, and the average physically active woman needs 2,000. This may sound high, but it can be easy to reach this limit if you eat certain types of food.

For example, eating a large take away hamburger, fries and a milkshake can add up to 1,500 calories of your intake.

Another problem is that many people are not physically active, so lots of the calories they consume end up being "wasted" and converted into fat cells.

Lifestyle choices

Obesity does not just happen overnight – it develops gradually from poor diet and lifestyle choices.

For example, unhealthy food choices could include:

  • eating processed or fast food high in fat
  • not eating fruit, vegetables and unrefined carbohydrates, such as wholemeal bread and brown rice
  • drinking too much alcohol – alcohol contains a lot of calories, and heavy drinkers are often overweight
  • eating out a lot – you may have a starter or dessert in a restaurant, and the food can be higher in fat and sugar
  • eating larger portions than you need – you may be encouraged to eat too much if your friends or relatives are also eating large portions
  • comfort eating – if you feel depressed or have low self-esteem, you may comfort eat to make yourself feel better

Unhealthy eating habits tend to run in families, as you learn bad eating habits from your parents.

Childhood obesity can be a strong indicator of weight-related health problems in later life, showing that learned unhealthy lifestyle choices continue into adulthood.

Lack of physical activity

Lack of physical activity is another important factor related to obesity. Many people have jobs that involve sitting at a desk most of the day. They also rely on their cars rather than walking or cycling.

When people relax, they tend to watch TV, browse the internet or play computer games, and rarely take regular exercise.

If you are not active enough, you do not use the energy provided by the food you eat and the extra calories are stored as fat instead.

The Department of Health recommends adults do at least 150 minutes (2 hours and 30 minutes) of moderate-intensity aerobic activity such as cycling or fast walking every week.

However, if you are obese and trying to lose weight, you may need to do more exercise – in some cases, up to 300 minutes (five hours) may be recommended.

Read more about the physical activity guidelines for adults.


Some people claim there is no point in losing weight because "it runs in my family" or "it's in my genes".

While there are some rare genetic conditions that can cause obesity, such as Prader-Willi syndrome, there is no reason why most people cannot lose weight.

It may be true that certain genetic traits inherited from your parents – such as taking longer to burn up calories (having a slow metabolism) or having a large appetite – can make losing weight more difficult, but it certainly does not make it impossible.

Many cases where obesity runs in families may be due to environmental factors, such as poor eating habits learned during childhood.

Medical reasons

Medical conditions that can cause weight gain include:

  • Cushing's syndrome, a rare disorder that causes the over-production of steroid hormones
  • an underactive thyroid gland (hypothyroidism), where your thyroid gland does not produce enough thyroid hormone

Certain medicines, including some corticosteroids and antidepressants, can also contribute to weight gain. Weight gain can also be a side effect of taking the combined contraceptive pill and from quitting smoking.

Page last reviewed: 05/03/2012

Next review due: 05/03/2014


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The 4 comments posted are personal views. Any information they give has not been checked and may not be accurate.

Mike Ospab said on 10 January 2014

Hi, eating a healthy diet and engaging in exercise are two things that need not be a significant burden on the wallet. Fresh fruits and vegetables are cheap in supermarkets and on high street markets, and a diet rich in these ingredients goes a long way towards becoming a healthy one. Sourcing your calories from primarily carbohydrates (these are the human body's preferred source of energy, as these are metabolised or stored as glycogen - a stored form of carbohydrate - long before fats, proteins, and alcohols are; the body will seek to burn off carbohydrates before trying to turn them into fat, as this is the most efficient method available to the body, even if it's only to produce dietary thermogenesis, the warming of the outer body). As for exercise: attending a gym is not mandatory. Engaging in simply physical activities even like just walking a couple of miles a day, or doing fifteen minutes worth of star jumps in your home, will promote the oxidisation of stored body fat. Please understand that your health is your responsibility, and not that of the government. As already stated, healthy fresh food is cheap and exercise is free.

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jennyrobson85 said on 21 May 2013

even if i wanted to eat healthy and do exercise its so expensive that this isn't really an option for me. the nhs and the government expect you to lose weight but they could make it a lot easier by making healthy food, gym memberships etc cheaper.

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paul parmenter said on 26 January 2013

Shame that what is supposed to be s public service advertisement by a body that is funded by taxpayers, should stoop to cheap and totally gratuitous misandry - depicting the man as stupid and the woman as superior. Presumably the NHS has done so in order to fall in line with so many ads that do exactly the same. Or maybe the NHS just doesn't have the imagination to break out of that abysmal and insulting stereotyping.

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2305 said on 30 October 2012

You state that coc can cause wt gain but if the link for coc is followed it say there is no evidence that the pill causes wt gain!

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