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Weight gain is occasionally simply due to eating more, or being less active, but usually it is more complicated than that. There are many possible causes of weight gain, and often a number of factors play a part. These may be social, psychological or medical. Maintaining weight within a normal range helps your overall health.

Most people reach a stable weight early in their adult life but over time, various things can cause you to lose or gain weight. There are several possible reasons why you might gain a significant amount of weight. Most simply, it might be due to taking in more calories through food and drink than your body needs, or not expending enough energy to use it up. However, other factors can also cause weight gain, including eating the wrong types of food, certain medical conditions or medications, genetics, stress, lack of sleep, and age. In certain circumstances weight gain is appropriate and needed. All these possible factors are discussed below, along with the more recent theory that there might be some relation to the normal germs that live in our guts.

Eating too much

Different people have different calorie requirements to keep their weight at the same level. A 20-year-old male athlete, for example, will need to eat more than a housebound elderly lady. Men and women have different calorie requirements but it also varies by age, activity level and other differences between individuals. However, for all individuals, regularly eating more calories than your body uses each day will, over time, cause you to gain weight. Broadly speaking, people need between 2,000 and 2,800 kilocalories (kcal) per day to be moderately active and maintain their weight at a stable level. Both food and drink (see below) contribute to your daily calorie intake.

Eating the wrong things

What you eat may be almost as important as how much you eat. A large plate of steamed broccoli does not provide the same number of calories as a large plate of cake! Experts are divided about whether fat or sugar is the biggest cause of weight gain, but the sensible option is a balanced diet. 'All things in moderation' is probably the best motto. See the separate leaflet called Healthy Eating.

However, even if your diet is entirely healthy, it is likely that if you eat more calories than your body is using up, you will gain weight. So keep portion sizes sensible.

There is much which is not yet understood about weight and weight gain. It can affect different people differently - possibly due to differences in metabolism or inherited factors. And there are certainly differences of expert opinion about exactly what constitutes a healthy diet. To a certain extent, however, most of us discover early on what causes us individually to put on weight.

Alcohol and other drinks

Drinking alcohol regularly adds significantly to your calorie intake. For example, there are approximately:

  • 159 kcal in a medium 175 ml glass of 13% wine.
  • 182 kcal in a pint of 4% lager or beer.
  • 244 kcal in a pint of 5% lager or beer.
  • 216 kcal in a pint of 4.5% cider.
  • 89 kcal in a flute (125 ml) of champagne.
  • 50-60 kcal in a single unit of a 40% spirit (plus any calories in the mixer).

So alcohol can be a cause of weight gain, and reducing or stopping what you drink may be one way of stopping weight gain in its tracks. However, alcohol is not the only drink that can contribute to your daily calorie intake. Soft drinks and fruit juices can also contain calories you didn't realise you were taking in. For example, there are approximately:

  • 139 kcal in a 330 ml can of Coke®.
  • 135 kcal in a 330 ml can of Pepsi®.
  • 55-80 kcal in a 150 ml glass of orange juice.
  • 134 kcal in a 250 ml Innocent® strawberry and banana smoothie.

Not exercising enough

Sedentary lifestyles are thought to be a contributing factor to the increasing problem of obesity in the developed world. Regular exercise has many health benefits and is a factor in maintaining weight. Public Health England advises 150 minutes of physical activity each week, in bouts of 10 minutes or more. This helps to protect you from gaining excessive weight. For more information, see the separate leaflet Physical Activity for Health.

Medical conditions causing weight gain

Some medical conditions make it more likely that you will gain weight. These include:

Normal weight can be maintained in most medical conditions, by treating the condition and/or adapting what you eat accordingly.

Side-effect of a medication

Some medicines may cause weight gain as a side-effect. If you think this is the case for you, consult your GP. In some cases it may be possible to swap to another medicine. If not, you can still control your weight by adapting what you eat. Some medicines which can cause weight gain include:

Although weight gain is sometimes thought to be a side-effect of the combined oral contraceptive (COC) pill and hormone replacement therapy (HRT), there is no evidence for this effect.


The number of calories your body needs drops as you get older - this may be partly due to a drop in level of activity and amount of muscle. So if you keep eating the same at 50 as you did at 20, the dreaded 'middle-aged spread' may occur. Weight distribution often changes at menopause in women, due to hormonal changes, causing more fat to settle around the tummy area.


Being overweight tends to run in families. Some of this may be due to a shared lifestyle, but inherited genes do have a part in weight gain. Nevertheless, a person who is genetically predisposed to being overweight can still control this through their lifestyle choices.

Lack of sleep

There is some evidence that tiredness and not getting enough sleep can result in weight gain. The time spent (or not) sleeping seems to affect hormones such as leptin and ghrelin which go on to have an effect on appetite.

Stress and lifestyle factors

Lifestyle factors can have a bearing on weight. For example, boredom or worries can lead to 'comfort eating'. We eat for many reasons other than hunger. Stress can cause some people to gain weight, although it may cause others to lose it.

Certain jobs which involve eating and drinking (bar work, corporate hospitality) might have an effect, as might changing from a job which involves walking around all day to one which involves sitting in an office chair all day. Social lives may involve eating and drinking, and if you associate with people who eat a lot, or eat out a lot, or drink a lot of alcohol, you may find yourself drawn into the same habits. Sometimes people who are giving up smoking put on weight as they replace their cigarette with a snack and consequently eat more.

Appropriate weight gain

At certain times, weight gain can be needed by your body, or can be appropriate. For example:

Recent research

Some recent studies suggest that weight gain may be related to the germs in our guts. We all have many millions of germs (microbes - bacteria, fungi, protozoa, viruses, etc) which live inside our guts all the time. It seems that the balance of these can affect how much food we absorb, and therefore possibly the amount of weight we put on in relation to the calories we eat. It may be that the balance can be changed by diet, by use of probiotics and prebiotics, or by medical means, which may in future possibly be used to help people lose weight. It is early days for this area of research, however, so there are more questions than answers so far.

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Up to a certain point, it doesn't. However if you gain too much weight and your BMI goes above 30, it can cause damage to your health. You can read about the ways in which your health can be damaged in the separate leaflet called Obesity and Overweight.

If you are still within a healthy weight range (if your BMI is between 18.5 and 25) then you don't need to do anything other than keep an eye on your weight. Within the healthy weight range, a lot of worry about weight and weight gain is fuelled by the media, social media, and (often photo-shopped) pictures of people with 'perfect bodies'. Much of this isn't necessarily realistic or healthy and can be very damaging to your physical and mental health. Being healthy is far more important to your happiness than having that perfect flat stomach - and people often don't realise that until they are not in good health, or someone close to them is not in good health.

However, if your weight gain is taking you out of a healthy weight range then you do need to try to take action in the interests of maintaining good health and being well. Read the separate leaflet called Weight Loss (Weight Reduction) for information about how to address weight gain in a healthy and sensible way.

Obesity and Weight Loss

Childhood Obesity

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