1. The main suspects

Many causes of obesity have been suggested. The most popular is eating too much and moving too little.

Other suggested causes include our genes, some medical conditions (such as an underactive thyroid gland), some medications (including anti- depressants), emotional factors, age, alcohol and lack of sleep.

2. The test of time

A useful check for any suggested cause of obesity is the test of time. What has changed significantly since the early 1980’s, when obesity rates first began their rapid rise in the UK, which might potentially explain that rise? For example, human genes are unlikely to have changed significantly since the 1980’s. There may be some people with a pre-existing genetic predisposition to excess weight when food is plentifully available – but if so, it is the unprecedented availability of calories which is the trigger.

• Levels of unhappiness, stress and depression among adults don’t seem to have risen significantly (despite media suggestions to the contrary). For example suicide rates have fallen in the UK for men and remained stable for women. 
• Medical conditions such as an underactive thyroid have been associated with obesity. However, these conditions are unlikely to have become more common - and research suggests that it is the obesity which triggers thyroid problems rather than the other way round.  
• Overall levels of physical activity have remained about the same. Work may now, on average, be more sedentary. However there has been an increase in leisure time physical activity – as seen in increased gym membership and the increased participation in marathons and fun runs.
• Some research has suggested that lack of sleep is a factor and an analysis of published research found poorer quality of sleep associated with overweight and obesity. However, a cause and effect relationship between poor quality of sleep and obesity hasn't yet been established.
• There has been a significant increase in anti-depressant prescriptions (as opposed to diagnoses of depression) – and anti-depressant use is associated with an increase in weight gain. However, there isn't yet a proven cause and effect relationship and the most rapid increase in prescriptions occurred when levels of obesity were levelling off, suggesting this may be a factor for some individuals but is not a significant factor overall. 

3. The test of time – is alcohol a possible factor?

In the UK alcohol consumption rose during the 1980’s and 1990’s but has since reduced over the last ten years or so. This roughly correlates with obesity levels which rose during the 80’s and 90’s but have since begun to level off (but not fall).

Alcoholics are rarely overweight, as they tend to neglect eating and the alcohol affects the way their liver digests food. However, regular alcohol consumption alongside food tends to add ‘hidden calories’ so is a possible contributory factor.

4. What happens to us early in life

The way in which we experience the early months and years of life has also changed. For example, there are fewer natural births, lower rates of breastfeeding and, as obesity rates have risen, more babies being born to obese parents. Research has shown that early life exposures, particularly in the first 1000 days (from conception up to 2 years of age), are a critical period that can significantly influence a child’s weight and future risk of disease. Research also indicates that pregnant women who are obese face increased health risks themselves, as well as increasing the risk of their child becoming obese. 

5. The test of time – a significant change in what we eat and drink?

What we eat and drink, when, where and how have all seen significant changes since the early 1980’s, so could be a potential cause.

Examples include significant increases in:

-      The availability and consumption of sugar sweetened drinks and snacks (which are often consumed in addition to the main meals of the day)

-      Ready meals (following the introduction of the microwave in 1984)

-      Takeaways and meals in fast food outlets (now a £30 billion a year market in the UK). 

What these trends have in common is a rise in food which is calorie rich but nutritionally poor (and often relatively cheap and widely available) - potentially leading to people becoming overweight but undernourished.

6. What research suggests

Research is increasingly confirming that, although exercise has many health benefits and has a part to play in controlling weight, it is what people eat and how much which is primarily causing obesity – with factors in the early months and years of life also important.

The key points here are that:  

Food high in sugar, salt, fat and refined carbohydrates is particularly likely to lead to weight gain.

This appears to be because:

• It is calorie rich (so you consume more calories for a given volume of food)
• It doesn't make you feel full for long (increasing the risk of overeating)
• Where sugar is concerned this may be ‘hidden’ (so people don’t realise how much they are consuming)
• It usually doesn't contain much fibre (which could help control weight) and so reduces the diversity of our gut microbiota (which is now being suggested as an another factor influencing weight).
• Where such food is relatively cheap and readily available this is likely to be a further factor leading to overconsumption.
• Salt contributes to weight gain in a different way. It increases thirst, leading to increased fluid consumption – and where the fluid is sugar sweetened beverages this contributes to weight gain as described earlier.

Conversely vegetables, fruit and whole grains are particularly likely to help control weight.

This appears to be because they are pretty much opposite in the way they work i.e. they

• Are not so calorie dense
• Make you feel fuller longer
• Don’t containing hidden ingredients likely to increase weight
• Contain fibre and encourage more diverse gut microbiota.

As regards the early months and years of life a range of interventions were proposed by Professor Mary Rudolf, in order to help tackle obesity, based on her review of the research - including the value of breastfeeding, of water instead of sugary drinks, of increasing the acceptance of healthy foods such as fruit and vegetables, of reducing the consumption of energy dense foods and finding alternatives to food for comfort and to encourage good behaviour. These again focus on the importance of food but shift attention to the earliest months of life, when food tastes, habits and preferences are being established. 

Some researchers would start the process even earlier. They argue that what mothers eat when they are pregnant and how much and whether or not they themselves are obese has significant influence on how their baby will develop.   

Conclusions

Research findings and the test of time agree on the key points:

• A diet high in sugar, salt, fat and refined carbohydrates increases the risk of obesity.
• A diet rich in vegetables, fruit and whole grains reduces the risk of obesity.
• Exercise has many health benefits and can help manage our weight - but what we eat and how much we eat is even more important.
• The first thousand days of life (from conception onwards) are particularly important in establishing food tastes, habits and preferences – meaning parents have a particularly important role to play.
• Maintaining a healthy weight before pregnancy can help reduce the risk of obesity in children (as well as health complications during pregnancy)

Reviewed and updated by Nicole Musuwo, November 2018. Next review date, October 2022.