It's said 'you are what you eat'. But why can some people eat whatever they like without putting on a pound, while others have to watch every morsel they put into their mouth and count each calorie they consume?
At first glance, the answer might appear simple: unhealthy junk food and takeaways are readily available and inexpensive these days. And many people consume more calories than they need while also doing little exercise. But it still doesn't explain why some people can eat whatever they like and not gain any weight.
"It is important to recognise that obesity is a complex disorder, as genetics, environment, development and behaviour all play a role in a person's likelihood of struggling with their weight," explains Dr Fatima Cody Stanford, an obesity medicine specialist at Massachusetts General Hospital and a researcher at Harvard Medical School.
"The idea of 'calories in/calories out' is too simplistic because it implies that we are not affected by other factors. We are so much more complex, and it is the oversimplification of weight equation that has likely contributed to our pandemic regarding obesity," she opines.
With the obesity crisis ever increasing - almost 1 in 3 children (36% of boys and 32.4% of girls) in their last year of primary school are classified as overweight or obese, according to Public Health England - researchers are investigating whether some individuals might have a genetic predisposition to weight gain.
A team from Imperial College London discovered mutations in a gene related to obesity in January 2018. They studied obese children in Pakistan and identified mutations in a gene called adenylate cyclase 3 (ADCY3), which leads to abnormalities in appetite control, type 2 diabetes and impaired sense of smell when functioning incorrectly.
Research teams in the Netherlands and Denmark have found similar results from their own genetic studies. These findings could potentially lead to medicines to specifically target obesity genes, offering new ways to treat the global epidemic in the future.
However not all scientists agree. Research from King's College London suggests that genetics do not have a significant role in the body's ability to gain - or indeed lose - weight. Indeed, our genes have changed very little in a generation, yet our rates of obesity have. Instead, they believe the environment in the gut plays a bigger role.
The King's College team examined stool samples from 393 pairs of twins and found several chemicals associated with an increase in visceral fat (the most unhealthy kind, which is typically found around the waist). The team examined how much these chemicals varied according to people's genes. They concluded that genetics does play a small role but that it only partially influences our waistlines. Instead, the bacteria living in people's guts may have a much greater part to play in weight gain and fat distribution.
"There are factors both inside and outside of an individual that contribute to weight struggles," says Stanford, who was not involved in this research.
"Factors that might increase an individual's food intake might include: delayed satiety [taking a long time to feel full], disordered eating, or large portion sizes. Factors that might decrease someone's ability to lose weight include: differences in gut bacteria, differences in a person's ability to burn calories, and how much exercise someone does. Things that might increase an individual's intake and decrease how much they can burn might include: genetic factors, age-related changes like menopause, medications which cause weight gain, or stress (to name only a few)."
The King's College research suggests tackling obesity from an alternative angle might be key - focusing on boosting our good gut bacteria.
"We know that our gut is home to approximately 100 trillion gut bacteria," says Shona Wilkinson, head nutritionist at Nature's Best. "We have always associated our gut bacteria purely with digestive problems such as constipation, IBS, diarrhoea, etc. We are only now beginning to realise the huge part our gut health plays in the health of our whole body."
"One study conducted on mice found that removing certain gut bacteria caused the mice to gain weight and develop insulin resistance [leading to type 2 diabetes]," she continues. "Although scientists are still unsure how gut bacteria actually affect our weight, it is thought that different species of gut bacteria seem to have different effects on appetite and metabolism. In the study above, the mice which had too much 'bad' bacteria ate more and had damaged metabolism."
By increasing the number of 'bad' bacteria in our guts, the rate at which fatty acids and carbohydrates are absorbed can be potentially increased, Wilkinson says. It means someone with excess bad gut bacteria can eat the same amount of food as someone with a healthy gut but gain more weight!
"Healthy gut bacteria levels are crucial for maintaining a normal weight. Things that detrimentally affect our gut bacteria include using antibiotics, diets high in refined carbs and sugar, diets low in fibre, and stress - amongst other things," Wilkinson says. "The best way to improve your gut bacteria health is to remove sugars and refined carbs from your diet, eat plenty of fibre, consider taking a probiotic supplement and manage your stress levels."
Read more about keeping your gut bacteria happy.
But again, other experts have their doubts about the gut bacteria and weight connection - and most don't believe it's the whole answer. It's important to know that, like the genetic studies, this type of research is still in its infancy. Far more evidence is needed before we fully understand how the environment in our guts might impact obesity.
While calories in/calories out may not be the whole solution to the weight loss puzzle, tried and tested advice still applies. As dull as it might sound, if you're looking to lose weight, your best bet is to follow a healthy balanced diet, watch your portion sizes, cut down on alcohol and increase your levels of physical activity. Making these small lifestyle changes will also reduce your risk of conditions such as heart disease, type 2 diabetes and some forms of cancer.
And if you'd like some extra support on your weight loss journey, it's well worth talking with your GP. They will be able to work out if there are any health issues preventing you from losing pounds, and discuss a plan that will work for you. Because even if genetics and other factors do influence your weight, it doesn't dictate what you can do about it.