Have you ever lost weight on a diet only to see those pounds pile back on? The unfortunate truth is that this happens to about 90 per cent of those trying to shed excess weight. 

And it's why people taking Ozempic jabs are predicted to need them for life – a potential issue as new side-effects are emerging, including joint pain and a higher risk than previously thought for pancreatitis (a potentially serious condition).

In the past, I'd have blamed lack of willpower – as a doctor I often used to privately judge my obese patients and blamed their lack of backbone for their inability to stick to a healthy diet.

But my own trouble with weight gain some years back led me to rethink this.

And now new evidence has emerged suggesting it's not 'just willpower' – and crucially, it shows how to keep the weight off, including after Ozempic (NHS patients get the drug for only two years).

In my case the weight started to pile on as I started comfort eating in response to stress both at work and in my private life. I was soon about 10kg above my ideal weight.

Thinking that to lose it was as simple as burning more calories, I started doing a daily 5k run. I felt fitter – but after six months I'd hardly lost any weight.

If I had understood the science then like I do now, I would've realised that although exercise is brilliant for many things – such as improving mood and lowering your risk of a heart attack – losing a significant amount of weight is not one of them.

That's because, essentially, if you regularly exercise your metabolic rate changes, so your body uses fewer calories at rest.

It is diet that's key – something I've only learnt in the past couple of years after reading new scientific studies that questioned what I had been taught at medical school.

So I decided to significantly reduce my calories – from about 2,750 to 2,000 daily. Sure enough, within a few months I lost about 8kg.

But then with a new baby, sleep deprivation, work-related stress – and not having time to cook – I slipped back into eating the ultra-processed comfort food I'd previously managed to cut out.

I was one of the 90 per cent of failed dieters.

Lack of willpower? Not according to an important study published a few weeks ago in the journal Nature – though this research went under the radar, so you may not have heard about it.

Essentially this study, by the Institute of Food, Nutrition and Health in Zurich, showed that fat cells have something akin to memory; those cells 'remember' being fat and, worse, they want to stay that way.

It's all thanks to epigenetics, the science of how genes are switched on or off by environmental factors such as diet.

For the study, researchers collected samples of fat cells from people who were obese and from those who'd maintained a stable weight. When they analysed the fat cells from the overweight people they found epigenetic changes that meant these cells were programmed to get bigger and bigger; they were more effective at absorbing fat molecules rather than breaking them down.

The researchers then looked at fat cells from people who'd previously been obese, but had lost significant weight – and found they looked like those of obese people, rather than those who had always had a stable weight.

In other words, the changes to the fat cells persisted even years after weight loss – they 'remembered' being obese and wanted to go back to their original weight, making it difficult for these people to maintain their weight loss.

To test this, the scientists did something very clever: they put mice on high-calorie diets, then a restricted diet so they lost weight.

As they gained the weight, the mice fat cells changed – certain genes were switched on and off. And when they lost weight, the cells carried on behaving as if they'd never lost weight. In other words, they too were primed to put weight back on when food was once again freely available.

This somewhat complicated scientific explanation is the reason why so many people end up yo-yo dieting: it's not their fault, but a biological phenomenon that means whenever they lose weight, they soon regain it.

Blame evolution. When food was scarce, our bodies adapted to hoard calories for times of famine. But in today's world there is food aplenty, and so much of it is designed to be irresistible (think ultra-processed food).

So how do we beat that evolutionary drive to hold on to weight? A balanced diet and patience are your best weapons. Maintaining a healthy weight for a prolonged period of time is the best way to change our fat cells back, so that they act like the fat cells in people who've always had a healthy weight.

The best foods to keep that weight stable are ones that are the most filling, but with the least amount of calories – those with a high protein and/or high fibre content, for instance, eggs, lean meat, fruit such as apples, pears and berries, and green vegetables such as broccoli and spinach.

And avoid those that have loads of calories but don't fill you up, such as sugary drinks, white bread, biscuits, crisps, chocolate and breakfast cereals.

Exercise is important for maintaining the weight loss rather than inducing it. Aerobic exercise such as walking, running, swimming or cycling counteracts the natural tendency of the fat cells to pile on the pounds because it pulls the nutrients away from the fat cells into the muscles.

Meanwhile, weight training helps as it builds up your muscles, which increases your metabolic rate and stops calories being stored in the fat cells.

But what about Ozempic and other similar injections?

In studies, obese patients have used these drugs for a maximum of two years and shown good amounts of weight loss. However, once they come off these medications, the weight invariably goes back on within 18 months.

And now we know that this will be because of fat cell memory.

These drugs can help many people. And as well as weight loss, they can have other benefits, as confirmed in a study just published in Nature Medicine.

Researchers at Washington University followed more than two million Americans prescribed the drugs for type 2 diabetes and found that they also reduced the risk of heart attack and dementia, with other unexpected benefits such as tackling addictions.

But they are not risk-free, doubling the risk of pancreatitis (serious inflammation of the pancreas)and increasing the risk of arthritis. And, most worryingly, they reduce muscle mass – which is dangerous as we age and need strong muscles to stop us falling over and breaking our hips.

Personally, I'd want to do everything I could to lose weight before going down the route of weight-loss jabs.

However, if you are already on Ozempic or are considering using it, you must ensure you undertake lifestyle changes – cutting down on ultra-processed food and eating good foods that fill you up, along with doing aerobic and weight-based exercises to help maintain muscle – if you don't want to pile on the pounds when you stop.

There are those who argue for just staying on these injections for life. But who knows what the longer-term risks are? All the studies were based on using them for two years at most and while people with type 2 diabetes have been on them longer, we haven't tested their use for weight loss alone. That's why NHS guidance is to use them for just two years.

Now that we know lack of willpower isn't why you pile on the kilos again, it's time to make those lifestyle changes, as they're the long-term solution to staying at a healthy weight.

@drrobgalloway