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Get startedThe one truth weight-loss diets, injections and pills share when it comes to keeping the pounds off
Bodyweight expert Professor Giles Yeo MBE on what we need to master to combat obesity
Professor of molecular neuroendocrinology at Cambridge and world-leading expert on obesity.
Is there a right way to lose weight? No. Everyone is different. But keeping it off without medical intervention requires you to retrain your brain as well as your gut.
To find out how, we reached out to Giles Yeo, a professor at the University of Cambridge and one of the world's leading experts on bodyweight and appetite.
For 30 years, Prof Yeo has been studying how our brains control food intake and has authored two books on the subject: Gene Eating: The Story of Human Appetite and Why Calories Don’t Count.
Why diets don't work
'One wet winter’s evening, I hopped into black cab in London. A couple of minutes into the journey, the driver peered at me in the rear-view mirror and asked what I did. I told him I studied food intake and obesity.
"Yeah? You know, I just started one of those diets two weeks ago. I’ve got two stone to lose."
'What kind of diet are you trying?'
"Doesn’t matter, does it? They’re all the same. The moment you stop, all the weight comes back on."
'My cabbie was, of course, entirely correct.
'They say that 95% of diets don’t work. That is not actually true – what is more accurate is that 95% of diets we can’t stick to.'
'In order to lose weight, you need to eat less.
'What about exercise, you say? It’s good for pretty much every health-related metric, except for weight loss, at least for the vast majority of us who are not elite athletes. We simply don’t exercise enough to lose weight without eating less.
'Exercise is, however, good to help you maintain your weight AFTER you have lost it, but that’s a story for another column.
'The easiest way to eat less is to feel fuller. The way to feel fuller is to eat food that takes longer to digest. Food that takes longer to digest travels further down our gut.'
Professor Giles Yeo MBE, professor at the University of Cambridge and honorary president of the British Dietetic Association.
The secret to feeling fuller
'Here’s how it works. Every time we take a mouthful of food, from the moment we begin chewing, till the moment it emerges from the other end, 20 different hormones are secreted at every step of the way, reflecting not only how many calories are in the meal, but also its ‘macronutrient content’; that is, how much protein, fat and carbohydrates are present.
'Food that travels further down the gut before being fully digested results in changes in the repertoire of hormone secretion, which then make you feel fuller.
'The two components of food which do this the best are protein and fibre.
A calorie of protein makes you feel fuller than a calorie of fat, than a calorie of carbohydrate, in that order.
'As for fibre – most of it we can’t digest at all, so it goes right through us, and has a similar effect to protein. You can read more from me about how all calories aren’t equal.
'That’s why the more popular diets are either high in protein (Atkins, keto, paleo) or high in fibre (plant-based, Mediterranean, low GI).
'However, they only work if you stick to them. The problem is your brain makes it difficult to maintain weight loss, because it considers it a reduction in the chances of survival. So the moment you lose weight, your brain makes you feel hungry and lowers your metabolism, which is what makes dietary interventions so difficult.
'For those who continuously try and struggle, however, there’s now the prospect of the latest class of weight-loss medications, exemplified by Ozempic/Wegovy (from Novo Nordisk) and Mounjaro (from Eli Lilly), which make you feel fuller on less food.'
The power of GLP1
'As mentioned above, protein and fibre travel further down the gut, causing hormones to be secreted, in particular one called GLP1 and another called GIP, both of which make you feel fuller. Ozempic/Wegovy is a modified version of GLP1, while Mounjaro mimics both GLP1 and GIP.
'The secret power of these medications is that they are modified to stick about in the blood for far longer than native GLP1 and GIP, which only have a half-lives of approximately two minutes. These drugs, on the other hand, have half-lives of more than a week, hence their effectiveness as once-weekly injections.
'In December 2025, the ‘Wegovy Pill’, was approved for sale in the USA. With the makers of Mounjaro likely to be following close behind with its own offering, the once-daily oral versions of these drugs will shift the weight-loss medication landscape yet again.
'As pills do not require a delivery system or refrigeration, they will be cheaper to manufacture and transport, thus increasing their accessibility.'
'But regardless of how you take these drugs, like with any diet, the shared truism is that once you stop, the weight will come back on, unless you make lasting behavioural changes.
'This implies two things, neither of which are mutually exclusive: that many people will be on some version of these medications for a pretty long time, or that people will have to take advantage of the time when on the medications to make those behavioural changes.
'That’s why the NHS requires patients to receive dietary and lifestyle support when prescribed weight-loss drugs.
To make a behavioural change last, it needs to become habitual. With the drugs removing feelings of hunger usually associated with weight loss, there are opportunities to build new habits around food that we can keep up, even if they’re sometimes difficult for us.
'This could be learning to cook healthier meals richer in protein and fibre, or adapting existing recipes to smaller portions.'
Here are 6 of the best breakfasts for energy and fibre
Increasing your physical activity could also be your key to weight maintenance. Although this would normally increase hunger, so you’re also going to need to build some new food habits in parallel.
Building new habits takes time. Tricks to help you get there include starting small and planning what you’ll repeat and when you’ll do it.
Try latching your new habit onto something you already do habitually to make it stick more easily, such as adding lentils to your weekly bolognese sauce to up its fibre.
'Aim for making it into a habit you repeat rather than beating yourself up if it’s not perfect every single time.
'I am not so naive as to think that this will be easy. However, if you're able to make these new behaviours part of your routine, you then have a greater chance of keeping the weight off.
'This is why there’s no right way to lose weight. It’s different for everyone.
'Obesity is perhaps best thought of as a chronic relapsing condition with a myriad of different causes. Depending on the severity and cause, it will require different approaches.
'At the less severe end of the spectrum, behavioural and dietary changes, of course, will continue to play a crucial role. Your biology and your life circumstances could mean that a dietary approach has a real possibility of success.
'What these new anti-obesity drugs provide is hope for those with more severe obesity. There is no morality here. People living with obesity need lifelong access to treatment and support, whether the intervention is behavioural or pharmaceutical.'
