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This week, I have been looking at the pros and cons of different diets after headline-grabbing research was published on two popular weight loss strategies, where there is more to the results than meets the eye.
First let’s look at the largest study of the two, which investigated an extreme meal-replacement diet, where people ate nothing but diet soups and shakes for three months, consuming just 900 calories a day. Then proper meals were gradually reintroduced, aiming to maintain the weight loss.
The approach was designed for people with type 2 diabetes, which involves poor blood sugar control and often seems to be caused by being overweight.
The study confirmed previous results showing that some people manage to lose so much weight their diabetes goes into remission.
Type 2 diabetes is usually seen as a condition that progresses inexorably over time, requiring increasing medication to control blood sugar. So, any diet that can put it into remission is great news. “If you can keep weight off, you can get rid of the diabetes,” said nutrition expert Professor Michael Lean at the University of Glasgow.
Indeed, i spoke to someone who tried this approach after being told they were “on the precipice” of diabetes. By the end of the year, they had lost about two stone and their blood sugar levels were back on track.
But sadly this approach does not work for most people.
Depending on how the numbers are crunched, one reading is that about a third of those who completed the programme were in remission from diabetes after a year.
But many people dropped out before the end of the trial. Only one in 10 of those who started the programme were still turning up for monitoring and were in remission from their diabetes by the end.
This is not surprising. It fits with what we already know about dieting – that while it is certainly possible to lose weight in the short term, it is very hard to keep it off.
According to one small Australian study, seven out of 10 people who lose weight end up putting it all back on after a few years.
Weight set point
Why is it so hard to maintain weight loss? One idea is that everyone has a “set point” for weight, which is probably mainly influenced by our genes. And if we become thinner, the body strives to regain that weight.
The set point theory is up for debate, but it is clear that when people are in a calorie deficit for any length of time, there is a drop in their metabolic rate, which is the amount of energy their bodies need to function at rest.
This may be one reason why people on a long-term diet often see their weight loss plateau, even while continuing with a calorie deficit. While at the start of the diet, consuming 1,500 calories a day, for instance, might have caused them to lose weight, but after six months, 1,500 calories a day could meet all their energy needs or even cause weight gain.
The drop in metabolic rate could last for a long time, according to a study of contestants on a US TV show called The Biggest Loser, where very overweight people followed an intensive diet and exercise programme for 30 weeks and lost on average 58kg.
When researchers returned to 14 contestants from one series, six years later, most of them still had a lower metabolic rate than would be expected – equivalent to needing 500 fewer calories a day.That could be partly why most of them had put a lot of their weight back on – and a few were even heavier than at the start of the programme.
Ozempic hack
As well as dieting causing changes to metabolic rate, there is also a fall in various gut hormones that are normally released after eating to promote fullness and dull the appetite.
These include a compound called GLP-1, which has been getting a lot of attention lately as it is the hormone mimicked by the startlingly successful weight loss drug semaglutide (also known as Ozempic and Wegovy).
Low levels of GLP-1 and similar gut hormones during dieting probably contribute to the constant hunger and thoughts of food.
This is probably why people who lose weight thanks to Ozempic are not bothered by such “food noise”, said metabolism expert Professor Javier Gonzalez at the University of Bath. “It’s partly why these new drugs seem to be so effective, they target that appetite system.”
Regardless of the mechanism, it is clear that many people find it very hard to stick to a diet for any length of time, whether in the calorie-deficit or weight maintenance phase.
Low-carbing
In the past decade, though, an alternative diet strategy has been gaining popularity: low-carbing. In other words, it means cutting down on food high in carbohydrates, namely sugar and starch.
As the typical British diet is based around starchy food, like bread, potatoes, pasta and rice, low-carbers tend to have a relatively higher-fat diet.
That’s highly controversial, as the medical orthodoxy says we should avoid fat for weight control, and that saturated fat – found mainly in red meat, and dairy products – raises cholesterol levels, so raising heart attack risk.
A further study out this week has shed more light on this question. Professor Gonzalez’s team randomised 53 people to either a very-low-carb (also called keto) diet, a low-sugar diet or a more typical diet for comparison. People could eat as much as they wanted, as long as their total food intake met the carbs and sugar criteria.
The study was widely claimed to find that low-carbing raised cholesterol levels – which certainly would be bad news. But while the low-carb diet did raise levels of a substance called bad cholesterol, as well as another blood compound linked to cholesterol called apolipoprotein B, that was just in the first four weeks.
By the end of the trial, after 12 weeks, both measures had returned closer to those of the control group’s diet, and in fact, the differences were so small they may have arisen by chance, according to the statistical analysis.
Low-carbing often leads to a transient cholesterol rise but this tends to improve as the metabolism adjusts to burning fat, said Dr David Unwin, a GP in Southport, who has pioneered offering low-carb diet advice as part of NHS care. “It can take several months for your enzymes to rev up.”
Last year, Dr Unwin published figures from his practice showing that, of 186 people who had low-carbed for an average of 2.5 years, they had improved on a range of cholesterol measures and other outcomes like blood sugar control and weight loss. In fact, half of his low-carbers had lost so much weight they achieved remission of their diabetes.
It would be unfair to compare Dr Unwin’s figures to those of the soup and shake diet as his practice offers intense patient support and the circumstances of the two studies could differ in other ways.
What we really need is a large and long-term randomised trial, comparing various different diet approaches under the same circumstances, to see which one works best for health and which people can stick to the longest.
Considering how much obesity rates have been rising over the past few decades, along with the rising rates of linked conditions such as diabetes and heart attacks, I can’t understand why the health service doesn’t make such a trial a priority.
It may be that different people tolerate different diets best, said Professor Gonzales. “Most of the health issues that we have at the moment are due to excess weight. It’s better to find a diet that you are able to stick to.”
Other things I’ve written recently
Have you been puzzled over the distinct lack of butterflies and bees in your garden lately? It’s because of the weird weather in the UK this summer and spring, as I explain here.
I’m reading
First-person accounts of working as a doctor are usually either funny or alarming. This is Going to Hurt, by Adam Kay, a tale of his time as an obstetrician, manages to be both. It was also turned into a BBC series, with the same name, but I prefer the book for its raw honesty.
Kay has taken some flak for the less-than-respectful way he and other doctors talked about patients out of earshot. But I am glad to have had this insight into how doctors really think, and Kay’s humanity shines through.
This is i on Science with Clare Wilson, a subscriber-only newsletter from i. If you’d like to get this direct to your inbox, every single week, you can sign up here.
