The 3 rules your diet needs to actually work

Fed up with navigating all the tips telling you how to lose weight? Us too. So we asked an obesity expert to reveal whether anything actually works for weight loss.

By Dr Giles Yeo

Published: Monday, 19 June 2023 at 12:00 am

Many of us would like to shed a few kilos, and keep them off. This has never been more relevant, as three years of pandemic living has resulted in some of us putting on unwanted weight.

But if you head online for some advice, prepare to be disappointed. Doctors, scientists and influencers seem to be locked in a tussle over what exactly works when it comes to shedding fat.

There’s the like of diet expert Prof Tim Spector, who set TikTok ‘influencers’ into a sweaty flurry over his soundbite on the Diary Of A CEO podcast when he said “exercise doesn’t work”.

To clarify, it is of course possible to lose weight through exercise. After all, Tour de France cyclists eat around 5,000 calories a day and still lose weight during the three-week race.

The problem is, most of us mere mortals don’t exercise anywhere near enough for this to be effective. Others, like myself, have argued that counting calories has its limitations, and we shouldn’t slavishly follow the calorie counts listed on food packaging.

Still more people quibble about which diet (of the hundreds out there) will actually get you looking lean, and permanently so.

I’m not in the business of endorsing any one of the many different dietary approaches that rear their heads online, because – whatever anyone tells you – there is no magical ‘one-size-fits-all’ solution.

But do any diets actually work? The answer is, surprisingly, yes, although perhaps not for the reason that is often being marketed.

In one respect, the truth about diets is a simple one: for a diet to work, there has to be a calorie deficit. If we forensically examine ALL the diets that show some evidence of working, the vast majority all share one (or more) of these three characteristics:

  1. They explicitly restrict calories
  2. They are high in protein
  3. They are high in fibre

Let’s look at these in a bit more detail:

1. Restrict your calories (the right way)

There are, of course, diets that simply restrict calories. This includes portion control – basically eating a little less of everything – which is very effective, but difficult to stick to.

The issue is we have recipes that we use, and it is difficult to reduce it by 20 per cent, say. Like, if the original recipe called for one or two eggs, how do you reduce that by 20 per cent? So that means you have to serve yourself less, but that would leave 20 per cent of the unfinished meal in the pot, tempting you all night!

That is why the use of meal-replacement shakes are popular. You simply make up an 800-calorie shake, and consume that. These shakes are nutritionally complete, and effective in the short-term.

It is, however, very difficult for most people to stick to these shakes for any length of time, if only because of the monotony. So I think these shakes are useful to lose weight initially, but alternative methods would be required to keep the weight off.

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With a constant stream of dieting advice through social media and books, the search for science-backed dieting tips can feel fruitless.
Image Credit: Grace Russell

Another popular approach is group support, including programmes such as Slimming World and Weight Watchers. While each has their own unique selling point, they all harness the fact that you are more likely to do something if you are doing it with somebody else.

It is important to remember that because public ‘weigh-ins’ are typically part of such programmes, they are going to be ill-suited to those who hate such events.

Then there are the intermittent fasting and time-restricted eating (TRE) approaches.

The most common variation of intermittent fasting is the 5:2 diet, where one eats normally for five days, and then restricts calorie intake to 500-600 calories for two days.

TRE limits the amount of time in the day that one consumes food, typically to a six- to eight-hour window. Both undoubtedly create a calorie deficit for many people.

The question is, aside from the reduction in calories, are there any additional metabolic benefits? The underlying concept is that during the fasting phase, you allow your body to use up the carbohydrates from your muscles and liver (stored in the form of glycogen) so you begin to burn fat.

While there is some evidence of the effectiveness of both diets in mice, the latest human studies do not see any significant advantage over and above the reduction in calories, for either intermittent fasting or TRE.

How about calorie counting, because aren’t all calories equal? Well, all calories are equal once they are in us, as little poofs of energy. However, remember that we eat food and not calories, and some food takes more energy for our bodies to extract the calories from.

This is why the source of the calories, whether from a steak, a carrot or a doughnut, makes an enormous difference. While calories are a useful reflection of portion size, they are not a marker of the nutritional content of the food.

Instead, we have to take into account ‘caloric availability’, which is the amount of energy we can extract from a food, as opposed to the total number of calories in it.

Digestion breaks food into its nutritional building blocks, all of which get moved across the gut wall into our blood. These building blocks are, however, just easily transportable intermediates that need to be metabolised to be converted into usable energy. This process of producing energy also costs energy.

The two elements of food that have the biggest influence on calorie availability are protein and fibre. If we zoom out and take a broader view, it becomes clear that this concept goes far beyond an esoteric piece of nutritional trivia; rather, it explains how many popular diets work.

Read more:

2. Eat meals high in protein

Of the three macronutrients – carbohydrates, fat and protein – protein is chemically the most complex. Fat and carbs are made entirely of carbon, hydrogen and oxygen atoms, just in a variety of different configurations, thus metabolising or storing them is energetically efficient.

Unlike fat and carbs, all protein in our body is there for a reason, either for building or for repair – there is no passive store of protein for a rainy day. So any excess protein that is not used immediately has to be metabolised into energy or converted into fat.

Second, while protein is, like fat and carbs, primarily composed of carbon, hydrogen and oxygen, it also contains a significant amount of nitrogen. This nitrogen needs to be removed, so is secreted as urea, primarily in our urine, before the remaining chemical structure of the amino acids can be converted into energy or fat.

This process takes a lot of words to explain, and even more energy to deliver. In fact, for every 100 calories of protein that we consume, we are only ever able to use 70 calories, with the other 30 calories needed to handle the protein.

Thus, protein has a caloric availability of 70 per cent. All the protein calorie counts we see today are 30 per cent out!

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It’s important to note that protein doesn’t only mean steaks! Fish, tofu, beans and nuts are all rich sources of protein, but without the saturated fat found in land-based animals. Consider including a wide range of protein in your diet.
Image Credit: Alamy

By comparison, fat has a caloric availability of 98 per cent, meaning that to convert it to energy costs next to nothing, and hence why it is such an efficient long-term fuel store.

As for carbs, it all depends on whether we are talking about the complex (90 per cent available) or refined (95 per cent available) variety. The difference in this availability is down to the presence of fibre, which we mostly cannot digest and therefore passes right through us.

Diets that are high in protein include the entire menagerie of ‘low carb, high fat’ diets, ranging in the severity of carb restriction from Atkins to keto to carnivore, but are all universally high in protein, defined as 16 per cent or more of total daily intake. For all intents and purposes, they also encompass diets like gluten-free (with the exception of those suffering from coeliac disease) and paleo.

These diets work for many people trying to lose weight because protein, from a chemical perspective, takes longer to digest and takes more energy to metabolise, so is more satiating than fat or carbs. You feel fuller, you eat less, you lose weight.

Read more:

3. Eat meals high in fibre

Fibre is actually a type of plant-based carbohydrate, although much of it is structured in a way that we humans cannot digest.

Fibre is, of course, vitally important for our gut health, keeping everything shipshape and, ahem, regular. On average, we each only consume 15 grams of fibre a day, whereas we need to try and achieve 30 grams a day.

From the perspective of caloric availability, fibre slows down the rate of digestion, resulting not only in the release of nutrients over a longer period of time, but ultimately in a reduction in the absolute amount of calories absorbed.

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There are a lot of posts on social media saying fruit is high in sugar, so should not be eaten in large amounts. The truth is, the presence of fibre means it is difficult for most people to eat too much fruit! So eat as many portions of fruit and vegetables as you like.
Image credit: Getty

An illustration of the impact of fibre can be seen when you compare drinking a glass of orange juice to eating a whole orange. When you drink juice, the sugar, which incidentally is at the same concentration as that of a fizzy drink, is absorbed by your body almost immediately.

Eating an orange, however, first involves chewing, which is sensed by your body and allows it to prepare for the arrival of nutrients; and second, because the sugar is interlocked in the fibre, it takes energy and time for your digestive system to extract, thus you feel fuller.

This is why dietary approaches that are high in fibre, including plant-based, low-GI and Mediterranean, as well as diets that are largely plant-based but with complex (and often false) back stories, such as alkali, work for weight loss.

Read more:

Prof Giles Yeo presents various BBC programmes including Horizon and Trust Me, I’m A Doctor.