Diet: how to follow a diet that lasts

The word ‘diet’ means something different to everyone. For one person it is a list of forbidden foods, for another two weeks of salads before the summer. In research it means something simpler: a way of eating with less energy than you need, which you can keep up for months. A diet like this has a few basic parts, and there is evidence for each of them.

The calorie deficit in plain words

Every day your body uses energy to keep running, to digest food and to move. If you eat less than you use, it takes the difference from its stores. This difference is called a calorie deficit.

The UK's NICE says it clearly in its 2025 guideline: any dietary approach to weight loss has to keep the energy you eat below the energy you use. This can be done with less fat, with fewer carbohydrates or in another way that limits calories.

How many fewer calories

The US guidelines on obesity (AHA/ACC/TOS, 2013) give two ways. One is a deficit of 500 to 750 kcal a day compared with what you need. The other is a target of 1,200 to 1,500 kcal a day for women and 1,500 to 1,800 for men, adjusted for body weight and activity. They accept diets under 800 kcal only in limited cases and with medical supervision.

Example. A woman aged 45, 1.65 m and 78 kg, with an office job. With the Mifflin-St Jeor equation her resting metabolic rate comes to about 1,425 kcal. With a factor of 1.40 for a sedentary life, her maintenance calories are about 2,000. With a deficit of 400 kcal, that is about 20%, her target comes to around 1,600 kcal a day. This is a slightly gentler deficit than the 500 kcal in the guidelines, and at the start it corresponds to about 0.35 kg a week. It is a starting point, not a measurement. We explain how the number is worked out, how far off it can be and how it is corrected in the article on daily calories. We show what a day of around 1,600 kcal with Greek food looks like for the same woman in the article with an example day.

Why there is no ‘best diet’

The large comparisons come to the same conclusion:

• 160 adults, 1 year. They followed the Atkins, Zone, Weight Watchers or Ornish diet and lost 2.1 to 3.3 kg on average. How much each person lost was related to how closely they followed their diet, not to which one they chose.
• DIETFITS, 609 adults, 12 months. On low fat they lost 5.3 kg, on low carbohydrate 6.0. The difference was not significant.
• 48 trials, 7,286 adults. The differences between the well-known diets were small. The researchers write that this supports the practice of recommending whichever diet the person can keep to.

It is fairly certain that keeping to a diet matters more than the type of diet. That is why NICE asks for the diet to be adapted to each person's preferences and daily life. It also advises avoiding strict diets that are not balanced, because they do not work in the long term and may cause harm. We explain what happens in the body on a strict diet of a few days in the article on the 5-day diet for 5 kg.

The Mediterranean diet as a base

If you grew up with Greek food, the Mediterranean diet has a practical advantage: it is already the food you eat at home. On weight, the trials say the following:

• In the DIRECT trial, 322 adults with obesity, mostly men, followed a Mediterranean, a low-carbohydrate or a low-fat diet for 2 years. They lost 4.4, 4.7 and 2.9 kg on average respectively.
• A review of 5 trials with 998 people found that at 12 months or more the Mediterranean diet gave greater weight loss than low-fat diets and similar loss to the others.

There are few trials, so the evidence is of low certainty. The Mediterranean diet does not make you lose weight on its own. It does what other diets do, with dishes you already know how to cook.

The Greek National Dietary Guidelines for Adults (2014) give the pattern for the week: pulses at least 3 times, fish 2 to 3, white meat 1 to 2, red meat up to 1, and every day 4 portions of vegetables and 3 of fruit. On a diet the pattern stays the same. The amounts change, mainly where calories add up easily, such as oil and bread.

Enough protein

An analysis of 24 trials with 1,063 adults compared diets with the same calories and either more or the usual amount of protein, for about 12 weeks. With more protein, participants lost 0.79 kg more weight and 0.87 kg more fat, and kept 0.43 kg more fat-free mass. In 3 of the 5 trials that measured it, they felt fuller. A 2024 review for the guidelines of the German Nutrition Society, however, judged the evidence insufficient because of the low quality of the studies. The benefit may exist, but it is small and of low certainty.

In practice, include protein in every main meal: yoghurt, eggs, pulses, fish, chicken or cheese. How much you need per kilo is in the article on protein.

Weighing and keeping a record

An analysis of randomised trials found that when weighing was added to a weight-loss programme, participants lost 1.7 kg more. Weighing every week or every day made no significant difference. A review of 22 studies found that recording food, activity or weight is consistently linked with greater weight loss, but the studies had weaknesses. Both findings are of low certainty.

Weigh yourself at least once a week, in the morning, under the same conditions. Look at the trend over the weeks, not at the number on a single day. Glycogen, the form in which carbohydrates are stored, holds water with it, and this makes the scales go up and down without any change in fat. The same water explains why the first week often shows more kilos, as we write in the article on the ‘2 kg a week diet’.

When the scales stop moving

Almost every diet reaches a plateau at some point, and the reason is mainly arithmetic. A smaller body needs less energy, while appetite goes up. In a study of 153 people with type 2 diabetes, appetite rose by about 100 kcal a day for every kilo lost. It concerns one specific group, so the figure is of low certainty. A mathematical model showed that the plateau that usually appears at around 6 months is largely explained by small departures from the diet that add up, and not by the metabolism ‘getting stuck’. We explain what to do then in the article on plateaus.

What to expect

• The first six months. The US guidelines set an initial goal of 5% to 10% of body weight in 6 months. For someone weighing 90 kg, this means 4.5 to 9 kg (example). In an analysis of 80 trials lasting at least a year, diets with fewer calories, alone or together with medicines, gave 5 to 8.5 kg on average in the first 6 months. After that, weight levelled off.
• In the long term. In the trials of the same analysis that lasted 4 years, participants weighed 3 to 6 kg less on average than at the start. A 2005 review estimates that about one in five people with overweight lose at least 10% of their weight and keep it off for at least a year.
• To keep the weight off. For maintenance, the US guidelines suggest about 200 to 300 minutes of activity a week, weighing yourself every week or more often, and fewer calories than before the diet, since a smaller body needs less energy.

Before you start

A general diet with a calorie deficit is not for everyone. Talk to your doctor first if you are pregnant or breastfeeding, if you are under 18, if you have or have had an eating disorder, if you take medicines for diabetes, or if you have chronic kidney, heart or liver disease. On pregnancy, for example, NICE says that deliberate weight loss is not recommended because of possible effects on the baby. If you have a lot of weight to lose, also read the article on losing a lot of weight, where we explain when to see a doctor first.

If you would rather not do the sums by hand, the MesogeaFit app, coming soon, will work out the grams of each meal with Greek food and correct them from your weigh-ins. Notify me when it launches.