Fad diets: how to spot them and their risks

How can you recognise a fad diet?

A fad diet is a weight-loss method that promises weight loss quickly and without effort, with unbalanced guidance or no scientific basis. The easiest sign to spot comes from the Agencia Española de Seguridad Alimentaria y Nutrición. They promise a loss of more than 5 kilos a month, claim that they need no effort and that they carry no health risk [1].

In addition, almost all of them share some of these clues.

• They cut out everyday foods for no reason or divide foods into good and bad
• They attribute extraordinary properties to one specific ingredient
• They say certain nutrients cannot be mixed in one meal
• They rely on testimonials and before-and-after photos
• They are tied to the sale of products or supplements
• They suggest you follow them on your own, without anyone assessing your case

Against this, the Organización Mundial de la Salud reminds us that healthy eating is based on a variety of fruit, vegetables, pulses, whole grains and lean protein sources [2].

Why is it not advisable to lose weight too quickly?

Because the first thing to go is not fat. When you suddenly eat much less, the body draws on its stores of glycogen, the sugar stored in the liver and muscle. Each gram of glycogen is stored with about three grams of water. That is why a good part of the first kilos that disappear is water.

Fat is lost more slowly. If the cut is excessive and goes on for a long time, the body also draws on muscle for energy. The Sociedad Española para el Estudio de la Obesidad warns of this risk when a very-low-calorie plan is continued without professional support [3]. With less muscle you burn slightly less at rest. If you then go back to eating as before, regaining weight is easier. This is the rebound effect.

That is why the same society proposes cutting between 500 and 1,000 kcal a day to lose between 0.5 and 1 kg a week [3]. This is a pace that allows you to preserve muscle and maintain the change over time. How long it takes depending on your goal is explained in the guides to losing 10 kilos and losing 15 kilos.

What types of fad diets are there?

New names appear every season, but almost all fit into four groups.

High-protein, very-low-carbohydrate diets, followed without supervision. They cut cereals, pulses and many fruits to a minimum so that the body uses ketone bodies, an energy source the liver makes from fat. They take away your appetite at first. They often cause tiredness, constipation and bad breath, and are soon abandoned because it is hard to keep up a social life while following them.

Single-food diets, based on one food, such as artichoke, pineapple or grapefruit. They provide very little energy and almost no protein or essential fats. They cause weakness, irritability and muscle loss.

Meal-replacement diets, which swap meals for commercial shakes, bars or powders that you take on your own. They mean a continuous expense and do not teach you how to shop or put a plate together. Replacing one or two meals a day within a supervised treatment is a different matter. The Spanish obesity guideline GIRO considers it a tool for losing weight [4]. The difference lies in who prescribes it and with what follow-up. See “Restrictive diets and weight regain”.

Food-combining diets, which separate carbohydrates and proteins into different meals because, they claim, eating them together makes you gain more weight. There is no basis for this. The digestive system digests both at the same time, and almost all foods, such as pulses, milk or grains, already contain both mixed together.

Why do juices and smoothies not detoxify the body?

Because the body already removes its own waste. The liver, kidneys, lungs, intestine and skin do this all the time. No juice or plant-based preparation replaces or speeds up this work.

When fruit is juiced, much of the fibre is lost and the sugar is freed. It is less filling and raises glucose faster than whole fruit. The Organización Mundial de la Salud counts the sugars in juices as free sugars, which it recommends limiting to less than 10% of daily energy [2]. In addition, a plan consisting only of juices provides very little protein. That favours muscle loss, tiredness and headaches.

What health risks do fad diets carry?

They can harm your health and your relationship with food. If you cut out whole food groups for weeks, it is easy to run short of iron, calcium or vitamins such as B12 or D. That can end in anaemia and tiredness.

Another risk is interactions with medicines, and the clearest example is the grapefruit diet. Grapefruit contains furanocoumarins, substances that block, in the intestine, the enzyme cytochrome P450 3A4, which breaks down many medicines [5]. As a result, more of the medicine enters the blood than expected. This happens with some statins for cholesterol, some blood pressure medicines and certain anxiety medicines [5]. It increases the risk of side effects, such as muscle damage with statins [5]. If you take medication, ask your pharmacist or doctor whether grapefruit interacts with it; do not change it on your own.

Very-low-calorie plans, of 800 kcal a day or less, are not something to do on your own. The GIRO guideline states that they require medical supervision within a programme that includes psychology, a dietitian-nutritionist and physical activity [4].

What mistakes do we see in consultations?

What we see most is the cycle of restriction and loss of control. It starts on Monday with a very strict plan, lasts a few days and hunger ends up winning. Long periods of deprivation can lead to overeating once you eat again and to focusing more on food [4]. Then come guilt and another, stricter plan.

If you recognise yourself in that cycle, help from specialist psychological support is as important as help from the nutrition team. If you are thinking of losing many kilos, the guide to losing 20 kilos explains where to start, with a medical assessment first. And if you have regained weight after giving up a method like that, that is what usually happens when nobody teaches you how to eat afterwards.

The second mistake is judging progress by the scales in the first week. What drops in those days is mostly water, so the number misleads you both when it goes up and when it goes down.

How to lose fat without starting from scratch every few months

Losing weight healthily means creating the conditions for the body to use its stored fat and protect muscle. It rests on three pillars.

• A moderate deficit, of 500 to 1,000 kcal a day less than you burn [3]. In consultations we tailor it to each person, without going below 1,200 kcal a day unless a doctor advises it.
• Enough protein at every meal, together with strength training, to preserve muscle.
• Fresh food as the basis, with pulses, vegetables, fruit, whole grains, nuts and extra virgin olive oil.

The full guidance for everyday life is in our weight-loss diet guide. If you are about to start, go through the 10 steps to start a successful weight-loss treatment. And for dinner, we have our dinner ideas for weight loss.