General principles
Nutritional therapy is a core component of comprehensive obesity management. Its purpose is not simply to prescribe a temporary “diet,” but to help the individual develop a nutritionally adequate, enjoyable and sustainable eating pattern that can be maintained over the long term.
The intervention should be individualised according to the person’s health status, obesity-related complications, medication, food preferences, cultural background, financial resources, cooking skills and previous experiences with weight-management programmes. Where possible, treatment should be supported by a registered dietitian or another appropriately qualified nutrition professional.
For weight reduction, a sustained energy deficit is required. The German S3 guideline recommends an individually determined deficit, commonly around 500–600 kcal per day. However, the appropriate deficit varies between individuals, and changes in body weight are not perfectly predictable from calorie calculations. As weight is lost, energy requirements decrease and the rate of further weight loss usually slows.
Unsupervised crash diets, prolonged fasting and highly restrictive or nutritionally unbalanced diets are generally unsuitable as long-term obesity treatment. They may result in:
Resting energy expenditure normally decreases during weight loss. This is partly explained by the reduction in body mass and metabolically active tissue and partly by adaptive metabolic responses. However, the claim that every period of dieting permanently and irreversibly lowers the metabolic rate is not supported by current evidence. Adaptive thermogenesis varies considerably between individuals and often becomes smaller or is no longer detectable after a period of weight stabilisation. (Cambridge University Press & Assessment)
Weight regain after a diet is therefore not simply the consequence of a permanently “damaged metabolism.” It reflects a combination of biological adaptations, increased appetite, a lower energy requirement at the reduced body weight, environmental influences, behavioural factors and the difficulty of maintaining a restrictive intervention. Weight regain should not be interpreted as a personal failure.
Low-energy and very-low-energy diets
Structured low-energy diets providing approximately 800–1,200 kcal per day may be considered for selected adults as part of a multicomponent programme with long-term support. Very-low-energy diets providing less than 800 kcal per day should be reserved for people with obesity who have a clinically assessed need for rapid weight loss.
Such programmes should:
They are not suitable as unsupervised or permanent eating patterns. Additional caution is required in people with eating disorders, relevant mental-health conditions, pregnancy, frailty or medical conditions that may be destabilised by rapid weight loss. (NICE)
Long-term treatment should focus on improving dietary quality while creating a manageable energy deficit. The person should not be instructed merely to “eat less.” A successful intervention should help them eat in a way that is satisfying, nutritionally balanced and compatible with everyday life.
There is no single macronutrient distribution that is optimal for every person. Energy-reduced low-fat, lower-carbohydrate, Mediterranean, vegetarian or vegan diets, structured meal-replacement strategies and some forms of intermittent fasting may all produce weight loss. When the energy deficit and treatment intensity are comparable, differences between dietary approaches are usually modest, particularly after the first year. Adherence, dietary quality, safety and personal preference are generally more important than the name of the diet.
Intermittent fasting is an optional organisational strategy rather than a metabolically superior treatment. On average, it does not appear to produce clearly greater long-term weight loss than continuous energy restriction. It may be suitable for some individuals but should not be presented as necessary for successful weight management. Clinical supervision is advisable for people taking insulin or medicines that can cause hypoglycaemia.
Some loss of fat-free mass may occur during weight reduction, especially during rapid or severe energy restriction. The treatment plan should therefore include:
Protein should not automatically be reduced together with fat and carbohydrate. Restricting all macronutrients indiscriminately may impair nutritional quality and make it more difficult to preserve muscle mass.
Fat provides approximately 9 kcal per gram and is therefore energy dense. Reducing excessive fat intake can help lower total energy intake, particularly when the diet contains large quantities of fried foods, processed meat, pastries, confectionery, creamy sauces or high-fat snack foods.
Nevertheless, dietary fat should not be eliminated. The quality and quantity of fat are both important. Unsaturated fats from foods such as olive oil, rapeseed oil, nuts, seeds, avocado and fish can form part of a healthy eating pattern. They should preferably replace foods rich in saturated or trans fats rather than simply being added to the existing diet.
Oils, nuts and seeds are nutrient-rich but energy dense, so appropriate portions remain relevant when weight reduction is an agreed goal. The energy-density concept should not be used to categorise all high-energy foods as unhealthy.
Carbohydrates do not need to be eliminated for weight loss. The dietary pattern should preferentially include carbohydrate sources that provide fibre and micronutrients, such as:
Highly refined grains, confectionery, products containing large amounts of added sugar and sugar-sweetened drinks should be limited.
The glycaemic index can provide some information about the postprandial glucose response to an individual carbohydrate food, but it should not be used as the sole measure of nutritional quality. Portion size, total carbohydrate quantity, fibre, food processing, meal composition and the person’s metabolic health also influence the glucose and insulin response.
Low-glycaemic-index diets may provide modest metabolic or weight-management benefits for some people, but the evidence does not justify a general recommendation to avoid all pasta, rice, potatoes, bread or fruit. Whole fruit should not be equated with fruit juice or foods containing added sugar.
There is no general requirement to stop eating carbohydrates after 18:00. Fat oxidation continues throughout the day and night and is determined by overall energy balance, physical activity, hormonal regulation and the composition of the diet over time—not by a universal evening carbohydrate cut-off.
Some evidence suggests that avoiding large late-night meals or consuming a greater proportion of daily energy earlier in the day may provide small benefits for weight or metabolic outcomes. However, these effects are modest and the certainty of evidence is limited. Total energy intake, dietary quality, regularity and long-term adherence remain more important than an inflexible clock-based rule. (PMC)
An evening meal may contain vegetables, a suitable protein source, whole grains or other high-fibre carbohydrates and an appropriate amount of unsaturated fat. Its composition should be based on the person’s preferences, activity, diabetes treatment, hunger pattern and total daily intake.
Hunger is not caused solely by an empty stomach, and satiation does not result from gastric distension alone. Appetite is regulated by a complex interaction between:
Consequently, a successful eating plan should aim to control excessive hunger rather than assuming that hunger must be completely avoided. Protein, dietary fibre and foods containing a high proportion of water can support satiation, although responses vary between individuals. (Frontiers)
Energy density describes the amount of energy provided per gram of food. Lowering the average energy density of meals can reduce energy intake while allowing a person to consume a satisfying volume of food.
Useful lower-energy-density foods include:
Randomised studies indicate that lowering dietary energy density reduces energy intake without necessarily reducing the total weight or volume of food consumed. (PMC)
Vegetables and salads can generally be eaten in generous portions, but “unlimited” consumption is not always an appropriate description. Oils, dressings, cheese, nuts, fried toppings and sauces can substantially increase the energy content of an otherwise low-energy-density meal.
Highly processed, energy-dense foods should be limited. In a controlled feeding trial, an ultra-processed diet resulted in greater spontaneous energy intake and weight gain than a minimally processed diet, even though the diets were designed to be similar in several presented nutrients.
Sugar-sweetened beverages can provide substantial energy without producing the same degree of satiation as many solid foods. Their intake is associated with higher body weight, and reducing their consumption can support weight management. Water and other unsweetened, low-energy non-alcoholic drinks should therefore be the preferred sources of hydration.
Relevant beverages include:
The statement that liquid calories produce no satiation at all is too absolute. Satiety depends on the type of beverage and its protein, fibre, viscosity and nutrient composition. Nutritionally formulated liquid meal replacements, for example, are different from sugar-sweetened soft drinks. Nevertheless, calories from clear sugary drinks and fruit juice are often poorly compensated for by a subsequent reduction in food intake.
Fruit juice contains vitamins and other compounds but also contains free sugars and less intact fibre than whole fruit. Whole fruit is therefore generally preferable. The energy content of an apple-juice spritzer varies according to the amount of juice used; the product label or recipe should be checked rather than assuming a fixed value.
The frequently repeated claim that an additional 100 kcal every day will inevitably produce exactly 6 kg of weight gain in one year is misleading. It is based on a static calculation that does not account for changes in energy expenditure, body composition and metabolic adaptation. A persistent energy surplus can cause weight gain, but the amount and time course vary between individuals.
Reducing foods and drinks high in added or free sugars is recommended. However, sugar does not have to be replaced automatically with non-sugar sweeteners.
Replacing a sugar-sweetened drink with a low- or no-calorie alternative may reduce energy intake in the short term when the remainder of the diet is unchanged. Nevertheless, the WHO advises against relying on non-sugar sweeteners as a long-term strategy for weight control or prevention of noncommunicable disease. A preferable long-term approach is often to reduce the overall preference for highly sweet foods and drinks and to choose water or unsweetened beverages more frequently. (World Health Organisation)
This WHO recommendation does not state that approved sweeteners are toxic at permitted intake levels. It concerns their use as a long-term weight-management strategy rather than their regulatory safety.
Alcohol provides approximately 7 kcal per gram, can contribute substantially to total energy intake and may reduce dietary self-regulation. Reducing alcohol consumption can therefore support weight reduction and improve other health outcomes.
The total amount and frequency of alcohol consumption are more important than beverage strength alone. Spirits may contain less energy per serving than some large beers or cocktails, but mixers and serving size can add considerable energy. Alcohol should not be recommended for cardiovascular protection.
A Mediterranean-style dietary pattern can be used to implement the above recommendations. It typically emphasises:
When it creates an energy deficit, a Mediterranean diet can support weight loss. Even without severe energy restriction, it can improve several cardiometabolic risk factors. In the PREDIMED randomised trial, Mediterranean dietary patterns supplemented with extra-virgin olive oil or nuts reduced the incidence of major cardiovascular events among people at high cardiovascular risk. This finding should be described as a reduction in cardiovascular events rather than as definitive proof that the diet reduces cardiovascular mortality in every population.
A sustainable intervention may include:
The intervention should be reviewed regularly. Outcomes may include not only body weight, but also waist circumference, glycaemic control, blood pressure, lipid levels, liver health, physical function, eating behaviour, quality of life and the feasibility of maintaining the dietary pattern.
Medication may need to be adjusted when energy or carbohydrate intake changes substantially, particularly in people treated with insulin, sulfonylureas or blood-pressure medication. More specialised nutritional advice is required for people with chronic kidney disease, liver disease, pregnancy, eating disorders, food allergies, malabsorption or obesity-related sarcopenia.