1. Weight loss, in the context of medicine, health, or physical fitness, refers to a reduction of the total body mass, by a mean loss of fluid, body fat (adipose tissue), or lean mass (namely bone mineral deposits, muscle, tendon, and other connective tissue). Weight loss can either occur unintentionally because of malnourishment or an underlying disease, or from a conscious effort to improve an actual or perceived overweight or obese state. “Unexplained” weight loss that is not caused by reduction in calorific intake or exercise is called cachexia and may be a symptom of a serious medical condition. Intentional weight loss is commonly referred to as slimming.

2. Unintentional weight loss may result from loss of body fats, loss of body fluids, muscle atrophy, or a combination of these. It is generally regarded as a medical problem when at least 10% of a person’s body weight has been lost in six months or 5% in the last month. Another criterion used for assessing weight that is too low is the body mass index (BMI). However, even lesser amounts of weight loss can be a cause for serious concern in a frail elderly person.

3. Unintentional weight loss can occur because of an inadequately nutritious diet relative to a person’s energy needs (generally called malnutrition). Disease processes, changes in metabolism, hormonal changes, medications or other treatments, disease- or treatment-related dietary changes, or reduced appetite associated with a disease or treatment can also cause unintentional weight loss. Poor nutrient utilization can lead to weight loss, and can be caused by fistulae in the gastrointestinal tract, diarrhea, drug-nutrient interaction, enzyme depletion and muscle atrophy.

4. Continuing weight loss may deteriorate into wasting, a vaguely defined condition called cachexia. Cachexia differs from starvation in part because it involves a systemic inflammatory response. It is associated with poorer outcomes. In the advanced stages of progressive disease, metabolism can change so that they lose weight even when they are getting what is normally regarded as adequate nutrition and the body cannot compensate. This leads to a condition called anorexia cachexia syndrome (ACS) and additional nutrition or supplementation is unlikely to help. Symptoms of weight loss from ACS include severe weight loss from muscle rather than body fat, loss of appetite and feeling full after eating small amounts, nausea, anemia, weakness and fatigue.

5. Serious weight loss may reduce quality of life, impair treatment effectiveness or recovery, worsen disease processes and be a risk factor for high mortality rates. Malnutrition can affect every function of the human body, from the cells to the most complex body functions, including:

– immune response;

– wound healing;

– muscle strength (including respiratory muscles);

– renal capacity and depletion leading to water and electrolyte disturbances;

– thermoregulation; and

– menstruation.

6. In addition, malnutrition can lead to vitamin and other deficiencies and to inactivity, which in turn may pre-dispose to other problems, such as pressure sores.

7. Unintentional weight loss can be the characteristic leading to diagnosis of diseases such as cancer and type 1 diabetes.

8. In the UK, up to 5% of the general population is underweight, but more than 10% of those with lung or gastrointestinal diseases and who have recently had surgery. According to data in the UK using the Malnutrition Universal Screening Tool (‘MUST’), which incorporates unintentional weight loss, more than 10% of the population over the age of 65 is at risk of malnutrition. A high proportion (10–60%) of hospital patients are also at risk, along with a similar proportion in care homes.

9. Intentional weight loss is the loss of total body mass as a result of efforts to improve fitness and health, or to change appearance through slimming. Weight loss is the main treatment for obesity, and there is substantial evidence this can prevent progression from prediabetes to type 2 diabetes with a 7-10% weight loss and manage cardiometabolic health for diabetic people with a 5-15% weight loss.

10. People who are attempting to lose weight can use the calorie allowance of the calculator to help decide how many calories they can consume per day in order to create the deficit needed for weight loss. The US National Institutes of Health provide a calculator that will give you the approximate daily calorie needs to maintain your current weight based on your sex, age, height, weight, and physical activity level. Reducing one’s daily calorie intake below this number can lead to weight loss.

11. Weight loss in individuals who are overweight or obese can reduce health risks, increase fitness, and may delay the onset of diabetes. It could reduce pain and increase movement in people with osteoarthritis of the knee. Weight loss can lead to a reduction in hypertension (high blood pressure), however whether this reduces hypertension-related harm is unclear.

12. Weight loss occurs when the body is expending more energy in work and metabolism than it is absorbing from food or other nutrients. It will then use stored reserves from fat or muscle, gradually leading to weight loss. For athletes seeking to improve performance or to meet required weight classification for participation in a sport, it is not uncommon to seek additional weight loss even if they are already at their ideal body weight. Others may be driven to lose weight to achieve an appearance they consider more attractive. However, being underweight is associated with health risks such as difficulty fighting off infection, osteoporosis, decreased muscle strength, trouble regulating body temperature and even increased risk of death.

13. Typical intentional weight loss methods include dietary changes, increased exercise, use of anti-obesity medications, and bariatric procedures. About 70% of Americans’ dieting attempts are of a self-help nature. However, only 20% of overweight individuals actually take part in a structured weight loss program involving medical care, and the use of anti-obesity medications or bariatric surgery is even lower.

14. Dietary changes are a common strategy for intentional weight loss. Calorie reduction is the underlying principle of most diets. For each individual, the results will be different, with some even regaining more weight than they lost, while a few others achieve a tremendous loss, so that the “average weight loss” of a diet is not indicative of the results other dieters may achieve. While some diets focus on decreasing appetite to reduce eating, many others focus on restricting calories, fat, or carbohydrates.

15. Dieting has been recommended for overweight people in general, and for overweight people with type 2 diabetes in particular, as most of these people are thought to have prediabetes or at increased risk of progressing to this condition. Several diets are effective for weight loss of obese individuals, with diet success most predicted by adherence and little effect resulting from the type or brand of diet. As weight maintenance depends on calorie intake, diets emphasizing certain macronutrients (low-fat, low-carbohydrate, etc.) have been shown to be no more effective than one another and no more effective than diets that maintain a typical mix of foods with smaller portions and perhaps some substitutions (e.g. low-fat milk, or less salad dressing).

16. A meta-analysis of six randomized controlled trials found no difference between low-calorie, low-carbohydrate, and low-fat diets, with a 2–4 kilogram weight loss over 12–18 months in all studies. Extreme diets may, in some cases, lead to malnutrition.

17. While low-carbohydrate diets are widely debated, authorities generally recommend adjusting eating habits to individual needs rather than engaging in a particular fad. Low-carbohydrate diets are among the most popular contemporary diets. Notably, many of the popular low-carbohydrate diets lack scientific evidence. Any weight-loss program must produce a calorie deficit to be effective in reducing weight, and low-carbohydrate diets are no exception.

18. Some experts recommend reducing calorie intake by around 500 calories per day for overweight individuals, as this should lead to a weight loss of about 0.5 kilograms (1.1 lb) per week. One kilogram of human adipose tissue contains approximately 7,000 calories.

19. A very low calorie diet is consuming fewer than 800 calories per day. Such diets are normally followed under the supervision of a doctor. Zero-calorie diets are also included.

Another approach to weight loss is following a low-carb diet. Low-carb diets restrict carbohydrate consumption, often focusing on protein and healthy fats instead. Some popular low-carb diets include the Atkins diet, the ketogenic diet, and the Paleo diet. Proponents argue that low-carb diets can lead to weight loss, reduced appetite, and improved blood sugar control and heart health markers. However, very low-carb diets may lack certain nutrients and can be difficult to sustain long-term. It’s important to consult with a healthcare professional before starting any new diet plan.

20. Excessive exercise and fasting can also put individuals at risk. Health professionals emphasize that any attempts to lose weight should be accompanied by a healthy, balanced diet and regular exercise and consultation with a physician is wise before beginning any new dietary or fitness regimen.