Weight-loss diets

Written by: 
Dr Sarah Brewer

Reasons to lose excess weight and how to go about it.

If you are obese you have an increased risk of developing a number of health problems, including type 2 diabetes, coronaryRelating to the arteries supplying the heart itself. heart disease, stroke and cancerAbnormal, uncontrolled cell division resulting in a malignant tumour that may invade surrounding tissues or spread to distant parts of the body..1 This is especially true if excess fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. is stored around your waist.

The good news is that if you can lose some of this excess weight, you can improve your bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. pressure, glucose toleranceThe ability of the body to maintain a normal glucose level following the ingestion of glucose. and cholesterolA substance present in many tissues and an important constituent of cell membranes although high concentrations of a certain type of cholesterol in the blood are unhealthy. levels and significantly improve your health.

It is estimated that every 1kg (2.2lb) in weight lost by people aged 65 with type 2 diabetes increases their lifespan by 3-4 months. Losing 10kg (22lb) in excess fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. can therefore wipe out the 35 per cent loss of life expectancy that is associated with a diagnosisThe process of determining which condition a patient may have. of diabetes after that age.2

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Types of diet available

Unfortunately, losing weight is not easy. It involves making big changes to your usual diet and lifestyle habits: essentially, this means you need to cut back on your food intake and exercise more.

A number of different weight-loss plans are available, and if you can find one that suits you, you are more likely to stick with it and succeed.

Low-calorie diets

A low-calorie diet restricts the amount of energy you eat and typically provides between 1,200 and 1,800kcal per day; women are allocated fewer kilocalories than men.

On average, adults following a low-calorie diet lose around 6.5kg (14lb) of weight over a 6-12 month period.3 After 12 months, however, the average weight loss is reduced to 3.4kg (7.5lb).

Counting calories can be time-consuming and it is easy to underestimate the amount you've eaten. Ideally, you need to weigh portions of food initially to help you gauge the calorie content, for example, the size of a piece of cheese containing 120kcal of energy.

Calories and kilocalories: 
Strictly speaking, one calorie is defined as the amount of heat energy needed to raise the temperature of 1g of water by 1 degree centigrade (from 15 degrees C to 16 degrees C). The unit commonly used in nutrition is the kilocalorie or kcalA unit of energy. (which, confusingly - and wrongly - is often referred to as a calorie). One kcalA unit of energy. is equivalent to 1,000 calories.

Very low-calorie diets

Very low-calorie diets (VLCDs) provide fewer than 800kcal per day. Those providing as little as 400 to 500kcal per day should be followed only under professional supervision by a trained counsellor.

VLCDs typically involve consuming vitamin and mineral-enriched drinks and meal-replacement products. These products are carefully balanced to provide all the necessary micronutrients.

VLCDs are more effective for rapid weight loss than low-calorie or low-fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. diets. They can produce weight losses of as much as 13.4kg (30lb) after 12 months.4

The combined results from six clinical trials found that VLCDs were more effective than low-calorie diets for short-term weight loss (people lost 16.1 per cent versus 9.7 per cent of their initial weight), but produced similar long-term results (6.3 per cent versus 5 per cent weight loss).5

Low-fat diets

A low-fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. diet is one in which 30 per cent or less of your daily energy comes from fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body.. This is the type of diet traditionally favoured by healthcare professionals. Such diets are based on the fact that, weight for weight, dietary fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. supplies more than twice as many calories (9kcal per gram) as either protein Compounds that form the structure of muscles and other tissues in the body, as well as comprising enzymes and hormones.(4kcal per gram) or carbohydrate (3.75kcal per gram).

Cutting back on your fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. intake therefore reduces your calorie intake more effectively than cutting back on other food groups. The average weight loss for someone on a low-fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. diet after 6 months is 5.08kg (11.2lb), but after 12 months this is reduced to 2.3kg (5lb) overall because some of the weight creeps back on.3 In those following a low-fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. diet in the long term, the average weight loss is 3.5kg (7.7lb) after a total period of 36 months.4

This type of diet can improve bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. pressure, cholesterolA substance present in many tissues and an important constituent of cell membranes although high concentrations of a certain type of cholesterol in the blood are unhealthy. balance and glucoseA simple sugar that is an important source of energy in the body. control, and may help to prevent type 2 diabetes and reduce the use of blood pressure medication.

See more information on blood pressure management and cholesterol.

Low-GI diets

If you follow a low glycaemic index (GI) diet, you restrict your intake of processed carbohydratesA group of compounds that are an important energy source, including sugars and starch. that are rapidly digested and absorbed. These carbohydratesA group of compounds that are an important energy source, including sugars and starch.:

  • produce rapid increases in bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. glucoseA simple sugar that is an important source of energy in the body. levels
  • increase insulinA hormone produced by the beta cells of the pancreas that acts to lower blood glucose levels. secretion
  • promote fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. storage.

This type of diet focuses instead on eating more slowly-digested carbohydratesA group of compounds that are an important energy source, including sugars and starch., such as those found in whole grains, fruits and vegetables.

The combined results of six trials suggest that people following a low GI diet for a period of between 5 weeks and 6 months lose 1.1kg (2.4lb) more weight than those following other types of diet. Greater improvements were also seen in their cholesterol levels.6

Low-carbohydrate, high-protein diets

In contrast to low-GI diets, low carbohydrate diets restrict your total intake of all carbohydratesA group of compounds that are an important energy source, including sugars and starch., including those that are allowed on a low GI diet such as the carbohydratesA group of compounds that are an important energy source, including sugars and starch. found in whole grains and fruits.

Initially, your carbohydrate intake is restricted to no more than 20g per day and is slowly increased, mainly in the form of low glycaemic carbohydratesA group of compounds that are an important energy source, including sugars and starch. such as salad ingredients, vegetables, nuts, seeds and berries. The high protein content of these diets helps to curb the appetite and reduce overall food intake. It may also increase the amount of energy lost as heat (a process called thermogenesisHeat production.).7

The combined results of five trials found that, after 6 months, those following a low-carb diet lost an average of 3.3kg (7.3lb) more than those following a low-fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. diet. However, after 12 months the difference was reduced to 1kg (2.2lb).8 Greater improvements were also seen in their triglycerideA type of fat in the bloodstream, formed from the digestion of fat in the diet. and 'good' HDL-cholesterolA substance that mainly exists to carry cholesterol from the circulation to the liver; commonly referred to as 'good cholesterol'. levels, while the total cholesterolA substance present in many tissues and an important constituent of cell membranes although high concentrations of a certain type of cholesterol in the blood are unhealthy. and 'bad' LDL-cholesterolSubstance that carries cholesterol around the bloodstream, a form of so-called 'bad cholesterol'. improved more in those on low-fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. diets.

See more information on 'good' and 'bad' cholesterol.

Drugs and psychotherapy

Weight-loss drugs can help some people to lose weight. One such drug, orlistat, blocks the action of digestive enzymes (lipases) needed for the breakdown of dietary fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body., with the result that less fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. is absorbed. Over 4 years, people using orlistat lose 2.2kg (4.8lb) more weight than those on a placebo, with significantly more people losing at least 10 per cent of their initial body weight.9

Another drug, called sibutramine, affects certain brain chemical messengers to reduce food intake, and also reduces the fall in metabolic rate associated with weight loss. Two-thirds of those using sibutramine lose at least 5 per cent of their initial body weight.10 On average, it produces a 2.2kg (4.8lb) greater weight loss than orlistat.11

The side effects of orlistat include oily leakage from the rectum, flatulence, a need to defecate urgently, oily stools, faecal incontinenceThe involuntary passage of urine or faeces., distension of the abdomenThe part of the body that contains the stomach, intestines, liver, gallbladder and other organs. and pain, while the side effects of sibutramine include constipation, dry mouth, nausea, taste disturbances, diarrhoeaWhen bowel evacuation happens more often than usual, or where the faeces are abnormally liquid. and vomiting.

Psychological techniques, such as cognitive behaviour therapy (CBT), can increase weight loss by 4.9kg (10.8lb) more than the use of diet and exercise alone.12

Exercise

Exercise:

  • increases your metabolic rate
  • mobilises fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body.
  • speeds up the rate at which fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. is burned in muscleTissue made up of cells that can contract to bring about movement. cells.

Although the combined results of 43 studies show that exercise on its own can result in small weight losses, when weight-loss diets and exercise are combined, the weight loss is increased by 1.1-1.5kg (2.4-3.3lb) compared with diet alone, depending on the intensity of the exercise.13

Exercise also reduces bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. pressure, triglycerides and glucose toleranceThe ability of the body to maintain a normal glucose level following the ingestion of glucose., even if no weight is lost.

References: 
  1. Huxley R, Mendis S, Zheleznyakov E, et al. 'Body mass index, waist circumference and waist:hip ratio as predictors of cardiovascular risk-a review of the literature.' Eur J Clin Nutr. 2009 Aug 5. [Epub]
  2. Lean ME, Powrie JK, Anderson AS, et al. 'Obesity, weight loss and prognosis in type 2 diabetes.' Diabetic Med. 1990; 7(3): 228-233.
  3. Summerbell CD, Cameron C, Glasziou PP. 'WITHDRAWN: Advice on low-fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. diets for obesityExcess accumulation of fat in the body..' Cochrane Database Syst Rev. 2008; (3): CD003640.
  4. Avenell A, Brown TJ, McGee MA, et al. 'What are the long-term benefits of weight reducing diets in adults? A systematic review of randomized controlled trials.' J Hum Nutr Diet. 2004; 17(4): 317-35.
  5. Tsai AG, Wadden TA. 'The evolution of very-low-calorie diets: an update and meta-analysis.' Obesity. 2006; 14(8): 1283-93.
  6. Thomas DE, Elliott EJ, Baur L. 'Low glycaemic index or low glycaemic load diets for overweight and obesityExcess accumulation of fat in the body..' Cochrane Database Syst Rev. 2007; (3): CD005105.
  7. Halton TL, Hu FB. 'The effects of high protein diets on thermogenesisHeat production., satiety and weight loss: a critical review.' J Am Coll Nutr. 2004; 23(5): 373-85.
  8. Nordmann AJ, Nordmann A, Briel M, et al. 'Effects of low-carbohydrate vs low-fatOne of the three main food constituents (with carbohydrate and protein), and the main form in which energy is stored in the body. diets on weight loss and cardiovascular risk factors: a meta-analysis of randomized controlled trials.' Arch Intern Med. 2006; 166(3): 285-93.
  9. Ioannides-Demos LL, Proietto J, McNeil JJ. 'Pharmacotherapy for obesityExcess accumulation of fat in the body..' Drugs. 2005; 65(10): 1391-418.
  10. Finer N. 'Sibutramine: its mode of action and efficacy.' Int J Obesity. 2002; 26(4): S29-S33.
  11. Neovius M, Johansson K, Rossner S. 'Head-to-head studies evaluating efficacy of pharmaco-therapy for obesityExcess accumulation of fat in the body.: a systematic review and meta-analysis.' Obes Rev. 2008; 9(5): 420-7.
  12. Shaw K, O'Rourke P, Del Mar C, et al. 'Psychological interventions for overweight or obesityExcess accumulation of fat in the body..' Cochrane Database Syst Rev. 2005; (2): CD003818.
  13. Shaw K, Gennat H, O'Rourke P, et al. 'Exercise for overweight or obesityExcess accumulation of fat in the body..' Cochrane Database Syst Rev. 2006; (4): CD003817.