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Abstract

• This article reviews popular diets for their ability to produce effective weight loss.
  
• Most of the "evidence" for fad diets is based on anecdotal findings, theories and testimonials of short term results.
  
• The most prominent elements of fad diets are those of ritual and sacrifice. These diets offer quick and painless weight loss while allowing consumption of favourite or tasty foods, but place severe restrictions on certain other foods or food categories.
  
• Fad diets often work in the short term because they are low-kilojoule diets in disguise; that is, energy intake as a result of the diet is lower than the person's requirements.
  
• Successful long term weight loss depends on the consumption over a long period of time of less energy than is expended. The ideal approach is to increase physical activity while modifying eating behaviour to achieve a nutritionally balanced intake.

As a nation, we are fat. According to the latest National Nutrition Survey,³ 64% of Australian men and 47% of women are overweight or obese. The situation is getting worse rather than better — the proportion of overweight or obese adults has increased since 1983⁴ by about 52% for men and 34% for women.

Personal efforts to address the overweight problem, while common, are apparently not working. One in three Australians claim they are on some type of "diet",³ yet energy intake has increased and physical activity levels have decreased.⁵ Mathematical modelling suggests that weight loss is a simple matter, with limited inputs and outputs to be controlled.⁶ Yet most people who successfully lose weight return to their old eating habits, and within two years regain most of the lost weight.⁷ The methodology and design of reported weight-loss studies have been questioned,⁸ especially in studies involving long term follow-up. Weight loss can be a treatment effect (weight loss to improve diabetes control) or it can be an outcome of some other treatment (medication used to produce weight loss), making randomised controlled trials difficult to interpret.

The lure of rapid weight loss promised by each new popular diet is undoubtedly compelling. A survey in the United States found that more than one in five dieters used fad diets.⁹ Fad diets feed into the psyche of people who seek to look better and feel better with the minimum of effort.

Sensible eating for weight loss often does not appeal to people who feel they are already doing the best they can. However, very simple dietary changes, followed diligently, can often produce effective weight loss at a rate that can be maintained over the longer term.¹⁰ Conversely, when weight loss is too fast, changes in body composition, especially the loss of lean body mass, can compound the problem of overweight in the longer term. One large study reported an overall increased risk of major weight gain in the long term (at 6 and 15 years) in those who undertook weight-loss attempts (dieting) at baseline.¹¹ However, these findings do not rule out the potential success and benefit of weight-loss programs which aim to encourage permanent changes in behaviour.

How to recognise a fad diet

The most prominent elements of fad diets are those of ritual and sacrifice. The ritual aspect is to always include, say, grapefruit daily but never add sugar to your beverage (sacrifice). These diets offer quick and painless weight loss while allowing consumption of favourite or tasty foods, but severely restrict certain other foods or food categories.

Why fad diets "work"

This is the only way to lose weight — to consume less energy than the body needs. No magic ingredients, strange food combinations or pseudoscientific formulas will alter this metabolic fact. The rate of weight loss (which reflects shifts in water equilibrium as well) varies depending on the relative proportions of the three major nutrients in the diet — carbohydrate, fat and protein. The macronutrient composition can also affect appetite: high-protein diets can suppress appetite, as can ketosis, which results from severe carbohydrate restriction.⁷

Because energy from food comes only from these nutrients (and alcohol), the number of dietary permutations and combinations is limited. Thus, most diets can be categorised into three main types:

• low carbohydrate with the emphasis on high protein;

• low carbohydrate with the emphasis on high fat; and

• high carbohydrate with an emphasis on low fat.

To complete the picture, some fad diets promote one food or a very limited range of foods, while others may be based on individual characteristics such as blood type or personality, or on an unproven physiological concept (such as cleansing "toxins" from the body).

Fad diets are generally nutritionally unbalanced and lack essential nutrients.¹³ They have the potential for health risks. A major problem is that the unfounded nutritional theories espoused with these diets undermine sound nutrition education and public awareness of the importance of healthy long term eating combined with regular physical activity.

Our metabolic flexibility has limits

With limited carbohydrate in the diet, once carbohydrate (glycogen) stores have been used the only source of glucose available to the body is that derived from the carbon skeletons of amino acids. In this situation, and in the absence of sufficient dietary protein, body protein (lean body mass) is catabolised to provide glucose. Muscle mass will therefore decline markedly on a very low carbohydrate, restricted protein diet. The ideal weight-loss diet should provide enough carbohydrate to prevent net protein catabolism, enough good quality protein to meet the normal needs of protein turnover, and enough fat to meet essential fatty acid requirements.

Low carbohydrate diets

Because the body's demand for glucose is constant, body glycogen stores are mobilised in the early phases of a low carbohydrate diet, and for each gram of glycogen lost two to four grams of intracellular water are lost (intracellular water maintains isotonicity). Consequently, there is greater water and hence weight loss in the early days of this type of diet. Water equilibrium is re-established in the second and subsequent weeks, so that, in the longer term, weight loss simply reflects the energy deficit. Energy-nitrogen balance studies have demonstrated that the greater weight loss on a low carbohydrate, high fat diet is accounted for by losses in body water.⁷

If carbohydrate restriction is severe (for example, less than 60 g), ketosis can result, which decreases appetite and causes nausea, but can also cause hyperuricaemia as ketones compete with uric acid for renal tubular excretion.⁷

Popular low carbohydrate, high protein diets include the Zone Diet,¹⁴ the Carbohydrate Addict's Diet,¹⁵ and the Sugar Busters! diet.¹⁶ A popular low carbohydrate, high fat diet that has been around since the 1970s is the Dr Atkins diet.¹⁷

Low carbohydrate, high protein diets

There is also evidence that higher-protein diets are more satiating. People feel fuller and eat less after a meal with a high protein content (31%-54% energy).^18-20 A low fat, higher-protein diet (25% of energy) has also been found to produce a significantly reduced energy intake and greater weight and fat loss over six months compared with a low fat diet with 12% energy from protein.²¹ However, energy restriction is responsible for the weight loss.

An additional problem of high protein diets is the extra solute load placed on the kidneys owing to greater production of nitrogen waste products, particularly in situations of high water loss from perspiration or inadequate fluid intake contributing to dehydration.⁷ In the long term, very high protein diets may increase the risk of osteoporosis in people with inadequate calcium intake by increasing calcium excretion.^7,22

Low carbohydrate, high fat diets

Common consequences of following this type of diet include dehydration, diarrhoea, weakness, headaches, dizziness and bad breath. Over the longer term, such a diet can increase the risk of atherosclerosis — one study has shown that this diet increases serum cholesterol levels and may increase the risk of coronary heart disease by more than 50% with long term use.²

This type of diet also does not include sufficient fruits and vegetables for good health and promotes the misconception that energy intake is not important.

High carbohydrate, very low fat diets

However, if lean meat and fish and low fat dairy products are allowed only in tiny amounts (eg, as "condiments" only), there is the risk of inadequate intakes of calcium, iron, zinc and high quality protein. The Pritikin diet,²⁴ for instance, recommends that fat intake be less than 10% of energy intake, which is likely to be unpalatable for many people used to a Western diet and is close to the lower limit of our requirement for essential fatty acids. The Pritikin diet is also quite low in protein in one of its forms (Maximum Wt Loss), so the quality of any protein present becomes important.

A US study of popular diets has demonstrated that diet quality (measured by dietary variety and intake of five food groups, fat, saturated fat and sodium) is higher in high carbohydrate diets and lowest in low carbohydrate diets.²⁵ The same study showed body mass index is lower in people following high carbohydrate diets and highest in people on low carbohydrate diets.

Other types of fad diets

How to advise patients

Energy needs for weight loss are best established by determining the energy needs of the person at their desired weight and then providing for a weekly energy deficit of about 30 MJ, or 4.2 MJ (1000 kcals) per day. This usually means a suggested energy intake of around 5 MJ (1200 kcals) per day for a woman and up to 8 MJ (1900 kcals) per day for a man.

All foods should be allowed, with an emphasis on fibre-rich carbohydrate foods (cereals, breads, fruit and vegetables), fish and other seafood, lean meat and low fat dairy foods, with small amounts of unsaturated fat as oil or margarine. Behaviour modification to help control impulsive eating is also useful.

The eating plan should be based on the principles of the Australian Guide to Healthy Eating.²⁸ Increased and regular physical activity adds substantially to the success of weight loss programs, so regular physical activity should be encouraged.²⁹

Acknowledgements

I wish to thank Ms Toni Irwin (APD), Dietitian/Nutritionist, for her help and assistance in the preparation of this article.

References

1. Egger G, Cameron-Smith D, Stanton R. The effectiveness of popular, non-prescription weight loss supplements. Med J Aust 1999; 171: 11-12.
2. Anderson JW, Konz EC, Jenkins DJ. Health advantages and disadvantages of weight-reducing diets: a computer analysis and critical review. J Am Coll Nutr 2000; 19: 578-590.
3. Australian Bureau of Statistics and Commonwealth Department of Health and Family Services. National nutrition survey: selected highlights, Australia. Canberra: ABS, 1997. (Catalogue no. 4802.0.)
4. National Heart Foundation of Australia. Risk factor prevalence study No. 2. Canberra: NHF 1983.
5. Armstrong T, Bauman A, Davies J. Physical activity patterns of Australian adults. Results of the 1999 National Physical Activity Survey. Canberra: Australian Institute of Health and Welfare, 2000. (Catalogue no. CVD-10.)
6. Kozusko F. A setpoint based dieting model. Math Comput Model 1999; 29: 1-7.
7. Denke M. Metabolic effects of high-protein, low-carbohydrate diets. Am J Cardiol 2001; 88: 59-61.
8. Lean ME. Is long-term weight loss possible? Br J Nutr 2000; 11 Suppl 1: s103-s111.
9. Jeffery RW, Folsom AR, Luepker RV, et al. Prevalence of overweight and weight loss behavior in a metropolitan adult population: the Minnesota Heart Survey experience. Am J Public Health 1984; 74: 349-352.
10. Goodrick G, Poston WS, Foreyt J. Methods for voluntary weight loss and control: update 1996. Nutrition 1996; 12: 672-676.
11. Korkeila M, Rissanen A, Kaprio J, et al. Weight-loss attempts and risk of major weight gain: a prospective study in Finnish adults. Am J Clin Nutr 1999; 70: 965-975.
12. Stein K. High-protein, low-carbohydrate diets: do they work? J Am Diet Assoc 2000; 100: 760-761.
13. Fisher MC, Lachance PA. Nutrition evaluation of published weight-reducing diets. J Am Diet Assoc 1985; 85: 450-454.
14. Sears B, Lawren B. The Zone — a dietary road map. New York: Harper Collins, 1995.
15. Heller RF, Heller RF. The carbohydrate addict's diet. The lifelong solution to yo-yo dieting. London: Reed International, 1992.
16. Steward HL, Bethea MC, Andrews SS, Balart LA. Sugar Busters! London: Random House, 1998.
17. Atkins R. Dr Atkins' new diet revolution. New York: Avon, 1992.
18. Stubbs RJ. Macronutrient effects on appetite. Int J Obes Relat Metab Disord 1995; 19 Suppl 5: s11-s19.
19. Stubbs RJ, Ritz P, Coward WA, Prentice AM. Covert manipulation of the ratio of dietary fat to carbohydrate and energy density: effect on food intake and energy balance in free-living men eating ad libitum. Am J Clin Nutr 1995; 62: 230-237.
20. Stubbs RJ, Harbron CG, Murgatroyd PR, Prentice AM. Covert manipulation of dietary fat and energy density: effect on substrate flux and food intake in men eating ad libitum. Am J Clin Nutr 1995; 62: 316-329.
21. Skov AR, Toubro S, Ronn B, et al. Randomized trial on protein vs carbohydrate in ad libitum fat reduced diet for the treatment of obesity. Int J Obes Relat Metab Disord 1999; 23: 528-536.
22. Osteoporosis prevention, diagnosis, and therapy. NIH Consensus Development Panel on Osteoporosis Prevention, Diagnosis, and Therapy. JAMA 2001; 285: 785-795.
23. Friedman RB, Kindy P Jr., Reinke JA. What to tell patients about weight-loss methods. 1. Diets. Postgrad Med 1982; 72: 73-80.
24. Pritikin N. The Pritikin permanent weight loss manual. New York: Grosset and Dunlap, 1981.
25. Kennedy ET, Bowman SA, Spence JT, et al. Popular diets: correlation to health, nutrition, and obesity. J Am Diet Assoc 2001; 101: 411-420.
26. D'Adamo P. Eat right for your type. London: Century, 1997.
27. Cabot S. The liver cleansing diet. Sydney: Women's Health Advisory Service, 1996.
28. Population Health Division, Commonwealth Department of Health and Aged Care. Australian Guide to Healthy Living. <http://www.health.gov.au/pubhlth/ strateg/food/guide>. Accessed 7 November 2001.
29. Population Health Division, Commonwealth Department of Health and Aged Care. National physical activity guidelines for Australians [brochure]. Available at <http://www.health.gov.au/pubhlth/publicat/document/physguide.pdf>.

Authors' details

Reprints will not be available from the author.
Correspondence: Professor D C K Roberts, School of Health Sciences, University of Newcastle, Newcastle, NSW 2308. david.robertsATnewcastle.edu.au

©MJA 2001
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