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Low carb diets

Supermarket shelves contain an increasing number of low carbohydrate products, because of the popularity of the Atkins and South Beach diets. Shairoz Samji looks at how low carbohydrate diets works and takes a look at their consequences for health

The Atkins approach is a low carbohydrate high fat high protein diet. Although named after Robert Atkins, its origins can be traced back to 1862.¹Dr Atkins' New Diet Revolution--a 504 page long guide--works on one major assumption. If insulin is the main hormone that causes the body to turn any excess sugars into fat, when its production is minimised, Atkins suggested that all fat production will cease. By preventing the body from being supplied primarily by sugars, it could instead begin to burn fats and proteins leading to a process called ketosis. If Dr Atkins' theory is right, someone following the Atkins diet will use their own fat as a source of energy and therefore lose weight.

Plan B

Physiological changes including ketosis induced by a low carbohydrate diet are similar to those induced by starvation. During the Atkins diet, the body is thought to switch to lipid metabolism--the body's "backup plan." Ketones are formed and used by the central nervous system and other organs. This yields the initial rapid weight loss that is seen on the Atkins diet. However, long term reliance by the body on lipid metabolism may have detrimental consequences (table).

Atkins recommends that sugars must be controlled, initially to 20 g a day.² Devotees are free to enjoy almost as much protein and fat as they wish, however.

Dish of the day

The book gives advice about menu ideas and what foods should and should not be avoided. Interestingly, Atkins recommends that all participants should take nutritional substitutes, which may be purchased online on the Atkins website. Other recommendations made include ceasing prescription medication,² for example, the pill, antidepressants, insulin, and antiarrhythmics, as he suggests that these may halt the switch to lipid metabolism. But is this really a good idea? The author does admit to some problems with the diet including bad breath, constipation (fibre supplements are recommended), and leg cramps,² but he does say that the diet will boost energy levels.

On page 12 of the book, Atkins claims, "The Atkins Nutritional Approach can and has corrected these serious risk factors [diabetes, heart disease and high blood pressure] associated with obesity."² Is this too good to be true? The Atkins diet is thought to work (like most diets) primarily by calorie restriction.³ If we consume fewer calories than we expend, we will by definition lose weight. Although it is well known that fats contain more energy than carbohydrates, if a diet is limited to primarily fats and proteins eventually the brain will suppress the appetite to such a level, that fewer calories will be consumed.

For better or worse

Two papers published in May 2003 trialled the low carbohydrate diet, comparing it with low fat diets using obese subjects.^{4 5} They found that people on the carbohydrate diet had lost significantly more weight at six months. Yet, there was no significant difference in weight loss between the two diets at 12 months.⁵ It was suggested that this could be attributed to people quitting the diet early, possibly because of the difficulties met in avoiding carbohydrates. Another explanation is that low fat diets gradually lead to the same weight loss rather than just the initial "burst" of weight loss experienced in the Atkins diet.

Both papers reported that the Atkins diet improved markers of coronary artery disease by increasing high density lipoprotein cholesterol concentrations and decreasing triglyceride concentrations.⁴⁵ They also showed an improvement in insulin sensitivity after six months and decreased satisfaction due to the effects of ketosis.

Long term risks are more difficult to determine directly as no studies have trialled the Atkins diet for longer than a year. By studying the diet, (focusing on the initial weight loss phase) and investigating each factor further, it may be possible to hypothesise potential risks.

Contradictions

The unlimited amount of fat in this diet is at direct odds with advice given by healthcare professionals, founded after decades of research.⁶ A high saturated fat diet raises cholesterol and low density lipoprotein levels. These factors inevitably lead to the development of hypertension, obesity, and an increased risk of cardiovascular disease⁶--all big killers in the west.

Obese people predisposed to hypertension and hypercholesterolaemia are far more likely to experience ill effects on the cardiovascular system and exacerbate their pathology further if they choose to follow this diet.

Certain aspects of this diet could predispose a person to malignancy. The lack of fruit and fibre may cause bowel cancer, but this is a contentious issue. One study showed a dramatic decrease in fibre intake contributed to a fivefold increase in the incidence of colorectal cancer in Japan.⁷ Other cohort studies have reported that consumption of fruit, vegetables and fibre does not seem to offer protection against colorectal cancers.⁸⁹ Research also points towards links between breast cancer and high fat diets.^{10 11}

Stones and bones

In healthy people, high protein diets may have little or no effect on renal function. Obese people, however, may lack normal renal function and a diet high in non-dairy animal protein has been shown to accelerate this decline.¹² Furthermore, the Atkins diet could cause a rise in the incidence of kidney stones.¹³

Experimental evidence using transgenic mice engineered to be predisposed to diabetes has shown that a high protein diet will accelerate the appearance of the disease.¹⁴ This increased risk is supported by epidemiological data.¹⁵ A high protein diet could also cause a build up of ammonia in the reproductive tract of mice.¹⁶ This increase in ammonia has been shown to harm fetal development in the womb.

The Atkins diet provides a lower than recommended calcium intake, which is detrimental to bone density.¹⁷ A high protein diet can cause an imbalance in the body's acid-base equilibrium resulting in bone that is chronically leached of calcium. Theoretically osteoporosis will ensue.¹⁸

This purine rich diet predisposes the individual to developing gout. It is also possible that the state of ketosis will result in decreased thyroid function.¹⁹

More than three million people in the United Kingdom are estimated to have tried the Atkins diet,²⁰ and a television programme using doctors as candidates suggested that incomplete adherence to the diet may have fatal consequences (The Real Story, BBC1, 1 Nov 2003).

The Atkins industry

Indications show that compliance to the diet is low. Like all diets it may not be suited to everyone. Most snack foods contain added sugars and other ingredients that do not uphold the diet. This has lead to the emergence of a new "Atkins industry" with the introduction of low carbohydrate biscuits, pasta, and other foods. And, as Atkins devotees have realised, this diet is more expensive than most.²¹

One important point to remember is that the diet does allow the follower to lose weight. This is believed to be crucial in decreasing the risk of many diseases. The diet is also well tailored, so that once someone has lost a desired amount of fat, this preferred weight can be maintained in the long term. The advantages of losing weight must be weighed up against the risks that this diet may have for clinically obese people.

Other diets have followed a similar "anti-carbohydrate" trend. One up and coming offshoot of the Atkins approach is the South Beach diet. It too bans "bad carbohydrates" in its early phase and then focuses on the glycaemic index--a measure of how quickly foods raise blood sugar concentrations. In general, "bad carbohydrates" (white bread, refined cereal, and potatoes) have a high glycaemic index whereas oats and sweet potato have a lower index.

The jury is still out on low carbohydrate diets. The National Institute for Clinical Excellence will be issuing its own advice as part of its obesity guidelines in January 2007.

Shairoz Samji, fourth year medical student, University of Bristol
Email: ss0746@bristol.ac.uk

I thank Dr Parfitt, Consultant Endocrinologist Frenchay Hospital, Bristol, Dr Sue Baic, Department of Nutrition, University of Bristol, and Lauren Hughes for all their time and help.

studentBMJ 2004;12:437-480 December ISSN 0966-6494

1. Groves B. William Banting: the father of the low carbohydrate diet. In: Weston A. Wise traditions in the food, farming and the healing arts. Price Foundation. 2002
2. Atkins R. Dr Atkins’ New Diet Revolution. London: Vermilion, 2003.
3. Jeor S, Howard B, Prewitt E, Bovees V, Bazzarre T, Eckel R. Dietary protein and weight reduction: a statement for healthcare professionals from the nutrition committee of the council on nutrition, physical activity and metabolism of the American Heart Association. Circulation 2001;104:1869-74.
4. Samaha F, Iqbal N, Seshadri P, Chicano K, Daily D, McGrory, et al. A low carbohydrate as compared with a low fat diet in severe obesity. N Eng J Med 2003;348:2074-81.
5. Foster G, Holly R, Hill J, McGuckin B, Brill C, Mohammed S, et al. A randomised trial of a low carbohydrate diet for obesity. N Eng J Med 2003;348:2082-90.
6. World Health Organization. Diet Nutrition and the prevention of chronic diseases. Geneva: WHO, 2003. (No. 916.)
7. Key T, Allen N, Spencer E, Travis R. The effect of diet on risk of cancer. Lancet 2002;360:86-68.
8. Michels K, Giovanucci E, Joshipura K, Rosner B, Stampfer M, Fuchs C. Prospective study of fruit and vegetable consumption and incidence of colon and rectal cancers. J Natl Cancer Inst 2000;92:1740-52.
9. Fuchs C, Giovanucci E, Colditz G, Hunter D, Stampfer M, Rosner B. Dietary fiber and the risk of colorectal cancer and adenoma in women. N Engl J Med 1999;340:169-76.
10. Boyd N, Stone J, Vogt K, Connelly B, Martin L, Minkin, S.. Dietary fat and breast cancer risk revisited: a meta-analysis of published literature. Br J Cancer 2003;89:1672-85.
11. Vlajinac H, Marinkovic J, Ilic M, Kocev N. Diet and prostate cancer: a case-control study. Eur J Cancer 1997;33:101-7.
12. Knight E, Stampfer M, Hankinson S, Spiegelman D, Curhan G.. The impact of protein intake on rena function decline in women with normal renal function or mild renal insufficiency. Ann Int Med 2003;138:460-67.
13. Dwyer J, Madans J, Turnbull B, Cornoni-Huntley J, Dresser C, Everett D, et al. Diet, indicators of kidney disease, ad later mortality among older persons in the NHANES I epidemiological follow-up study. Am J Pub Health 1994;84:1299-1303.
14. Schneider K, Laube H, Linn T. A diet enriched in protein accelerates diabetes maisfestation in NOD mice. Acta Diab 1996 33;236-40.
15. Tsunehara C, Leonetti D, Fujimoto W. Diet of second generation Japanese-American men with and without non-insulin-dependent diabetes. Am J Clin Nutr 1990;52:731-8.
16. Colorado Center for Reproductive Medicine. Study shows high-protein diets reduce fertility in mice. Colorado: CCfRM, 2004. www.colocrm.com/high_protein_diet.htm (accessed 9 Nov 2004).
17. Freedman M, King J, Kennedy E. Popular diets: a scientific review. Obesity Res 2001;9(suppl 1):S1-40.
18. Barzel U, Massey L. Excess dietary protein can adversely affect bone. J Nutr 1998;128:1051-3.
19. Brungger M, Hulter H, Krapf R. Effect of chronic metabolic acidosis on thyroid hormone homeostasis in humans. Am J Physiol 272 Renal Physiol 1997;41:F648-53.
20. BBC News. Government wades into Atkins row. London: BBC, 2003. http://news.bbc.co.uk/1/hi/health/3128188.stm (accessed 9 Nov 2004).
21. Greenwood H, Truby H. Pound for pound? Comparing the costs incurred by subjects following four commercially available weight loss programmes. J Hum Nutr Dietet 2003;16:365-70.