Low Carbohydrate Diets – The Facts

by Dr David Ashton, Medical Director, Healthier Weight

Summary - Low-Carbohydrate Diets

Low-carbohydrate (low carb) diets have been very popular, but what does the scientific evidence say about them?  

 

Here are the facts:

• Low-carb diets do not result in greater long-term weight loss than more    conventional calorie-restricted diets

• Whilst initial weight-loss is greater with low-carb diets, this is due to loss of water,  glycogen and muscle mass, not fat

• Low-carb diets may result in greater loss of lean (muscle) mass than standard diets

• Low-carb diets do not result in better weight maintenance than other low-calorie  diets

• Low-carb diets may increase the risk of various cancers – especially bowel cancer

• Low-carb diets may increase the risk of kidney stones and osteoporosis

• Low-carb diets have a beneficial impact on some heart disease risk factors, but a  negative effect on others.  The long term implications for heart health are  unknown.

• The long-term safety of low-carb diets has not been established

Introduction

Carbohydrate is the big buzz word in dieting.  Both low-carbohydrate (CHO) and low-glycaemic index (GI) diets have been enormously popular in recent years, though interest in some versions – notably Atkins – is clearly beginning to wane.  Despite their popularity and the inevitable celebrity endorsement the long-term efficacy and safety of these diets has yet to be established.  

There are two important questions to ask about low carbohydrate or low GI diets:

1.     Do they work?
2.     Are they safe?

Low Carbohydrate Diets
Low carbohydrate diets are considered to contain less than 100g of carbohydrate/day. 
Whilst this type of diet is hardly new, recent versions are Atkins, The CHO Addict’s Diet, Protein Power Diet, Sugar Busters Diet and the South Beach Diet. 

When dietary carbohydrate is restricted the body begins to break down fat stores in order to meet its energy requirements.  When fatty acids are broken down they produce various chemicals called ketones, which are present in the blood, urine and breath. (NB: If you have been near anyone on the Atkins diet, you will know only too well what ketotic breath smells like).
 

1.     Effect of low carbohydrate diets on nutrition
One of the problems with weight reduction by the use of low-carb diets is that they restrict food choices and may not, therefore, provide properly balanced nutrition.  The generally low consumption of fruits, vegetables and whole grain products reduces the overall intake of dietary fibre, vitamins, calcium, potassium, magnesium and iron. 

The Continuing Survey of Food Intake by Individuals Study (CSF II 1994-1996) examined the relationship between popular diets and diet quality.  The study showed that high carbohydrate diets (>55% energy from Carbohydrates) gave the highest diet adequacy score (82.9) whereas low-carb diets (<30% of energy from carbohydrates) gave the lowest score (44.6)1,2
The increase in meat consumption, together with the low intake of fruits and vegetables often associated with low-carb diets, could theoretically place an individual at increased risk of certain types of cancer – especially bowel cancer -     if such a diet is followed long-term3-4

2.    Effects on kidneys and bone health
There is some evidence that ketones (see above) may help to suppress appetite which may account for the rapid weight loss often associated with low-carb diets5.  However, there is also a concern that ketosis can have adverse effects on calcium metabolism leading to an increased risk of kidney stones and osteoporosis 6-8

3.    Effects on mood, fatigue and exercise capacity
A recent study showed that blood ketones may also increase fatigue, alter mood and reduce exercise capacity in individuals on low-carb diets9

4.    Effects on weight loss and body composition.
In the short term, most people who go on low-carb diets do lose weight and they lose it very quickly. However, most of the weight loss in the early stages comes from loss of glycogen, water and muscle tissue, not fat which is what you need to lose to keep the weight off.  There is some evidence to suggest that loss of muscle mass is more pronounced in those on low-carb diets than on conventional low-fat diets10.  This loss of lean mass is important, because if you are trying to lose weight permanently, losing precious lean muscle tissue is like sabotaging your own body. Muscle tissue is metabolically active, and burns calories even when you are at rest.  So if you lose muscle, you are in effect reducing the number of calories you need each day to maintain your weight, making it much harder to keep your weight under control when you stop following the low-carb diet. 

Moreover, even though low-carb diets may produce more weight loss initially, in the longer term there is no evidence that they produce greater weight loss than standard low-calorie diets.  Two recent randomised trials found that after 1 year, weight loss in individuals who followed either a high or a low-carb diet was similar11,12
   
In addition, a major systematic review of studies published between 1996 and 2003 on the safety and efficacy of low-carb diets concluded that participant weight loss was primarily due to reduced calorie intake and duration of the diet, but had almost nothing to do with the carbohydrate content.  Accordingly the authors concluded that there is insufficient evidence to recommend such diets13
   
5.    Effects on weight maintenance
Low-carb diets do not appear to be any more effective in maintaining weight loss than a conventional low-fat diet14.

6.    Effects on cardiovascular health
Current evidence suggests that whilst low-carb diets result in improvements in some cardiovascular risk factors such as insulin, glucose, blood pressure and some blood fats (primarily triglycerides), there are negative effects on others15,16.  In addition, blood ketones resulting from low-carb diets have been shown to accelerate damage to blood vessels and other tissues, which in the long term may increase the risk of heart disease17.  Other studies of low-carb diets have shown an increase in inflammation of the blood vessels which may also adversely impact on heart health18. 

 A British Heart Foundation (BHF) study of the impact of low-carb diets on the heart was carried out by researchers at Oxford University.  They found that energy stores in the heart were reduced by around 16% in those who followed a high fat, low-carb diet.  In some people the energy reduction was as much as a third.  The hearts of those on the diet also became slightly ‘stiffer’ – not relaxing as well as before the diet.  Commenting on the findings, BHF Medical Director Professor Peter Weissberg said:
   
 "We would certainly not recommend high-fat low-carb diets to anyone who wants to lose weight and look after their heart”.
   

Summary
There are no long-term data (no longer than 12 months) concerning the efficacy and safety of weight reduction using low-carb diets.  Whilst the available scientific evidence suggests that low-carb diets may have favourable effects on some cardiovascular risk factors, they may have negative effects on others.  In addition, low-carb diets may increase loss of lean body mass and increase the risk of kidney stones and osteoporosis.  Furthermore, because low-carb diets tend to have low levels of fruits, vegetables and whole grains, they may increase the risk of various cancers, especially bowel cancer in both sexes.  Finally, it should be noted that there is no evidence that low-carb diets result in greater long-term weight loss than more conventional calorie-restricted diets and no evidence that they produce better weight maintenance.  

Given that low-carb diets appear to offer no clear advantages over more conventional calorie-restricted diets and the serious lack of scientific evidence regarding their long-term effects, low-carb diets cannot be recommended. 

Scientific References
1.    Kennedy ET, Bowman SA et al.  Popular diets: correlation to health, nutrition and obesity.  J Am Diet Assoc 2001; 101:411-420
2.    U.S. Department of Agriculture ARS, Food and Nutrient Intakes by Individuals in the United States by Sex and by Age, 1994-1996.  Nationwide Food Surveys, report no.96-2, 1998.
3.    Norat T, Bingham S, Ferrari P et al.  Meat, fish and colorectal cancer risk: the European Prospective Investigation into cancer and nutrition.  J Natl Cancer Inst 2005; 97: 906-916
4.    Campos FG, Logullo Waitzburg AG, et al.  Diet and colorectal cancer: current evidence for aetiology and prevention.  Nutr Hosp 2005; 20:18-25
5.    Meckling KA et al.  Effects of a hypocaloric, low-carbohydrate diet on weight loss, blood lipids, blood pressure, glucose tolerance and body composition in free living overweight women.  Can J Physiol Pharmacol 2002; 80: 1095-1105
6.    Westman EC, Yancy WS, Edman JS et al.  Effect of 6-month adherence to a very low-carbohydrate diet program.  Am J Med 2002; 113:30-36
7.    Furth SL, Casey JC, Pyzik PL et al.  Risk factors for urolithiasis in children on the ketogenic diet.  Pediatr Nephrol 2000; 15:125-128
8.    Reddy ST, Wang CY, et al.  Effect of low-carbohydrate high protein diets on acid base balance, stone forming propensity and calcium metabolism.  Am J Kidney Dis 2002; 40:265-274
9.    White AM, Johnston CS, Swan PD et al.  Blood ketones are directly related to fatigue and perceived effort during exercise in overweight adults adhering to low-carbohydrate diets for weight loss: a pilot study.  J Am Diet Assoc 2007; 107:1792-6
10.    Meckling KA, O’Sullivan C, Saari D.  Comparison of a low-fat diet to a low-carbohydrate diet on weight loss, body composition and risk factors for diabetes and cardiovascular disease in fee-living, overweight men and women.  J Clin Endocrinol Metab 2004; 89: 2717-2723
11.    Stern L, Iqbal N, et al. The effects of low-carbohydrate versus conventional weight loss diets in severely obese adults; one-year follow-up of a randomized trial.  Ann Intern Med 2004; 140:778-785
12.    Foster GD, Wyatt HR et al.  A randomized trial of a low-carbohydrate diet for obesity.  New Engl J Med 2003; 348: 2082-2090
13.    Bravata DM, Sanders L, et al.  Efficacy and safety of low-carbohydrate diets: a systematic review.  JAMA 2003; 289:1837-1850
14.    Lecheminant JD, Gibson CA, et al.  Comparison of a low carbohydrate and low fat diet for weight maintenance in overweight or obese adults enrolled in a clinical weight management programme.  Nutr J 2007; 6: 36
15.    Adam-Perrot A, Clifton P, Brouns F.  Low-carbohydrate diets: nutritional and physiological aspects.  Obesity Rev 2006; 7: 49-58
16.    Nordmann AJ, Nordmann A, Briel M et al. Effects of low-carbohydrate vs low-fat diets on weight loss and cardiovascular risk factors; a meta-analysis of randomized trials. 
    Arch Intern Med 2006; 166:285-93
17.    Beisswenger BG, Delucia EM, Lapoint N et al.  Ketosis leads to increased methylglyoxal production on the Artkins diet.  Ann N Y Acad Sci 2005; 1043:201-10
18.    Rankin JW, Turpyn AD.  Low carbohydrate, high fat increases C reactive protein during weight loss.  J Am Coll Nutr 2007; 26: 163-9