Low-carbohydrate diets have been a subject of discussion for over two decades. They have attracted attention as a means of losing weight and optimising blood glucose control, particularly in people with Type 2 diabetes. However, debate has arisen about whether this approach is both safe and effective.
Diabetes UK has reviewed the evidence from 1998 to 2009 relating to low-carbohydrate diets, weight control, blood glucose management and Type 2 diabetes.
Diabetes UK has concluded that:
- Evidence exists suggesting that low-carbohydrate diets can lead to improvements in HbA1c and reductions in body weight in the short term (less than one year).
- Weight loss from a low-carbohydrate diet may be due to a reduced calorie intake and not specifically as a result of the carbohydrate reduction associated with this diet.
- Despite the short-term benefit there is a lack of evidence related to long-term safety and benefit of following this diet.
Diabetes UK recommends that:
- A range of approaches to weight loss should be considered with the overall aim of energy intake being less than energy expenditure; and that the most appropriate method to achieve this is identified between the person with diabetes and their dietitian.
- When considering a low-carbohydrate diet as an option for weight loss, people with diabetes should be made aware of possible side effects such as the risk of hypoglycaemia, head-aches, lack of concentration and constipation.
- Diabetes control should be considered and blood glucose levels need to be closely monitored with adjustments to medications as required.
- Nutritional adequacy should be considered ensuring that optimal amounts of vitamins, minerals and fibre are supplied by the diet.
- The amount of carbohydrate to be restricted should be agreed between the person with diabetes and their dietitian.
Why is carbohydrate important?
Carbohydrate is a nutrient that is an important source of energy in the diet. All carbohydrates are broken down into glucose which is essential fuel for the body, especially the brain. The body will always try to keep a minimum amount of glucose circulating at all times. Glucose is also stored in the liver as glycogen in case it is required immediately. When these stores become saturated, any excess glucose is stored as fat for the long term.
If carbohydrate intake is severely restricted and glucose stores are exhausted, the fat stores will be broken down and used as energy. During this process ketones are produced and excreted in the urine: this is known as ketosis. Approximately 50–70g per day of carbohydrate is required to prevent ketosis (1).
How much carbohydrate is in a low-carbohydrate diet?
For a 2000kcal diet, the recommendation for carbohydrate is 225–300g per day (45–60%) (2).
There is no agreed or set definition amongst researchers regarding the amount of carbohydrate in low-carbohydrate diets, but a critical appraisal by Accurso et al (2008) (3) suggested the following definitions:
- Moderate-carbohydrate diet: 130–225g per day (26–45%) of a 2000kcal diet
- Low-carbohydrate diet: less than 130g per day (26%) of a 2000kcal diet
- Very low-carbohydrate ketogenic diet: less than 30g per day (6%) of a 2000kcal diet
For the purpose of this position statement, the term “low-carbohydrate” is used as a collective term to describe any amount of carbohydrate restriction which is less than the dietary reference value of 45% of total energy.
What are the risks associated with low-carbohydrate diets?
Nutrition adequacy of the diet needs to be assessed; one of the main side effects associated with a low carbohydrate diet is the risk of hypoglycaemia, which can be even greater during physical activity. It is therefore important to consider overall diabetes control and monitor blood glucose levels closely and adjust medication accordingly. Other reported side effects include headaches, lack of concentration and constipation.
What does the evidence show?
Although there is substantial evidence regarding low-carbohydrate diets, the strength of the studies vary. Many trials lack a control group or are of short duration (less than one year) or have varying degrees of carbohydrate restriction or include a combination of these shortcomings.
There is evidence that low-carbohydrate diets lead to improvements in HbA1c and/or reductions in body weight, but this needs to be interpreted with caution (4, 5, 6, 7, 8). This benefit is mainly observed in short-term trials lasting six months or less. Trials lasting longer than six months reported rapid weight loss at the start (9) or in the first six months, which was then followed by a period of partial rebound and plateau (10). Despite the increases in weight and HbA1c values, these values remained lower than those at the start of the trial which could suggest that low carbohydrate diets may have lasting effects (11,12,13). Some studies have reported that weight loss is due to a reduced calorie intake and not specifically to the carbohydrate restriction (5, 14, 15).
A review of low- and reduced-carbohydrate diets in 2008 reported no deleterious effect on glycaemic control or cardiovascular risk factors, and again reported that these findings should be interpreted with caution because the majority of studies lacked a control group and were of short duration (16).
There is little evidence to support the use of low-carbohydrate diets in people with Type 1 diabetes. Five-year cohort evidence from people previously treated in the Diabetes Control and Complication Trial show that lower carbohydrate, and higher saturated, monounsaturated and total fat intake were associated with higher HbA1c levels (17).
In conclusion, low-carbohydrate diets may be effective in facilitating weight loss in people with Type 2 diabetes in the short term, but there is no evidence that this approach is more successful in the long term than any other approach (9). More research is needed to assess the effectiveness of varying degrees of low-carbohydrate diet on weight, glycaemic control, hypertension and lipid profile in people with Type 2 diabetes (18) as well as to investigate the long term effects of these diets (9).
- Thomas B, Bishop J (2007). Manual of dietetic practice, 4th ed. Oxford: Blackwell Publishing ltd.
- Department of Health (1991). Report of the Panel on Dietary Reference Values of the Committee on Medical Aspects of Food and Policy (COMA). Dietary Reference Values for Food Energy and Nutrients of the United Kingdom. Report on Health and Social Subjects 41. London: HMSO
- Accurso A, Bernstein R, Dahlqvist A, Drazini B, Finman R, Fine E, Gleed A, Jacobs D, Larson G, Lustig R, Manninen A, Mcfarlane S, Morrison K, Nielsen J, Ravnskov U, Roth K, Silvestre R, Sowers J, Sundberg R, Volvek J, Westman E, Wood R, Wortman J and Vernon M (2008). Dietary carbohydrate restriction in type 2 diabetes mellitus and metabolic syndrome: time for a critical appraisal. Nutrition and Metabolism 5; 9
- Gutierrez M, Akhavan M, Jovanovic L, Peterson C (1998). Utility of a short-term 25% carbohydrate diet on improving glycemic control in type 2 diabetes mellitus. Journal of the American College of Nutrition 17; 595–600
- Boden G, Sargrad K, Homko C, Mozzoli M and Stein T (2005). Effect of a low-carbohydrate diet on appetite, blood glucose levels, and insulin resistance in obese patients with type 2 diabetes. Annals of Internal Medicine 142; 403–411
- Yancy W, Foy M, Chalecki A, Vernon M and Westman E (2005). A low-carbohydrate, ketogenic diet to treat type 2 diabetes. Nutrition and Metabolism 2; 34
- Westman E, Yancy W, Mavroplous J, Marquart M and Duffle J (2008). The effect of a low-carbohydrate, ketogenic diet versus a low-glycaemic index diet on glycaemic control in type 2 diabetes mellitus. Nutrition and Metabolism 5; 36
- Daly M, Paisey P, Millwards B, Eccles C, Williams K, Hammersley S, Macleod K and Gale T (2006). Short-term effects of severe dietary carbohydrate-restriction advice in type 2 diabetes – a randomized controlled trial. Diabetic Medicine 23; 15–20
- Davis N, Tomuta N, Clyde S, Schechter C, Isasi C, Segal-Isaacson C, Stein D, Zonszein J and Wylie-Rosett J (2009). Comparative study of the effects of a 1-year dietary intervention of a low-carbohydrate diet versus a low-fat diet on weight and glycemic control in type 2 diabetes. Diabetes Care 32; 1147–1152
- Shai I, Schwarzfuchs D, Henkin Y, Shahar D, Witkow S, Greenberg I, Golan R, Fraser D, Bolotin A, Vardi H, Tangi-Rozental O, Zuk-Ramot R, Sarusi B, Brickner D, Schwartz Z, Sheiner E, Marko R, Katorza E, Thiery J, Fielder M, Bluher M, Stumvoll M, Stampfer M (2008). Weight loss with a low-carbohydrate, Mediterranean, or low-fat diet. The New England Journal of Medicine. 359; 229–241
- Nielsen J, Joensson E and Nilsson A (2005). Lasting improvement of hyperglycaemia and body weight: low carbohydrate diet in type 2 diabetes: a brief report. Ups J. Med. Sci. 110; 179–183
- Nielsen J and Joensson E (2006). Low-carbohydrate diet in type 2 diabetes: stable improvement of bodyweight and glycaemic control during 22 months follow up. Nutrition and Metabolism 3; 22
- Nielsen J and Joensson E (2008). Low-carbohydrate diet in type 2 diabetes: stable improvement of bodyweight and glycaemic control during 44 months follow up. Nutrition and Metabolism 5; 14
- Bravata D, Saunders L, Huang, J, Krumholz H, Olkin I, Gardener C and Bravata D (2003). Efficacy and safety of low-carbohydrate Diets: a systematic review. JAMA, 280; 1837–1850
- Kirk J, Graves D, Craven T, Lipkin E, Austin M, Margolis K (2008). Restricted-carbohydrate diets in patients with type 2 diabetes: a meta-analysis. Journal of the American Dietetic Association 108; 91–100
- Dyson P (2008) A review of low and reduced carbohydrate diets in weight loss in type 2 diabetes. Journal of Human Nutrition and Dietetics, 21; 550–538
- Delahanty L, Nathan D, Lachin; Hu B; Cleary P; Ziegler G; Wylie-Rosett J; Wexler D (2009). Association of diet and glycated hemoglobin during intensive treatment for type 1 diabetes in the Diabetes Control and Complications Trial. American Journal of Clinical Nutrition 89; 518–524
- Worth J, Soran H (2007). Is there a role for low carbohydrate diets in the management of type 2 diabetes? Q J Med 100; 659–663