Carbohydrate tolerance in perspective – any justification for low carbohydrate diets?

Carbohydrate tolerance in perspective – any justification for low carbohydrate diets?

Dr Louise van den Berg and Prof Corinna Walsh, University of the Free State

Digestible dietary carbohydrates constitute different saccharides and sugars in food, thus starch, fruits and some vegetables, dairy products and refined sugars contribute to carbohydrate intake.1A diet that includes carbohydrates is encouraged for general health.2The brain and nervous system is particularly reliant on a steady blood supply of glucose derived from carbohydrates in the diet at levels of at least 130g/day or >45% of total daily energy intake.1,2,3

Carbohydrates are digested to simple monosaccharides in the small intestines and absorbed into the blood as glucose. The pancreas responds to the rise in blood glucose by releasing the hormone, insulin into the blood. Insulin binds to insulin receptors on the cell membranes, allowing cells to take up glucose from the blood and to normalize the blood glucose levels1,4.

The term “carbohydrate tolerance” or “glucose tolerance” refers to this ability to normalize blood glucose levels after carbohydrates are consumed. This ability is directly related to how sensitive the cell receptors are to insulin (insulin sensitivity). If a person is insulin resistant (thus the receptors response to insulin is not optimal), blood glucose levels tend to remain elevated after a carbohydrate containing meal or snack4,5.Insulin sensitivity is negatively influenced by being overweight and obese, as well as by being inactive4,5.

A number of tests can be used to determine whether a person is insulin resistant. The gold standard is hyperinsulinemic euglycemic clamping, while several indexes based on fasting blood insulin and/or glucose levels are also available for screening6.

The first line of defense in increasing insulin sensitivity is weight loss. Studies show that even modest weight loss of 5-10% of body weight (achieved by reducing energy and fat intake) in obese subjects, and increased physical activity (to 150 minutes/week) significantly improve insulin sensitivity and glucose tolerance.2,7,8,9Gradual weight loss of about 250g to 500g per week for adults with a BMI of 27 kg/m2 to 35kg/m2, and 500g to 1kg per week for those with BMIs ≥35kg/m2 is currently recommended for the general population. This rate of weight loss shouldbe planned to be achieved within 6 months and results in ±10% loss of body weight. During the following 6 months the new weight should be maintained, where after further weight reduction may be considered.10

Evidence suggests that both low-carbohydrate and low-fat energy restricted diets may be effective for weight reduction in the short term.2,9A meta-analysis of studies recently demonstrated that for the first 6 months of dietary intervention, low-carbohydrate diets proved more beneficial to triglyceride- and HDL cholesterol concentrations, but were associated with significantly higher LDL cholesterol concentrations, than low-fat diets.11Low-carbohydrate diets necessitate careful monitoring of lipid profiles, renal function, and protein intake (in individuals with nephropathy), as well as adjustment of hypoglycemic therapy as needed. The long-term safety and efficacy of low-carbohydrate diets have also not been adequately studied. Carbohydrate-intake below the Dietary Reference Intake (DRI) of 130g/day is not recommended based on the nervous system’s reliance on glucose. Carbohydrate-rich foods are vital sources of micronutrients and fiber, and contribute to palatability and variety in the diet. Low-fat, moderate protein diets are therefore still considered the safer option.2,9

Glycaemic index (GI) is defined as the effect that a food has on blood glucose levels after consuming a standard amount, expressed as a percentage of the effect of an equal amount of glucose12.Unrefined carbohydrates have lower GI than refined carbohydratesand tend to decrease cholesterol levels and increase satiety.13A variety of unrefined carbohydrates including whole grains, cereals, fruits, starchy vegetables and legumes shouldtherefore be consumed daily.1As fibre lowers GI, intakes of 15-25g/4200kJ (1000ckal) /day of which 50% should be soluble, is advised.2,14 The GI of a food is somewhat influenced by the variability in the glycemic response of individuals to a specific type of food, as well as the variability of GI within similar foods14(for example the GI of different types of bread varies considerably).However, GI is useful for fine-tuning postprandial response after first focusing on total carbohydrate intakes and overall dietary composition.1Some studies have also demonstrated a beneficial effect of diets with a low GI on insulin resistance, as well as glycemic and lipid control and satiety.15,16,17

References

1. Franz MJ. Medical Nutrition Therapy for Diabetes Mellitus and Hypoglycemia of Nondiabetic Origin. In Krause’s food & nutrition therapy, Ed:Mahan LK & Escott-Stump S.12th edition. Saunders Elsevier: International Edition, 2008: 773-781.

2. American Dietetic Association: Nutrition recommendations and interventions for diabetes.Diabetes Care 2008a; 31(Supplement 1): S61-S78.

3. Nathan DM, et al. Management of hyperglycemia in type 2 diabetes: a consensus algoritm for the initiation and adjustment of therapy. A consensus statement from the American Diabetes Association and the European Association for the study of diabetes.Diabetologia 2006; 49: 1711-1721.

4. Eckel RH, Grundy SM, Zimmet PZ. The metabolic syndrome.Lancet 2005, 365 (9468): 1415-1428.

5. Reaven GM. The insulin resisitance syndrome: Definition and dietary approaches to treatment. Annual Review of Nutrition 2003, 25:391-406.

6. Straczkowski M, Stepien A, Kowalska I and Kinalska I. Comparison of simple indices of insulin sensitivity using euglycemichyperinsulinemic clamp technique. Medical Science Monitor 2004, 10(8):CR480-4.

7. KnowlerWX, et al. Reduction of the incidence of type 2 diabetes withlifestyle intervention or metformin.New Engl JMed 2002; 346: 393-403.

8. The Diabetes Prevention Program Research Group. Achieving weight and activity goals among diabetespreventionprogram lifestyle participants.Obesity Research2004; 12(9): 1426-1434.

9. American Dietetic Association:Standards of Medical Care in Diabetes – 2008.Diabetes Care2008; 31(Supplement 1): S4-S41.

10. Gee M, et al. Weight management.In Krause’s food & nutrition therapy, Ed: Mahan LK & Escott-Stump S. 12th edition. Saunders Elsevier:International Edition,2008: 544.

11. NordmannAJ. Effects of low-carbohydrate vs low-fat diets on weight loss and cardiovascular risk factors: a meta-analysis of randomized controlled trials.Archives of Internal Medicine 2006; 166: 285-293.

12. Monro J. Redefining the Glycemic Index for Dietary management of postprandial glycemia. Journal of Nutrition 2003, 133:4256-42.

13. Anderson JW. Diabetes Mellitus: Medical Nutrition Therapy. In: Modern nutrition in health and disease. Edited by: ME Shils, M Shike, AC Ross, B Caballero & RJ Cousins.10th edition.Lippincott Wialliams&Wilken, 2006: 1043-1061.

14. Franz M.Nutritional management of diabetes mellitus and the dysmetabolic syndrome.Clin Nutr Highlights2006; 2(1): 2-7.

15. Augustine LS,et al.Glycemic index in chronic disease: a review. Eur J ClinNutr2002; 56(11): 1049-1071.

16. Brand-Miller J, et al. Low–Glycemic Index Diets in the Management of Diabetes: A meta-analysis of controlled randomized trials. Diabetes Care 2003;26: 2261-2267.

17. RiccardiG, et al.Role of glycemic index and glycemic load in the healthy state, in prediabetes, and in diabetes.Am J Clin Nutr 2008; 87(1): 269S-274S.