Are Carbohydrates Worse Than Saturated Fat? A New Paradigm
Abstract & Commentary
By Ralph R Hall, MD, FACP, FACSM, Professor of Medicine Emeritus, University of Missouri-Kansas City. Dr. Hall reports no financial relationship to this field of study.
Synopsis: Replacing saturated fatty acids with carbohydrates with low glycemic index values is associated with a lower risk of myocardial infarction.
Source: Jakobsen, MU, et al. Intake of carbohydrates compared with intake of saturated fatty acids and risk of myocardial infarction: Importance of the glycemic index. Am J Clin Nutr 2010;91:1764-1768.
The authors of the study note that research has suggested that replacing saturated fatty acids (SFAs) with carbohydrates is modestly associated with a higher risk of ischemic heart disease, whereas replacing SFAs with polyunsaturated fatty acids is associated with a lower risk of ischemic heart disease. They suggest that the effects of carbohydrates, however, may depend on the type consumed.
The objective of their study was to investigate the risk of myocardial infarction (MI) associated with a higher energy intake from carbohydrates and a concomitant lower energy intake from SFAs. They also investigated the effects of carbohydrates with different glycemic index (GI) values.
The design of the investigation used a prospective cohort of 53,644 women and men free of MI at baseline. During 12 years of follow-up, 1943 incident MI cases occurred. There was a nonsignificant inverse association between substitution of carbohydrates with low-GI values for SFAs and risk of MI. In contrast there was a statistically positive association between the substitution of carbohydrates with high-GI values for SFAs and the risk of MI. No effect modification by sex was observed.
The authors concluded that replacing SFAs with carbohydrates with low-GI values is associated with a lower risk of MI, whereas replacing SFAs with carbohydrates with high-GI values is associated with a higher risk of MI.
The accompanying editorial, by one of the world's experts in this field, Frank Hu,1 is a new paradigm for the management of lipid disorders. He notes that the attempt to improve lipid profiles with low-fat, low-cholesterol diets promotes the use of diets that are high in complex carbohydrates. This "has spurred a compensatory increase in the consumption of refined carbohydrates and added sugar a dietary shift that may be contributing to the current twin epidemics of obesity and diabetes."
The glycemic index, which measures the blood glucose concentration after a test load of 50 g of the reference carbohydrate, is a better guide to the quality of the carbohydrates rather than the complexity, which is based on chemical structure rather than its effect on blood glucose levels. Foods such as potatoes are complex carbohydrates but have a high GI. An excellent review, suitable for both patients and physicians, which lists the low and high glycemic foods, is available on Wikipedia.
The study by Jakobsen et al is the first study of sufficient size and duration to examine the effects of replacing dietary fats with either high- or low-quality carbohydrates. As Hu notes, the study provides direct evidence that substituting high GI-value carbohydrates for saturated fat actually increases the risk of ischemic heart disease.
It is interesting that in 1998, Knopp et al studied the long-term effect of four levels of fat restriction on a total of 444 men for 1 year. They found that diets with fat restriction below 26% of fat in hypercholesterolemic subjects and below 30% in subjects with combined hyperlipidemia were of little benefit in lowering serum cholesterol and that the more restricted diets resulted in significantly higher triglycerides and lower high-density lipoproteins. The most unfavorable effects of the low-fat and thus high-carbohydrate diets occurred most often in the subjects who were the most overweight. It was also of note that, despite intense indoctrination, most subjects were unable to maintain the prescribed diets. This is not news to most practicing physicians.
We seem to be slow learners. Raven and associates demonstrated the relationship between carbohydrate intake and triglyceride elevations 40 years ago.2 More recently Sacks and Campos noted (in an article that should be read by all physicians), "In two controlled trials involving patients with the metabolic syndrome3 or type 2 diabetes,4 a reduced carbohydrate Mediterranean diet lowered blood pressure and improved serum lipid levels more than a low fat diet."5
It is likely that we have more to learn about saturated fats. A study from Sweden and published in the July issue of the American Journal of Clinical Nutrition found a decreased incidence of MI in women with high intakes of saturated dairy fats.6
In summary, the Jakobsen et al study plus the analysis by Hu provides evidence that suggests the combination of substitutions of low-GI carbohydrates and polyunsaturated and monounsaturated fatty acids for saturated fats provides a more flexible, as well as a more effective, approach to lowering the incidence of ischemic heart disease.
1. Hu FB. Are refined carbohydrates worse than saturated fat. Am J Clin Nutr 2010;91:1541-1542
2. Raven GR, et al. Role of insulin in endogenous hypertriglyceridemia. J Clin Invest 1967;46:1756-1767.
3. Esposito K, et al. Effect of a Mediterranean-style diet on endothelial dysfunction and markers of vascular inflammation in the metabolic syndrome: A randomized trial. JAMA 2004;292:1440-1446.
4. Esposito K, et al. Effects of a Mediterranean-style diet on the need for antihyperglycemic drug therapy in patients with newly diagnosed type 2 diabetes. Ann Intern Med 2009;151:306-314.
5. Sacks FM, Campos H. Dietary therapy in hypertension. N Engl J Med 2010;362:2102-2112.
6. Warensjo E, et al. Biomarkers of milk fat and the risk of myocardial infarction in men and women. Am J Clin Nutr 2010;92:194-202.