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Feature Article Atkin's Diet and Cardiovascular Risk by Steve Lome D.O. Coronary Heart Disease (CHD) is well established as the leading cause of death in the United States and as well as other industrialized countries. In order to decrease the incidence of CHD, the incidence of modifiable cardiovascular risk factors must also be reduced. Cardiologists universally recommend dietary changes as a means to lessen these cardiovascular risk factors and ultimately prevent CHD. The American Heart Association (AHA) specifically recommends a diet of 30% of calories from fat or less as a healthy dietary modification.[1] While low-fat diets are established to reduce lipid concentrations in patients with hyperlipidemia, they are associated with poor long-term compliance and only modest weight loss.[2-3] Knowing that the prevalence of obesity has continued to increase over the past 3 decades[3] and that low-fat diets have limited efficacy in weight loss, the public’s interests have turned towards low-carbohydrate diets such as the Atikns or South Beach diets. However, little is known regarding how these low-carbohydrate diets affect known cardiovascular risk factors and no data exists explore these diets in patients with known coronary heart disease. This review focuses strictly on the existing research data and expert analyses on how these popular low-carbohydrate diets affect cardiovascular risk. Cardiovascular risk factors potentially modified by dietary modalities include elevated low-density lipoprotein (LDL) cholesterol, reduced high-density lipoprotein (HDL) cholesterol, elevated triglycerides, hypertension, diabetes, and obesity. Diets high in fat have been correlated with increased lipid levels, insulin resistance, and obesity. It should then be of no surprise that a low-carbohydrate, high fat diet may be concerning in regards to lipid levels and cardiovascular risk. Some of the most important clinical studies involving low-carbohydrate diets will now be discussed focusing specifically on their influence on the aforementioned cardiovascular risk factors. Low-Carbohydrate Diets and the Lipid Profile It is counterintuitive to advise a patient with cardiovascular risk factors, especially one with hyperlipidemia, to follow a high-fat diet. A low fat diet to promote weight loss, reduce total cholesterol and LDL levels (the atherogenic cholesterol) and increase HDL levels (the athero-protective cholesterol) is usually advocated. Recently, a number of clinical trials have been published to assess the safety and efficacy of low-carbohydrate diets.[4-8] These trials indicate that low-carbohydrate diets are not only safe in regards to lipid profiles, but may in fact have beneficial effects (see figure 1). For example, 3 of the 5 studies in figure 1 show an increase in HDL cholesterol while on a low-carbohydrate diet and 3 of the 5 studies show no significant change in lipid profiles whole on a low-carbohydrate diet and 2 of the 5 studies show an increase in HDL cholesterol. One of the latter studies in figure 1 actually shows improvement in all lipid profile parameters.[8] Bravata from Stanford University confirmed that low-carbohydrate diets do not have a detrimental effect on lipid parameters via a meta-analysis of 107 trials.[17] Caution must be taken, however, not to over interpret these results. For example, it is known that diets high in atherogenic saturated fats (like most low-carbohydrate diets) increase specific subfractions of HDLs that have not been proven to have protective anti-atherogenic properties.[1] This may explain the increase in HDLs seen while on a low-carbohydrate diet and this increase would not prevent CHD. From the above studies, it can be concluded that a diet low in carbohydrate and high in fat does not have detrimental affects on the lipid profile and may actually have beneficial effects. However, the AHA issued a statement in 2001 claiming that the beneficial effects on the lipid profile are due to the weight loss and not the content of the low-carbohydrate diet, thus any diet producing significant weight loss would have this effect.[9] Nevertheless, further investigation is needed to determine whether these apparent benefits persist in the long term (greater than one year). More trials testing various patient populations are needed to sort out which groups of people can improve their lipid profiles by following a low-carbohydrate diet. Perhaps future studies can also focus on the treatment of drug-resistant hyperlipidemia with low-carbohydrate diets in patients with known CHD where other pharmacological and dietary measures have failed.  Figure 1 – Five recent studies on low-carbohydrate and their effects on total cholesterol, LDL cholesterol, and HDL cholesterol. Low-Carbohydrate Diets and Insulin Sensitivity Diabetes mellitus is perhaps the most important risk factor for developing CHD.[10] Unfortunately, studies have shown that despite strict control of glucose levels and maintaining HgA1C levels in acceptable ranges, there is no significant decrease in cardiovascular mortality.[11] It was concluded that the CHD risk is not secondary to elevated glucose levels, but instead a result of the decreased insulin sensitivity that exists in a diabetic state. The influence that a low-carbohydrate diet has on insulin sensitivity has been minimally investigated, however available data show possible beneficial effects. Samaha demonstrated that, in non-diabetic subjects, a significant increase in insulin sensitivity occurred while following a low-carbohydrate diet. This increase was significant even after adjusting for the amount of weight lost. Samaha concluded that the amount of weight lost and maintaining a low-carbohydrate diet were each independent determinants of increasing insulin sensitivity.[5] Brehm confirmed this finding, however Foster did not demonstrate a significant change in insulin sensitivity in his study.[6] The data is limited in regard to low-carbohydrate diets and insulin sensitivity. Recommendations cannot be made for or against these diets based on current data. Nevertheless, low-carbohydrate diets appear safe in the short term and may in fact improve glucose control in diabetics.[21] Larger studies are again needed to confirm the effects that a low-carbohydrate diet may have on both non-diabetic and diabetic patients. Low-Carbohydrate Diets and Blood Pressure Hypertension is another well-established risk factor for CHD.[10] In general, a diet low in sodium is recommended to patients with high blood pressure. No other recommendations regarding dietary content are specifically given to these individuals. Many of the same studies investigating low-carbohydrate diets on lipid profiles and weight loss also measured blood pressure. According to these studies, the effects that low-carbohydrate diets have on blood pressure appear minimal. Most trials report no significant change in systolic or diastolic blood pressures in subjects maintaining a low-carbohydrate diet.[5-8] Foster showed that both a low-carbohydrate and a low-fat diet have a beneficial effect on diastolic blood pressure.[4] This effect, however, may have been secondary to the weight lost and not necessarily the dietary composition. Low-Carbohydrate Diets and Triglyceride Levels Elevated triglyceride levels have recently been established as an independent risk factor for CHD.[10] A series of 17 studies following over 50,000 individuals over an average of 8-11 years showed that even a small increase in triglyceride levels leads to a 32 percent increased risk of CHD in men and a surprising 76 percent increased risk in women.[12] Triglyceride levels have subsequently become another important target of lipid lowering therapy. Recent studies have also elucidated that diets higher in carbohydrates may significantly increase triglyceride levels.[13] On the other hand, a 1966 Harvard study by P.K. Reissell showed that a low-carbohydrate diet reduced triglyceride levels by 70 percent in patients whose baseline levels were greater than 500 mg/dL.[14] The same 5 studies sited above for their effects on the lipid profile show a universal decrease in triglyceride levels in those patients following a low-carbohydrate diet, at least in the short term.[4-8] Anecdotally, some cardiologists have indeed been advising patients with drug-resistant hypertriglyceridemia to follow a low-carbohydrate diet or at least reduce dietary carbohydrates to some extent.[15] Based on the above limited evidence, this practice seems logical. Whether all patients with significant hypertriglyceridemia should follow a low-carbohydrate diet is still undetermined. At this point, it is not recommended that any individual with elevated triglycerides adhere to a low-carbohydrate diet. Long-term studies of greater than one year following a larger number of individuals are needed to definitively determine the benefits that low-carbohydrate diets may provide in respect to triglycerides levels. Low-Carbohydrate Diets and Obesity The incidence of obesity in the United States has risen significantly over the past decades. Moreover, obesity in the child and adolescent population has more than doubled in this same time period.[16] CHD has been strongly linked with this increasing incidence of obesity and it is therefore considered a risk factor for developing heart disease.[10] A significant part of CHD risk reduction includes weight loss in those who are overweight. However, few efficacious modalities exist for significant long-term weight loss. Currently, a low-fat, low-calorie diet in combination with exercise is recommended. Unfortunately, this method produces only modest weight loss and has poor long-term compliance.[2-3] Therefore, diets such as the low-carbohydrate Atkins diet have become an acceptable alternative in the public’s eye. The efficacy of low-carbohydrate diets on weight loss has recently received significant attention. A study by Westman showed that obese individuals placed on a low-carbohydrate diet without caloric restriction lost an average of 9 kilograms over a 6-month period.[8] Many other studies have indeed confirmed the efficacy of low-carbohydrate diets on weight loss.[17] Despite this positive data, low-carbohydrate diets are not flawless. Like any diet, long-term compliance to a low-carbohydrate diet is poor.[1] Moreover, a meta-analysis of 107 articles describing trials of low-carbohydrate diets performed by Bravata concluded that people following a low-carbohydrate diet lost weight not because of the content of the diet, but instead because of lower calorie consumption (probably induced by ketosis) and longer diet duration.[17] How low-carbohydrate diets compare to low-fat diets in this regard will be discussed below. Low-Carbohydrate Diets vs Low-Fat Diets The question remains which diet is better for your heart? Four studies previously discussed for their findings regarding low-carbohydrate diets and CHD risk factor modification asked the same question.[4-7] They will be summarized below. Foster randomized 63 obese men and women to a low-carbohydrate diet or a calorie restricted low-fat diet. Weight loss at 3 and 6 months was significantly greater in the low-carbohydrate group, however an important finding was that this difference did not persist to 12 months. No difference in total cholesterol or LDL was seen, however a greater increase in HDL cholesterol and a greater decrease in triglycerides were observed in the low-carbohydrate group versus the low-fat group. Both groups showed a significant decrease in diastolic blood pressure and increased insulin sensitivity.[4] A study by Samaha included a high percentage of patients with either diabetes (39%) or metabolic syndrome (43%). A total of 132 severely obese individuals were randomized to a calorie non-restricted low-carbohydrate diet or a calorie restricted low-fat diet for 6 months. Subjects on the low-carbohydrate diet lost more weight, had greater decreases in triglycerides, and, in non-diabetic participants, insulin sensitivity significantly improved compared to those on the low-fat diet. These latter two findings were significant even after adjusting for the amount of weight lost.[5] In a study by Brehm, 53 obese females were randomized to 6 months of a non-calorie restricted low- carbohydrate diet or a calorie restricted low-fat diet consisting of 30% of calories from fat. Subjects on the low-carbohydrate diet lost more weight and body fat than the low-fat group. There were no significant differences in blood pressure, lipid profiles, or fasting glucose levels, although these parameters were normal at baseline. In the low-carbohydrate group, a significant decrease in triglycerides occurred.[6] Sondike studied 30 overweight adolescents over a 12 week period randomizing them to either a low- carbohydrate diet or a lot-fat diet. After 12 weeks, the low-carbohydrate group lost more weight despite a higher reported energy intake, however no significant change in total cholesterol, LDL, or HDL levels was seen. The low-carbohydrate group did, however, realize a significant decrease in triglycerides as compared to the low-fat group. The low-fat group had a significant decrease in LDL cholesterol, which was not seen in the low- carbohydrate group.[7] The studies reviewed, while important, represent only a minority of all studies involving low-carbohydrate diets. It may be concluded that low-carbohydrate diets are not harmful in the short term in respect to the lipid profile as compared to low-fat diets. Benefits of a low-carbohydrate diet as compared to the low-fat diet may include more weight loss, an increase in insulin sensitivity, an equal or perhaps better efficacy in controlling cholesterol parameters, and a significantly better reduction of triglyceride levels. It is important to note that the above affects have only been evaluated in the short term and the available studies are small. The longest study lasted only 12 months.[4] Although lipid parameters seemed to remain improved at this duration, it was found that the weight loss was not significantly better with a low-carbohydrate diet versus a low-fat diet. Bravata’s finding supports this notion that dietary content was not associated with the amount of weight loss in study participants, however, the number of calories consumed and duration of the diet may be more important.[17] In regard to insulin sensitivity, data does exist showing that low-fat diets in combination with exercise can indeed slow or prevent the onset of diabetes, while no data exist regarding low-carbohydrate diets.[18-19] However many experts believe that the dramatically increasing incidence of diabetes mellitus in the U.S. may be due to the high carbohydrate diets that many Americans follow. Perhaps a low-carbohydrate diet can play a future role in preventing the development of diabetes mellitus in people with a family history of diabetes. Once again, no data exists to support or refute this hypothesis and more research is needed. Conclusion Based on the above evidence, it appears as though there is indeed reason to prescribe a low-carbohydrate diet to certain patient populations. Currently the main use of a low-carbohydrate diet is for healthy, obese individuals who have failed to lose weight on a low-fat diet, which seems reasonable. Whether a low-carbohydrate diet should be recommended to all obese individuals as a weight loss option to reduce risk for CHD is still under much debate and no recommendations can currently be made. Perhaps another indication for low-carbohydrate diets should be for the treatment of resistant hypertriglyceridemia, although further research is needed to establish efficacy and safety in the long-term. Lipid profiles do not appear to be adversely affected by a low-carbohydrate diet in the short-term and a beneficial effect may actually exist. Whether this positive effect is secondary to weight loss or dietary content is undetermined. Nevertheless, no recommendations can be made for or against the use of low-carbohydrate diets in the treatment of elevated LDL or low HDL cholesterol levels. Furthermore, trials measuring the development of CHD other cardiovascular endpoints while on a low-carbohydrate diet would provide definitive data. A positive effect on insulin sensitivity in the short-term may occur while on a low-carbohydrate diet. However, limited data once again does not allow a low-carbohydrate diet to be recommended to diabetic patients or to those at increased risk of developing diabetes. No harmful or beneficial effects on blood pressure occur while on a low-carbohydrate diet. As compared to a low-fat diet, low-carbohydrate diets appear to result in greater short-term weight loss, however this advantage may not persist after 12 months. Low-carbohydrate diets lower triglyceride levels to a greater degree than low-fat diets. Moreover, a low-carbohydrate diet, according to available limited data, may be of similar efficacy to a low-fat diet in improving the cholesterol profile. Despite all of the hoopla surrounding low-carbohydrate diets and CV risk, an analysis of 147 original investigations by Hu determined that only 3 dietary interventions are efficacious in reducing the risk of CHD. Reducing the intake of saturated fat and instead consuming unsaturated fat, increasing intake of omega-3 fatty acids, and maintaining a diet high it fruits, vegetables, and nuts appear to prevent CHD. Hu concluded, on the other hand, that lipid profiles and CHD incidence do not improve simply by lowering dietary fat intake, as is currently recommended.[20] Based on the above evidence, it should be recommended that people on low-carbohydrate diets attempt to adhere to the above 3 principles. Low-carbohydrate diets can easily incorporate omega-3 fatty acids, fruits, vegetables, nuts, and replace saturated fatty acids with unsaturated fatty acids. While some low-carbohydrate diets, such as the Atkins diet, restrict consumption of fruits and vegetables (which is not recommended), other low-carbohydrate diets, such as the South Beach diet, do incorporate CHD risk reducing fruits and vegetables. The role of low-carbohydrate diets is continuing to be elucidated. Current data has established their short-term safety and efficacy of low-carbohydrate diets for weight loss and control of lipid profiles, however long-term studies and studies with cardiovascular endpoints are needed. REFERENCES 1. Bonow RO, Eckel RH. Diet, obesity, and cardiovascular risk. N Engl J Med. 2003 May 22;348(21):2057-8. 2. Toubro S, Astrup A 1997 Randomized comparison of diets for maintaining obese subjects’ weight after major weight loss: ad lib, low fat, high carbohydrate diet v fixed energy intake. Br Med J 314:29-34. 3. Willett WC. Dietary fat plays a major role in obesity: no. Obes Rev. 2002;3:59-68. 4. Foster GD, Wyatt HR, Hill JO, et al. A randomized trial of a low-carbohydrate diet for obesity. N Engl J Med 2003 May 22; 348(21):2082-90. 5. Samaha FF, Iqbal N, Seshadri P, et al. A low-carbohydrate as compared with a low-fat diet in severe obesity. N Engl J Med 2003 May 22; 348(21):2074-81. 6. Brehm BJ, Seeley RJ, Daniels SR. A randomized trial comparing a very low carbohydrate diet and a calorie-restricted low fat diet on body weight and cardiovascular risk factors in healthy women. J Clin Endocrinol Metab 2003 Apr; 88(4):1617-23. 7. Sondike, S.B., Copperman, N.M., Jacobson, M.S., "Low Carbohydrate Dieting Increases Weight Loss but not Cardiovascular Risk in Obese Adolescents: A Randomized Controlled Trial" Journal of Adolescent Health, 26, 2000, page 91. 8. Westman EC, Yancy WS, Edman JS, et al. Effect of 6-month adherence to a very low carbohydrate diet program. Am J Med 2002 Jul; 113(1):30-6. 9. St Jeor ST. Dietary protein and weight reduction: a statement for healthcare professionals from the Nutrition Committee of the Council on Nutrition, Physical Actvity, and Metabolism of the American Heart Association. Circulation 2001 Oct 9; 104(15):1869-74. 10. Ridker PM, Genest J, Libby P. Risk factors for atherosclerotic disease. In: Braunwald E, ed. Heart Disease: A Textbook of Cardiovascular Medicine. Philidelphia: WB Saunders, 2001:1010-1039 11. Beckman JA, Creager MA, Libby P. Diabetes and atherosclerosis. JAMA, 2002; 287:2570-81 12. Austin, M.A., Hokanson, J.E., Edwards, K.L., "Hypertriglyceridemia as a Cardiovascular Risk Factor" The American Journal of Cardiology, 81(4A), 1998, pages 7B-12B. 13. McLaughlin, T., Abbasi, F., Lamendola, C., et al., "Carbohydrate-Induced Hypertriglyceridemia: An Insight Into the Link Between Plasma Insulin and Triglyceride Concentrations" Journal of Clinical Endocrinology and Metabolism, 85(9), 2000, pages 3085-3088 14. Reissell, P.K., Mandella, P.A., Poon-King, T.M.W., et al., "Treatment of Hypertriglyceridemia," The American Journal of Clinical Nutrition, 19, 1966, pages 84-98. 15. Heartwire. Diet dilemma: Are cardiologists going loco for low-carb? TheHeart.org 2004 Feb 24 16. Hill JO, Peters JC 1998 Environmental contributions to the obesity epidemic. Science 280:1371-1374. 17. Bravata DM, Sanders L, Efficacy and safety of low-carbohydrate diets: a systematic review. JAMA. 2003 Apr 9;289(14):1837-50. 18. Tuomilehto J, Lindström J, Eriksson JG, et al. Prevention of type 2 diabetes mellitus by changes in lifestyle among subjects with impaired glucose tolerance. N Engl J Med 2001;344:1343-1350. 19. Diabetes Prevention Program Research Group. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med 2002;346:393-403. 20. Hu FB, Willett WC. Optimal diets for prevention of coronary heart disease. JAMA. 2002 Nov 27;288(20):2569-78. 21. Ressiter J, Atkin’s diet helps combat diabetes. Healthc Foodserv Mag Jan 2000;10(1):5 |
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