By: Neal D. Barnard
Remember the big picture: Populations that stick to traditional high-carbohydrate diets (for example, Asian rice-based diets) typically have low rates of obesity and diabetes. When they abandon traditional rice-based diets in favor of meatier Western fare, carbohydrate intake falls, but weight problems and diabetes increase.
In clinical trials, plant-based diets—low in fat and high in complex carbohydrate and fiber—facilitate weight loss. Our research team showed that even in the absence of exercise, a low-fat vegan diet causes weight loss and improves insulin sensitivity. (1) And although this may seem counterintuitive, our study participants found the vegan diet easier to stick with than the standard—or American Diabetes Association—diet. That's because no one has to cut calories, watch portion sizes, or limit carbohydrates.
For diabetes management, the big concern is cardiovascular damage, which is the combined result of several factors. For the heart, elevated LDL (bad cholesterol) levels, hypertension, smoking, and other factors appear to play larger roles than hyperglycemia. For the microvasculature of the eyes, kidneys, and nerves, hyperglycemia plays a larger role. The bottom line is that we want to control all of these risk factors.
Vegan Diet Tackles A1c, LDL, Blood Pressure, and Weight
In our recent study published in Diabetes Care in August 2006, we found that, in participants whose medications and exercise were held constant, a low-fat, low-glycemic-index, vegan diet lowered A1c’s by 1.2% on average, which is greater than the effect of typical oral medications.(2) Generally speaking, A1c changes were greater among those whose initial A1c values were higher or who had a great deal of weight to lose. We also found an average 21% drop in LDL (bad cholesterol). The LDL/HDL ratio improved, and blood pressure fell somewhat. This vegan diet also caused significant weight loss, despite the fact that it allowed unlimited carbohydrate intake.
Low-Carb Diet Sacks A1c, But What About Cholesterol and Kidney Function?
Low-carb diets typically reduce A1c, but have an unpredictable effect on cholesterol. Normally, weight loss reduces cholesterol levels. But while some low-carbohydrate dieters experience a cholesterol reduction, about one-third of low-carb dieters have just the opposite result: a cholesterol increase, sometimes quite big. In some reports, individuals have had to drop out of low-carbohydrate research studies because of rapidly rising LDL.(3) This probably occurs because when carbohydrate-rich foods are limited, people typically replace them with meats, dairy products, or other foods that are high in saturated fat and cholesterol. Such foods are also linked with higher risk of cancer and other health problems.
Low-carbohydrate diets also may contain animal protein, which can be taxing to the kidneys and lead to renal damage over the long run. Harvard researchers reported in the Annals of Internal Medicine that high-protein diets were associated with a significant decline in kidney function among individuals whose kidney function was mildly impaired to begin with. This was based on observations in 1,624 women participating in the Nurses’ Health Study.(4) Mild renal impairment is found in approximately 40% of individuals with diabetes.(5) For them, a high-protein diet accelerates the loss of kidney function. There are, to my knowledge, no long-term studies of the effect of low-carbohydrate diets on renal function.
For further details, readers may wish to consult the Diabetes Care article or the other research details provided in Dr. Neal Barnard’s Program for Reversing Diabetes (Rodale 2007). For details on beginning such diets, you may wish to visit http://www.pcrm.org/.
1. Barnard ND, Scialli AR, Turner-McGrievy G, Lanou AJ, Glass J. The effects of a low-fat, plant-based dietary intervention on body weight, metabolism, and insulin sensitivity. Am J Med 2005;118:991-997.
2. Barnard ND, Cohen J, Jenkins DJ, Turner-McGrievy G, Gloede L, Jaster B, Seidl K, Green AA, Talpers S. A low-fat, vegan diet improves glycemic control and cardiovascular risk factors in a randomized clinical trial in individuals with type 2 diabetes. Diab Care 2006;29:1777-1783.
3. Westman EC, Yancy WS, Edman JS, Tomlin KF, Perkins CE. Effect of a 6-month adherence to a very low carbohydrate diet program. Am J Med 2002;113:30-6.
4. Knight EL, Stampfer MJ, Hankinson SE, Spiegelman D, Curhan GC. The Impact of Protein Intake on Renal Function Decline in Women with Normal Renal Function or Mild Renal Insufficiency Ann Int Med 2003;138:460-7.
5. Coresh J, Astor BC, Greene T, Eknoyan G, Levey AS. Prevalence of chronic kidney disease and decreased kidney function in the adult US population: Third National Health and Nutrition Examination Survey.Am J Kidney Dis 2003;41:1-12.
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