Thursday, April 22, 2010
Is Diabetes Caused by Refined Carbohydrates?
Last week we criticized Good Calories Bad Calories. This week we shall praise it. In chapter 6 of GCBC, Gary Taubes discusses Captain Thomas Latimore Cleave, a physician who believed that the common chronic diseases of Western civilization could be linked to the consumption of refined carbohydrates. Cleave had observed that non-Western societies tended to remain healthy even if they ate relatively large amounts of low glycemic index carbohydrates such as brown rice, wholemeal flour, non-starchy vegetables and nuts. But when a cultural group switched from traditional foods to white rice, white flour and sugar, the chronic diseases of civilization would begin to appear. To illustrate this, Cleave prepared the chart at the top of this post, which has been scanned from page 116 of GCBC. The dashed line shows per capita sugar consumption in England and Wales from just before 1905 to just after 1945. Sugar consumption increased during prosperous times and decreased during periods of wartime rationing. If diabetes had no relation to sugar intake, one would expect that deaths from diabetes (diabetic mortality) would gradually decrease as (1) injectable insulin was introduced and (2) medical treatments in general improved. Instead, until 1945 the index of diabetic mortality increased and declined in parallel with the consumption of sugar. Correlation is not causation, but the close relationship between sugar consumption and deaths from diabetes bears serious consideration.
Since 1945, the use of antibiotics to treat infection, the widespread use of home blood glucose monitors and the advent of new drugs to treat diabetes has dramatically reduced the death rate from diabetes. Nevertheless, there seems to be a steadily-increasing incidence of diabetes, particularly of type 2 diabetes. A recent article in Science Daily describes a study showing that type 2 diabetes has reached epidemic proportions in China. The scientists estimated that 9.7% of adult Chinese have diabetes and 15.5% have prediabetes. The prevalence of both conditions is higher in urban areas. Possible causes may include longer lifespans, increased smoking, decreased physical activity, increased air pollution, increased food consumption and decreased food quality.
Along the lines of Dr. Cleave's hypothesis about the relationship of refined carbohydrates and diabetes, in 2007 the Archives of Internal Medicine published an article suggesting one possible cause for the increase of diabetes in China. Its title was "Prospective Study of Dietary Carbohydrates, Glycemic Index, Glycemic Load, and Incidence of Type 2 Diabetes Mellitus in Middle-aged Chinese Women".
The study spent 4.6 years observing a cohort of about 64000 Chinese women with no history of diabetes or other chronic disease at baseline. These women were between 40 and 70 years old and lived in seven communities in urban Shanghai. They were divided into sets of quintiles according to several measures of carbohydrate intake. Adjustments were made for possible confounding factors including age, education, income, occupation, smoking status, alcohol consumption, total daily energy intake, physical activity, body mass index, waist-to-hip ratio and presence or absence of hypertension.
When confounding factors were eliminated, it was found that in middle-aged Chinese women, the percentage of carbohydrate in the diet was positively associated with the risk of developing type 2 diabetes. When glycemic index was considered, the higher the glycemic index of the food eaten, the more likely the women were to develop type 2 diabetes. In Shanghai, rice is a main staple food, contributing 73.9% of dietary glucose load (calculated by multiplying the total carbohydrate of a food by the glycemic index of the food and summing the values for all foods over a day). When women were stratified according to the amount of rice they ate, the group eating the most rice (over three cups of cooked rice per day) had a relative risk of 1.78 of developing diabetes as compared with those eating the least rice (less than two cups of rice per day).
For this group of Chinese women living in an urban area, carbohydrate intake averaged between about 260 and 340 grams per day. The largest part of their diet consisted of rice, which has a glycemic index of 55 (glucose=100). In this population, when adjusted for other factors predisposing to diabetes, a diet high in carbohydrates with a high glycemic index was associated with a higher risk of type 2 diabetes. Does this mean that diabetes is caused by refined carbohydrates? No, but once again, the close association between a higher intake of refined carbohydrates and a higher incidence of type 2 diabetes is worth serious consideration.