Dietary habits and patterns that promote excess glucose (sugar) in the blood (hyperglycemia), excess insulin in the blood (hyperinsulinemia), and excess body fat also increase the risk for several chronic diseases including type 2 diabetes, cardiovascular disease, and cancer.
How much a food or meal raises blood sugar depends on the glycemic index of the food consumed and the amount of carbohydrate in the food or meal. Here are some definitions that can provide more clarity:
- Glycemic Index is a ratio of how much blood sugar rises after consuming a standard amount of available carbohydrate compared to a control (usually glucose or white bread).
- Glycemic Load is calculated by multiplying the glycemic index of a food by the quantity of available carbohydrate eaten in grams.
- Glycemic Load of a Meal is computed by summing the glycemic loads of all foods consumed.
A lot of research has been devoted to studying the impact of consuming foods with a low glycemic index compared to high glycemic index foods.
Livesey et al (2008) used meta-analysis and meta-regression to examine data from 45 prospective studies to determine the outcome of substituting low glycemic food for high glycemic foods, as well as the impact of glycemic index and glycemic load on fasting blood sugar and insulin, blood sugar control, and body weight. The following paragraphs provide detailed information on what these researchers discovered.
Data was analyzed from 36 studies in the meta-analysis and showed that eating a diet with a low glycemic index significantly reduces fasting blood sugar in proportion to the reduction in glycemic index. Almost all low glycemic index foods are good sources of fiber, and therefore, as the glycemic index of the diet was reduced, the amount of fiber was increased. It was determined that both a reduction in glycemic load as well as an increase in fiber was important in reducing fasting blood glucose. A reduced glycemic load can be achieved by simply reducing the amount of carbohydrate-containing foods that are consumed, and/or by including low glycemic index foods and increasing fiber. It was determined that a reduction in fasting blood glucose was better achieved by including more low glycemic index foods (like beans), rather than decreasing glycemic load by reducing carbohydrate intake.
Blood concentrations of glycated proteins (fructosamine and HbA1c) reflect overall blood sugar control. Fifteen of the 36 studies provided information about fructosamine and/or HbA1c concentrations. These studies showed that overall control of blood glucose is strongly related to the glycemic index and glycemic load of the diet and the amount of fiber consumed. It was suggested that optimum control of blood glucose is achieved when the diet has a glycemic index < 45, a glycemic load < 100g per day and a fiber intake of ≥ 25g per day.
The amount of insulin required to promote glucose uptake by tissues (insulin sensitivity) is an important aspect of blood glucose control. Eighteen of the 45 studies reported measurements of insulin sensitivity. There was an average of 20% improvement in insulin sensitivity for the 18 studies when low glycemic index foods were substituted for high glycemic index foods. A reduction in insulin concentrations in fasting blood samples by switching from a high to a low glycemic index diet was found only when subjects had hyperinsulinemia (higher than normal insulin levels circulating in the blood). People without diabetes improved their insulin sensitivity by 25% and individuals with type-2 diabetes improved their insulin sensitivity by 12%. Normal weight individuals did not achieve a significant improvement in insulin sensitivity while overweight and obese individuals had a 14% improvement in insulin sensitivity.
Taken together, people with Type 2 diabetes (but not those with Type 1 diabetes) can expect an improvement in insulin sensitivity by switching from a high glycemic index diet to a low glycemic index diet.
The percentage of overweight and obese individuals in the United States has been steadily increasing. Today, more than 67% of the adult population is either overweight or obese (NIH, NHANES 2009-2010). Obesity and overweight account for more than 300,000 deaths per year in North America (McGinnis, 1993; Allison, 1999), and the medical costs associated with excess body fat is estimated to be greater than $147 billion per year (CDC). Most of the costs associated with excess body fat are related to type 2 diabetes, heart disease, and high blood pressure (Wolf & Colditz, 1998).
Twenty-three studies examined changes in body weight that occurred when subjects changed from a high to a low glycemic index diet. It goes without saying that a reduction in body weight can occur only if there is a reduction in energy intake. On the average, the glycemic load needed to be decreased by 17g/day before weight loss would occur. Consistent weight loss was not reported until the glycemic load was reduced by >42g/day. If a reduction in glycemic load by substituting low glycemic index foods for high glycemic index foods resulted in less available carbohydrate (therefore a lower energy intake), weight loss occurred. The only significant factor related to weight loss was a reduced glycemic load and caloric intake; changes in fat, protein, and fiber intake that occur by substituting low glycemic index foods for high glycemic index foods could not explain the weight loss.
Clearly, if bean consumption could be increased and if there was a related decrease in body weight, the public health benefit would be enormous. Since increasing bean consumption would not increase the cost of the diet, it is hard to imagine a more cost effective intervention.
Beans, glycemic index, and glycemic load
The study by Livesey (2008) provides very strong evidence that eating diets with a low glycemic index (< 45), a low glycemic load (<100 g equivalents per day), and more than 25g per day of fiber will help normalize blood glucose, blood insulin, and body weight. Controlling blood glucose, blood insulin, and body weight in turn will reduce the risk of type 2 diabetes and cardiovascular disease, as well as certain types of cancer.
Beans are the perfect food to improve glycemic control. Beans have a low glycemic index, varying from 27-42% relative to glucose and 40-59% that of white bread (Table 1). Beans are also high in non-starch polysaccharides (typically 18-20%), 5% resistant starch, and 4% oligosaccharides to give a dietary fiber value of 27 – 29%. Consuming beans may significantly increase dietary fiber intake, and that is particularly important for blood sugar control.
|Table 1: Glycemic Indices of Various Beans (Foster-Powel, 1995)|
|GI (White Bread)||GI (Glucose)|
|Baked beans, canned||57||40|
|*Calculated glycemic index when white bread or glucose were used as a reference food. Expressed as a percentage of the reference food.|
Allison DB, Fontaine KR, Manson JE, Stevens J, VanItallie TB. Annual deaths attributable to obesity in the United States. Jama-Journal Of The American Medical Association. 1999;282(16):1530-1538.
Center for Disease Control & Prevention, Adult Obesity Causes and Consequences
Foster-Powell K, Miller JB. International Tables of Glycemic Index. American Journal of Clinical Nutrition. 1995;62(4):S871-S890.
Livesey G, Taylor R, Hulshof T, Howlett J. Glycemic response and health – a systematic review and meta- analysis: relations between dietary glycemic properties and health outcomes. American Journal Of Clinical Nutrition. 2008;87(1):258S-268S.
McGinnis JM, Foege WH. Actual Causes Of Death In The United-States. Jama-Journal Of The American Medical Association. 1993;270(18):2207-2212.
National Health and Nutrition Examination Survey, 2009–2010, http://www.niddk.nih.gov/health-information/health-statistics/Pages/overweight-obesity-statistics.aspx
Ogden CL, Yanovski SZ, Carroll MD, Flegal KM. The epidemiology of obesity. Gastroenterology. 2007;132(6):2087-2102.
Wolf AM. Impact of obesity on healthcare delivery costs. American Journal Of Managed Care. 1998;4(3):S141-S145.
Wolf AM, Colditz GA. Current estimates of the economic cost of obesity in the United States. Obesity Research. 1998;6(2):97-106.