This second in a two-part series on the glycemic index (see the January issue of The Natural Foods Merchandiser for part one) examines the evidence for the argument that eating a diet of foods low on the glycemic index can reduce the risk of cardiovascular disease.
As explained in Part 1, the GI of a particular food represents the relative rate of entry of glucose into the bloodstream over a two-hour period after eating 50 grams of the food, compared with 50 grams of a reference carbohydrate, typically glucose or white bread.1 The greater the rise in blood sugar, the higher the GI of the food.
The glycemic-load concept is based on the same idea, but takes serving size into account. Instead of using the standard 50 grams of carbohydrate of a food, the GL takes the typical carbohydrate serving size of that food and multiplies it by the GI of that food. The resulting number is the GL. Many experts say the GL is a more realistic reflection of the impact carbohydrate-containing foods have on blood sugar because it measures both the quality and quantity of that carbohydrate.2
How a high-GI/GL diet affects heart health has not been studied as much as how it relates to obesity. But because obesity and the factors associated with it are intimately related to heart-disease risk, it can be difficult to completely separate the two.3,4 Insulin resistance (the forerunner to diabetes), diabetes and obesity, all of which have been linked to high-GI/GL diets, also increase the likelihood of developing cardiovascular disease.
GI and lipid risk factors
One of the first studies to find that a low-GI diet reduced CVD risk factors was published in 1985. Researchers studied 12 people with elevated lipids and found that by reducing the GI of the diet by about 20 percent for one month, total cholesterol, low-density lipoprotein cholesterol and triglycerides were significantly reduced, compared with when the subjects ate a control diet.5 Elevated LDL and triglycerides are established risk factors for CVD.
A study from Denmark in which 45 women ate either a low-GI diet or a high-GI diet for 10 weeks showed that LDL decreased by 10 percent on the low-GI diet and increased by about 2 percent on the high-GI diet.6 Most recently, after testing four diets with different GIs (two high-GI and two low-GI) in 129 overweight or obese young adults, an Australian study concluded that the GI had a significant effect on LDL levels.7 Those on a high-GI, high-protein diet experienced an increase in LDL, while those on the two low-GI diets (high-protein or high-carbohydrate) experienced a decrease. However, there was little change in LDL in those following a high-GL, high-carbohydrate diet. These trials are small, however, and more research in this area is needed to clarify the effects of GI on LDL.
Several studies have concluded that people who eat high-GI/GL diets have lower levels of high-density lipoproteins (good cholesterol).8,9,10,11 Higher levels of HDL are associated with a reduced risk of CVD. Low HDL levels and elevated triglycerides are surrogate markers for insulin resistance, the forerunner to metabolic syndrome and diabetes, both risk factors for CVD.
Using data from the Third National Health and Nutrition Examination Survey (1988-1994), researchers found that high dietary GI and GL were associated with lower HDL levels.8 In a study of more than 1,000 patients with existing CVD, dietary GI was calculated from food-frequency questionnaires and compared with HDL concentrations. Significant inverse relationships were seen between dietary GI and GL and HDL concentrations. 9
Using data from more than 1,400 men and women in the 1986-1987 Survey of British Adults, researchers found that dietary GI was the only dietary factor significantly related to HDL levels.10
A Japanese study of 32 women (average age of 53) showed that those who consumed a diet with the lowest GL had the highest HDL and the lowest triglyceride levels.11
And in a recent study of more than 1,000 middle-aged adults with normal or impaired glucose tolerance, researchers found that as GL or GI went up, so did total cholesterol, LDL and triglycerides, while levels of HDL went down.12
In the Australian study above, researchers also concluded that the diet with the lowest GL produced the greatest improvement in the ratio of total cholesterol to HDL cholesterol; a high TC/HDL ratio is another recognized risk factor for CVD.7
C-reactive protein is a compound found in the blood that is a marker for inflammation. As the level of CRP increases, so does the risk for CVD. Researchers measured CRP concentrations and determined average dietary GL in a group of 244 healthy women from the Women's Health Study; they found that as GL went up, so did levels of CRP.13
Overall, most studies have found that following a low-GI/GL diet, at least over the short term, improves recognized CVD risk factors, such as levels of LDL and HDL.
Insulin sensitivity and CVD risk
Insulin resistance and diabetes are well-known risk factors for CVD. A high-GI diet may affect the risk for CVD by increasing insulin levels, which in turn lowers HDL and elevates triglycerides (two recognized risk factors for CVD).14 While several studies demonstrate that a low-GI/GL diet can improve glucose metabolism in people with existing diabetes, it is less clear whether a low-GI/GL diet can prevent the development of type 2 diabetes. Two large population studies found a two- to 2.5-fold increase in diabetes risk in people consuming high-GL diets.15 However, another large study of 36,000 postmenopausal women found no such link.16 The World Health Organization has stated that the evidence suggests a possible link.17
Anything that improves insulin resist?ance or helps control glucose metabolism in people with insulin resistance or diabetes reduces the risk of CVD, and several studies have shown that low-GI/GL diets do both. One study found that incremental increases in GL for a range of foods produced proportional increases in blood glucose and insulin.18 In an analysis of 12 studies, Australian researchers concluded that eating a low-GI/GL diet improved insulin resistance and helped lower blood-sugar levels in people with both type 1 and 2 diabetes.19
A small French study involving 12 men with type 2 diabetes found that just four weeks of a low-GI diet was enough to improve glycemic control.20 Reducing dietary GI from an average of 70 to 60 has been shown to improve insulin resistance as well as markers of risk for CVD.3
(See Part 1 for a table listing the GL of some common foods.)
However, not all studies show an association between lowered dietary GI/GL and improved insulin sensitivity.21,22,23 Some of the inconsistencies among study findings might be caused by the use of food-frequency questionnaires that were not designed to measure GI or GL. The American Diabetes Association states that there is insufficient evidence to determine whether there is a relationship between the GI or GL of diets and the development of diabetes.24
In Australia, however, experts are taking a more proactive stance. The University of Sydney, Diabetes Australia and the Juvenile Diabetes Research Foundation have established the Glycemic Index Symbol Program, which encourages food companies to include GI on food packaging and label them as high-, medium- or low-GI.25
GI/GL link to CVD
There have been few studies that have taken the link one step further to investigate whether there is a direct association between a high-GI/GL diet and actual cardiovascular disease, not just CVD risk factors. Evidence from population studies suggesting that a high-GL diet increases the risk of cardiovascular disease was first reported in 2000 by a group of Harvard researchers.4 As part of the Nurses Health Study, more than 75,000 women between the ages of 38 and 63 were followed for 10 years; researchers recorded their dietary intakes as well as the incidence of CVD.
The study group found that women consuming diets with the highest GL had twice the risk of CVD compared with those consuming diets with the lowest GL, even after controlling for age, smoking, calorie intake and other heart-disease risk factors. Such studies may be few, but the large size of the study populations means that they carry more weight than if they had included few subjects.
In an Italian study of 443 nondiabetic subjects who had suffered a first heart attack and 448 control subjects, re?searchers found that while a high-GI/GL diet was not strongly linked with overall heart-attack risk, a link was seen among the older and more overweight subjects.26
However, some studies have found no association between high-GI/GL diets and CVD. In the Zutphen Elderly Study, a prospective Dutch study of elderly men, researchers found no link between a high-GI diet and an increased risk of CVD in those with or without diabetes. Neither did they find a link between high-GI diets and CVD risk factors, such as elevated cholesterol, insu?lin, glucose, triglycerides or low levels of HDL.27 And no well-controlled, long-term clinical trials have examined whether consuming a low-GI/GL diet affects the risk of having a heart attack or dying from CVD.28
Among studies that have investigated potential links between GI/GL and CVD, most have looked at whether the GI/GL was linked to CVD risk factors, such as insulin resistance and elevated LDL. In studies that conclude low-GI/GL diets improve CVD risk factors, women sometimes appear to benefit more than men. And overall, people who are obese or overweight and those with elevated triglycerides also may benefit the most from following low-GI/GL diets.
While most studies indicate that a low-GI/GL diet improves risk factors for CVD, some experts have suggested that a low-GI/GL diet may simply be a marker for a healthier lifestyle.3 Before low-GI diets will be declared an effective treatment for the prevention of cardiovascular disease by health professionals, long-term clinical trials in people at risk for developing CVD will be needed.
Densie Webb, Ph.D., R.D., is a freelance writer and industry consultant based in Austin, Texas.
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