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With one in five Americans at high risk of developing diabetes, consumers and scientists alike are thinking a great deal about the glycemic index (GI). Many consumers see the GI as a convenient way to plan diets that have a minimal impact on blood sugar levels. While many scientists also view the GI favorably, several recent studies have questioned some common assumptions about the GI, opening the concept up for debate.
The uncertainty about the GI hasn't stopped consumers from pursuing low-GI products, however. Sales are on track to reach nearly $2 billion by 2011. Now that low-GI products are indicative of a trend, more than a fad, future research could help clear up some lingering questions and safeguard consumer confidence in the years ahead.
WHAT IS THE GI?
The GI measures the effects of carbohydrates on blood glucose levels. To calculate an item's GI value, researchers typically feed a specific amount of the item to volunteers and then take blood samples at regular intervals for two hours. After plotting a blood sugar response curve and measuring the area under the curve (AUC), the researchers divide the AUC by the AUC of a standard reference item, such as white bread. A GI score of 55 or less is considered low, while a score of 70 or more is considered high.
It took a while for the concept to catch on, but consumer awareness of the GI has skyrocketed in recent years. According to the International Food Information Council's (Washington, DC) 2007 Foundation Food and Health Survey, 46% of consumers knew about the GI in 2007, compared with 32% in 2006. Moreover, nearly half of the survey respondents with diabetes or a family history of diabetes were familiar with the term in 2007, compared with about a third in 2006.
One reason for the growing awareness has been a wave of of new low-GI foods.
The Emerging Role of Dairy Foods in Reducing the Risk of Metabolic Syndrome and Type 2 Diabetes
By Lisa A. Spence
Metabolic syndrome describes a cluster of metabolic abnormalities that are risk factors for cardiovascular disease and type 2 diabetes, including abdominal obesity, hypertension, elevated fasting glucose, elevated triglycerides, and low high-density lipoprotein (HDL) cholesterol.
Worldwide, 197 million people have impaired glucose tolerance due to obesity and metabolic syndrome, and it is estimated that by 2025, this number will rise to 420 million. Key recommendations for preventing and managing metabolic syndrome include weight loss, increased physical activity, and dietary changes that follow the 2005 Dietary Guidelines, including a reduction in calories within a diet low in saturated fat, trans fat, and cholesterol, and an increase in consumption of low-fat dairy products, fruits, vegetables, and whole grains.
The role of specific foods and/or nutrients as important factors in the prevention and management of metabolic syndrome and type 2 diabetes is an emerging area of research. A growing body of evidence demonstrates that dairy food consumption may not only lower elevated blood pressure and adiposity, two components of metabolic syndrome, but also may reduce the incidence of metabolic syndrome and type 2 diabetes, which is often a subsequent consequence.
Dietary patterns with high dairy intake are associated with reduced risk of the components of metabolic syndrome, which is in contrast to the traditional belief that dairy foods contribute to development of risk factors for cardiovascular disease due to the association with high dietary cholesterol, saturated fat, and protein. Populations consuming dairy products are at lower risk of developing type 2 diabetes as well as metabolic syndrome than populations consuming a low dairy food diet. A recent review reported that the risk for incidence of type 2 diabetes was 14% lower for those individuals consuming greater dairy intakes (three to five servings per day) compared with those with lower dairy intakes (<1.5 servings/day).
Several reports have shown an inverse association between dairy consumption and incidence of metabolic syndrome and type 2 diabetes in various population groups and in both men and women. Greater dairy consumption was associated with reduced incidence of metabolic syndrome in a cohort of adults from Iran and the UK. A study of more than 10,000 middle-aged and older U.S. women reported that those consuming the highest dairy intakes had a 34% reduction in risk of metabolic syndrome versus those consuming the lowest dairy intakes. Similar findings have been reported in men with dietary patterns including milk/dairy products, bread, cereal grains, and fish being associated with a lower risk of metabolic syndrome.
Additionally, the Coronary Artery Risk Development in Young Adults study, which studied ~3000 black and white men and women over a 10-year period, reported a 72% lower risk of developing metabolic syndrome in individuals overweight at baseline who consumed five or more dairy servings per day compared with those consuming only 1.5 servings. In the Malmo Diet and Cancer cohort in Sweden, increased dairy food consumption in women provided a protective effect against the development of type 2 diabetes. In two large prospective studies, each additional daily serving of dairy was associated with a 4% lower risk in women followed for 10 years and a 9% lower risk in men followed for 12 years of developing type 2 diabetes.
While the above-mentioned studies have shown associations, some studies have demonstrated direct impact of dairy consumption and components of metabolic syndrome, e.g., high blood pressure and adiposity, which are also risk factors for type 2 diabetes. The DASH study (Dietary Approaches to Stop Hypertension) evaluated the effects of a healthy diet including low-fat dairy products (milk, yogurt, and cheese), fruits, and vegetables on blood pressure in ~450 subjects for eight weeks. Results showed that the combination of fruits, vegetables, and low-fat dairy, "the DASH diet," resulted in the greatest reductions in blood pressure compared with the "typical American diet," whereas the fruit and vegetable diet that excluded dairy products was about half as effective as the DASH diet.
Another study examined the effects of the DASH diet in subjects with metabolic syndrome. Compared with the control diet, the DASH diet led to increased HDL, lower triglycerides, lower blood pressure, weight loss, and reduced fasting blood glucose in both men and women. Clinical, observational, and animal model studies support a relationship between the consumption of dairy foods and weight management.
Although much remains to be learned about the role of dietary factors in the prevention and treatment of metabolic syndrome, the potential protective role of dairy products on metabolic syndrome and type 2 diabetes is emerging. Observational studies and clinical trials have indicated that dairy consumption may have beneficial effects on reducing body weight, body fat, abdominal fat, blood pressure, and possibly incidence of metabolic syndrome and risk for type 2 diabetes. For future research, the focus should be placed on randomized clinical trials to investigate the direct effects of dairy and its components on metabolic syndrome and risk of type 2 diabetes.
Lisa A. Spence, PhD, RD, is vice president of nutrition research for the National Dairy Council, which is managed by Dairy Management Inc. (Rosemont, IL). She is a member of the Obesity Society and the International Society of Nutrigenetics and Nutrigenomics. She is also a contributor to scientific texts, journals, and trade publications. For more information, visit www.innovatewithdairy.com.
"In the face of a worldwide epidemic of type 2 diabetes, many or most food companies are looking for food products that can aid in managing diabetes," says James Elliott, PhD, director of nutritional sciences at DSM Nutritional Products Inc. (Parsippany, NJ), which recently introduced a new blood sugar–balancing ingredient called InsuVital. "Although there are a number of approaches, marketing products as having a low GI can be an attractive strategy."
Saul Katz, president of low-GI nutrition bar manufacturer Solo GI Nutrition (Edmonton, AB, Canada), agrees that shoppers are catching on to the idea. "We see more and more consumers making the connection between the GI and blood sugar management, with application to weight management, sustained energy, and disease risk reduction," Katz says. "Consumers are learning that carbohydrate quality, not carbohydrate elimination or reduction, is a more sensible and sustainable approach to healthy living."
HOW DO LOW-GI PRODUCTS WORK?
Unlike low-carb diet products that simply center around the avoidance of carbohydrates, low-GI products attempt to deliver sustained energy and satiety while keeping blood sugar at healthy levels. Popular ingredients in low-GI products include proteins, slowly digestible carbohydrates, fiber, and alternative sweeteners.
For instance, DSM's InsuVital, which launched last year, contains a mixture of dipeptides and tripeptides extracted from selectively hydrolyzed casein, a milk protein.
"To be a low-GI food, the carbohydrate must be digested slowly or absorbed slowly to lead to a low glycemic response," Elliott explains. "InsuVital has the ability to stimulate the pancreas to increase its production of insulin when given in combination with carbohydrates. Thus, in a carbohydrate-rich meal, it will enable people to maintain their postprandial blood glucose in the normal range without hypoglycemia."
Clinical studies have shown that InsuVital won't produce hypoglycemia in people with normal blood sugar levels, Elliott adds. "Thus, the entire family may enjoy foods containing InsuVital without worrying that their blood glucose will drop below the normal range," he says.
Solo GI Nutrition's bars also aim to get blood sugar levels within safe parameters. Available in five flavors, the bars offer balanced amounts of grain- and fruit-based carbohydrates, proteins, fibers, and healthy fats.
"It is the exact nature and source of the carbohydrates, combined with the overall blend of ingredients, that contribute to the clinically validated low GI values and glucose-stabilizing capability of the SoLo bars," Katz says.
Another approach is through the use of alternative sweeteners, such as Gadot Biochemical Industries Ltd.'s (Haifa, Israel) NRGylose isomaltulose. The noncariogenic sweetener has the same caloric value as sugar but is digested at a slower pace. Gadot manufactures NRGylose by fermenting refined sugar with non-GMO bacteria.
"Since the enzymatic hydrolysis of NRGylose is slower than that of sucrose, the increase in blood glucose levels will be significantly slower," says Ronny Hacham, vice president of business development and marketing at Gadot. "As a result, glucose levels in the blood will also be lower." Hacham adds that while the sweetener is useful for people with diabetes, it may also have sports nutrition applications. "A marathon runner can maintain a constant blood sugar level more easily with our isomaltulose," he says.
MAKING SENSE OF CONFLICTING DATA
The GI can be a helpful tool for measuring food's impact on glucose. However, there is some uncertainty about its scope and usefulness. Several recent studies suggest that the GI's effects may be highly variable.
One small but provocative study appeared last June in the journal Diabetes Care. In that study, researchers from the Jean Meyer USDA Human Nutrition Research Center on Aging at Tufts University (Medford, MA), working under a grant from NIH's (Bethesda, MD) National Institute of Digestive and Kidney Diseases, reported that individuals have a wide range of responses to glucose.
For their experiment, the USDA researchers gave 14 volunteers 50 g of carbohydrates on three separate days and measured their glycemic responses. The researchers found that the volunteers, who received the carbohydrates in forms of white bread and glucose dissolved in water, each had different outcomes. Moreover, the responses varied not only among individuals, but also within individuals.
"Using glucose as the control food, previous studies indicate that white bread has a GI of about 70," wrote study author Alice Lichtenstein, DSc. "In our study, the combined average was 71, virtually identical to the published value. However, quite strikingly, individual values ranged from 44 to 132." She added that test values for each individual varied as much as 42%.
"These results show that perhaps using the GI for groups is a reasonable indicator to predict chronic disease risk, but there is still considerable uncertainty when applying the GI to individuals," Lichtenstein noted.
More recently, in the January 2008 issue of the American Journal of Clinical Nutrition (AJCN), two studies found little or no association between adherence to low-GI diets and weight loss or glucose control.
In one of these studies, researchers from the University of Maryland (College Park, MD) examined data from the Health, Aging, and Body Composition Study, which involved 3075 adults aged 70–79. After measuring the volunteers' dietary intakes of specific nutrients and food groups over a four-year period and calculating their risk of developing type 2 diabetes, the researchers found no correlation between the GI and diabetes risk.
In the other study, the Canadian Trial of Carbohydrates in Diabetes (CCD), University of Toronto researchers recruited 162 volunteers with type 2 diabetes and assigned them to either a high-carbohydrate, high-GI diet; a high-carbohydrate, low-GI diet; or a low-carbohydrate, high-monounsaturated fat diet for one year. The researchers reported finding no difference among the three diets for body weight and hemoglobin A1c, which measures the amount of glucose circulating in the blood. However, the researchers also reported that the low-GI diet produced lower postload glucose after two hours, as well as lower C-reactive protein (CRP), a marker for inflammation. Additionally, the researchers noted that volunteers on the low-GI diet had altered cholesterol levels that disappeared after six months.
The CCD results have promoted a great deal of discussion. In one commentary that appeared in the same January 2008 issue of AJCN, Xavier Pi-Sunyer, MD, professor of medicine at Columbia University College of Physicians and Surgeons (New York City), wrote that "it seems unwise at this point to burden type 2 diabetes patients with trying to pick and choose among different high- and low-GI foods."
However, in another commentary, Mayo Clinic (Rochester, NY) endocrinologist John Miles, MD, wrote that while the latest findings "will be disappointing to some advocates of low-GI diets," the diet's effects on CRP and blood pressure warrant further investigation.
These studies aren't the final word on the GI, however, and many advocates of the GI point to other studies with different conclusions. The November 2007 issue of the Archives of Internal Medicine, for instance, contains two studies that imply an inverse association between GI scores and diabetes risk.
In one of those studies, data from 40,078 food frequency questionnaires suggested that women who eat high-GI foods are at higher risk of developing diabetes. In the other, researchers who tracked the health of 64,227 Chinese women for five years found that the women who ate the most carbohydrates had a 28% higher risk of developing the disease than those who had the least; moreover, women who ate at least 300 g of rice per day were 78% more likely to develop diabetes than women who ate less than 200 g.
Solo GI Nutrition's Katz notes that even when studies like the AJCN research fail to find a link between the GI and weight loss, the same data often support other benefits, such as higher high-density lipoprotein cholesterol and lower total cholesterol, triglycerides, low-density lipoprotein cholesterol, blood pressure, and CRP.
Moreover, in March, AJCN published a meta-analysis of 37 prospective cohort studies involving more than 40,000 subjects. The authors of that meta-analysis did conclude that low-GI diets offer some protection from diabetes and related conditions.
"The general consensus, despite the recent articles published in AJCN, is that for people who are overweight or insulin resistant, low-GI diets may confer benefits," Katz says.
DSM's Elliott agrees, adding that there is likely to be controversy over the use of any index. "Clearly, the GI is not perfect," he says. "The methodology is not standardized, and values for common foods vary with the source of the information."
Elliott adds that some researchers have concluded that the best evidence for the clinical usefulness of the GI is with diabetic patients. "The value of the GI for normal individuals who want to lower their risk of developing diabetes is less certain, since no intervention studies have been conducted that showed a reduced risk of developing diabetes from eating a low-GI diet," Elliott says. "However, the consumption of a low-GI diet in normal individuals would appear to be of value."
A LOW-GI FUTURE
Debate about the GI is likely to persist as long as researchers continue to report conflicting data. While some experts will point out the GI's shortcomings, others will highlight its potential to offer benefits beyond weight loss and glucose management.
On the other hand, consumer demand for low-GI products should remain healthy, regardless of the debate. Both Katz and Elliott point to recent market research by Packaged Facts (Rockville, MD) that predicts a compound annual growth rate (CAGR) of 45.7% through 2011. The report estimates that low-GI sales hit $359 million in 2006 and could reach $1.8 billion by 2011.
"GI awareness is gaining momentum across all ages, income levels, and demographics," Katz says, adding that a major public policy initiative involving low-GI foods may be necessary to stem the rising diabesity epidemic.
Even with the extra momentum, there is still room for more growth, adds Gadot's Hacham, who notes that "awareness of low-GI products is still not wide enough to reach the point where market potential is exhausted."
With CAGR of 55.4% from 2003 to 2005 and double-digit growth expected in the years ahead, consumers seem ready for a low-GI future, even if some researchers remain unconvinced. Says Elliott: "The continuing growth indicates that it has a long way to go before the trend starts to run out of steam."