July 18, 2006
Time was when rake-thin 18-year old males and underweight boxers would be given the same admonishment by anxious parents and professional coaches: “Bulk up—eat plenty of carbs.”
Fast-forward a few decades to around 2003, and Americans were on a tear to rid their diets of every last carbohydrate. According to a survey by ACNielson, New York, $1.3 billion worth of low-carb products were purchased during the year preceding July 2004.
Meanwhile, nutritionists were spending long hours in their labs challenging both the pro- and anti-carb movements. Their general conclusions echoed the innate wisdom followed by generations of mothers: everything in moderation.
In many ways, the carbohydrate issue paralleled that of fat. Initially given a blanket condemnation, some fats then received qualified endorsements and were finally stratified into the good, the bad and the indescribably ugly. The same is true of carbohydrates—they are complex and neither entirely beneficial nor completely harmful. Some support wellness, others can raise the risk factors for coronary disease and diabetes.
When the body digests carbohydrates, it breaks them down into their component monosaccharides, which then enter the bloodstream. Each individual food ingredient has a specific rate at which its carbohydrates pass into the bloodstream and cause blood-sugar levels to rise.
At St. Michael’s Hospital in Toronto in the early 1980s, initial work was done on classifying carbohydrates in terms of their effect on blood-sugar rates. This study formed the basis of what became known as the glycemic index (GI), which is actually a clinical blood-sugar test. Jennie Brand-Miller, professor of human nutrition at the University of Sydney in Australia is best-known around the world for extending the discovery of how the glycemic process actually works and for expanding the GI to cover more than 1,000 food products. Researchers constantly add new food ingredients to the list.
Indexed for life
Originally developed as a system to help diabetics decide which carbohydrates they might safely consume, and for athletes to choose which carbohydrates could improve performance, GI operates on a scale of 0 to 100 based on the rate of a carbohydrate’s glycemic response (in effect, its transmutation into glucose in the bloodstream). In the Brand-Miller system and most other GI systems, pure glucose is defined as the standard reference with a GI of 100. Most sources including the Harvard School of Public Health, Boston, refer to the Brand-Miller GI system. The higher the ranking, the faster the particular food causes blood sugar to increase.
For example, plain popcorn undergoes an almost instantaneous conversion to blood sugar and, therefore has a high GI of 72. Whole-grain rye bread, on the other hand, tends to affect blood sugar levels more slowly and, therefore, has a lower GI—between 66 and 72. Pearled barley has a GI of only 36.
In general, the consensus among nutritionists is that foods with a GI rating below 55 are considered “better for you” than those above 55. Often low-GI foods are all-around healthier choices, because they typically correlate with high fiber and tend to be low in low-density lipoprotein (LDL) cholesterol, calories and fat. The Harvard School of Public Health recommends that people moderate their intake of high-GI foods while eating plenty of whole-grain foods and fruits and vegetables (see www.hsph.harvard.edu/nutritionsource/carbohydrates.html). Also, “Canada’s Food Guide to Healthy Eating” states that most low-GI foods are all-around healthier choices and that “choosing low-GI foods more often may help you increase levels of HDL (healthy) cholesterol.”
While many food components are easily identified as having a high GI, others are more surprising. Thus, bran cereals have a low GI, but a baked potato has a high rating (85) due to its high starch content. Perhaps more surprising is that both carrots and parsnips have high GIs, 92 and 97, respectively.
Among the most-important considerations governing a food’s GI is the degree of processing. When subjected to refining, a grain’s bran, which contains the highest fiber content in the food, is stripped away, leaving the starchy interior of the carbohydrate. Without its barrier, the body digests this endospermic carbohydrate more quickly, and it rapidly passes into the bloodstream. Cooking and starch gelatinization can also influence GI. For example, al dente pasta has a lower GI than fully cooked pasta.
Other factors affect the speed with which carbohydrates increase bloodglucose levels. The physical structure can also come into play. For example, finely ground grains have a higher GI than coarser versions. Also, the type of starch present, the ripeness of the food and even its fat or protein content, which tends to retard carbohydrate digestion, can influence the carbohydrate’s speed of breakdown In reality, GI’s function is purely qualitative: measuring how quickly equivalent quantities of carbohydrates increase blood glucose. Obviously, it is just as important to consider the quantity of carbohydrate ingested and its consequent impact on both the blood-glucose level and the body’s insulin response. This consideration led researchers at Harvard University, Boston, in 1997 to evolve the concept of the glycemic load (GL), a formula that calculates the quantity of available carbohydrate in a serving.
Taken together, GI and GL show fairly conclusively that a high-GL diet causes blood-sugar levels and insulin requirements to increase more rapidly than would a low-GL diet. Several studies have revealed a link between high bloodglucose levels and large insulin demands, the stress of which diminishes the insulin-manufacturing capability of the pancreas, which can lead to irreversible diabetes. Among the foods most-frequently identified with a heightened possibility of contracting diabetes are sugary carbonated beverages, potatoes and white bread—all foods with a high GI.
At the other end of the scale, a low- GL diet is generally associated with better diabetes management. As many nutritionists and cardiologists suggest, using GI to manage glucose levels and limit insulin resistance also has a measurable effect on cardiovascular disease and obesity. In fact, high-fiber, low-GI diets tend to reduce obesity by extending satiety.
GI under attack
GI has been controversial for the 25 years of its existence, and it does have serious limitations. For example, GI critics note its lack of comparative data, pointing to the fact that the food components so far rated represent only a small proportion of total common foods—a situation that will worsen as new food products come out each year.
Also, critics charge that there’s insufficient agreement on universal GI ratings. Since no one accepts any one list, the same food item can have several ratings. No less serious, many observers criticize GI for its lack of sophistication—for example by not accounting for the combination effect of several food components on a plate, each with its own GI rating. The total GI of the whole plate is very different than the sum GI of the individual components.
Consumers think GI is too complicated, leaving them feeling confused and frustrated. Many have stopped using it as it was intended: as a guide. Instead, they’ve turned GI into yet another diet aid, even co-opting it to resurrect failing diet regimes, such as low-carb diets which have survived only by morphing into low-GI diets.
“Nutritionists are not unified. There’s so much debate about GI among nutritionists that it just turns consumers off,” says Ron Deis, vice president of technology, SPI Polyols, New Castle, DE. “Consumers always respond better when they have a clear-cut reason they can understand. Such as, ‘I have diabetes and need this data to help me control my blood sugar.’ What I don’t like is the concentration on GI to the exclusion of other factors involved in the glycemic response. It’s a very individualized process.”
Everyone’s looking for an easy way of dieting, “when they should be following, say, the federal dietary guidelines,” suggests Deis. “If they actually followed that, and increased their fiber intake, they’d gain a lot of real benefit from dieting.”
For all of its faults, Ross Craig, product manager for sweeteners, Danisco, Ardsley, NY, offers opportunity for the GI. “GI is a complex topic, but with a better explanation to consumers of its purpose, along with more easily understandable communications, consumers could be motivated by GI,” he suggests.
The low-carb diets were less sustainable, says Craig, precisely because they kept hammering on the low-carb message without differentiating between carbohydrates. “Their longevity would have been much greater,” he notes, “if they had premised the diet on GI, and the consumption of lower-GI carbohydrates —much more substance.”
Craig notes that “there’s too much squabbling within the food industry. We should agree on an ideal way to communicate with consumers about carbs and calories. We should help consumers understand that GI, ultimately, is a food-management tool, a guide. Information on which type of foods increase satiety would provide another tool. Such dietary tools can help consumers make more-informed decisions, but they cannot be used instead of common sense.”
Martin Schultz is an experienced consumer and trade magazine writer with a special interest in food and food-technology topics. He can be contacted at
The Role of Dietary Fiber in Glycemic Management
Blood sugar is becoming an important biomarker for health, wellness and energy management. However, considering the benefits of glycemic management without also considering the inclusion of dietary fiber and resistant starch would be incomplete at best, and misleading at worst.
Studies, such as those documented by Alldrick et al. have shown a relationship between dietary fiber and the glycemic index (GI) values of some starchy foods (“Dietary Fibre and the Glycaemic Index: Technological and Physiological Aspects,” Campden & Chorleywood Food Research Association Group Review No. 49, May 2006). In these cases, the greater the dietary fiber content, the lower the GI value.
However, dietary fiber is only one approach to lowering the glycemic response of foods. Other approaches include sugar alcohols, as well as minimizing carbohydrates in favor of protein and fat. Thus, it is entirely possible that evidence for glycemic management may be built upon low-glycemic foods containing dietary fiber, but applied to low-glycemic foods lacking dietary fiber. Careful consideration of the complete carbohydrate profile, including both glycemic impact and dietary-fiber content, is needed to ensure the delivery of promised or implied benefits.
The real benefit of glycemic management probably resides in foods that improve and help to maintain insulin sensitivity. In other words, glycemic management may be attainable through foods that assist in maintaining insulin sensitivity (a long-term benefit) and less by individual foods with reduced glycemic impact (a short-term benefit).
The distinction is important. A recent published study in Diabetes Care (2005; 28(12):2,832-2,838) demonstrated that fiber, but not GI and glycemic load,was related to measures of insulin sensitivity and insulin secretion. Another study, published in The American Journal of Clinical Nutrition (2006; 83:817-822), demonstrated that fermentable carbohydrates assist in regulating glucose responses to a second meal.
The benefits of dietary fiber are well known and include: prolonged satiety (Pharmacology & Therapeutics, 1994; 62:407-427); energy intake reduction and weight management (WHO Technical Report Series 916, 2003); as well as reduced risk of cardiovascular disease (Archives of Internal Medicine, 2004;164:370-376). In fact, the World Health Organization, Geneva, has identified dietary fiber as the only dietary ingredient with “convincing evidence” showing a protective effect against weight gain and obesity. In addition, high-fiber diets and resistant starch from high-amylose corn has also been shown to increase insulin sensitivity in healthy populations, which are entirely relevant for glycemic management (The American Journal of Clinical Nutrition, 1990; 52:524-528 and 2005; 82:559-567).
The evidence is further confounded by physiological differences between the three different types of fiber: insoluble, soluble and resistant starch. While insoluble fiber treats and prevents constipation, there is limited evidence supporting additional health benefits of individual insoluble fibers. Beta-glucan reduces blood cholesterol levels, but the benefit is not delivered by all soluble fibers. Fermentable soluble fibers, i.e. inulin and fructo-oligosaccharides, increase bulking and promote the growth of bifidobacteria. Insoluble natural resistant starch delivers some of the benefits of both soluble and insoluble fiber: it increases bulking, is fermented, promotes digestive health via the growth of bifidobacteria, and increases insulin sensitivity and lipid oxidation. Clearly, not all fibers are the same, and specific fibers such as natural resistant starch will have a role to play in glycemic management.
Short-term glycemic moderation isn’t the whole story. On the other hand, generic dietary fiber isn’t the answer either. Only further research will clarify the exact role of specific ingredients, but dietary fiber, and which type of dietary fiber, must be part of the equation and emphasis.
—Rhonda Witwer, National Starch Food Innovation
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