See How the Carbs You Eat Make a Difference

Glycemic Control May
Protect Your Vision

A high dietary glycemic index is associated with
increased risk for age-related macular degeneration
By Will Block


© iStockphoto.com/PhotographerOlympus

mong life’s tragic ironies, one of the greatest is that blind people say, “I see.” They’re using that phrase, of course, in its most common sense of “I understand.” But equating seeing with understanding does make a certain sense, doesn’t it? It has been said that our vision accounts for about 80% of everything we learn. What we can see, we can more readily understand and believe (“seeing is believing”) than what we learn in more indirect ways.

Albert Einstein once said, “If I can’t picture it, I can’t understand it.” He was talking about picturing, in his mind’s eye, abstract concepts in theoretical physics, but the principle is the same for us mere mortals. Imagine trying to describe the eerie beauty of a jellyfish or the dazzling excitement of a fireworks display to a person who has never had sight. It’s a sobering thought, isn’t it?

Glycemic Index Is a Measure of Digestive Speed

Also sobering is the realization that many cases of blindness in the elderly could probably have been averted through better adherence to the twin pillars of good health: diet and exercise. Here we are concerned with diet—but not just with the use of nutritional supplements that are known to help prevent certain eye diseases, notably cataracts and age-related macular degeneration. Also important, as new research has shown, is the glycemic index of the foods we eat.

In case you’ve forgotten (or never knew), glycemic index is a measure of the rate at which glucose from the digestion of carbohydrates in our food enters our bloodstream after a meal. The more rapidly the carbs from a given food are digested and converted to glucose, the more rapidly the glucose enters the blood, and the higher the blood glucose levels go before enough insulin can be released by the pancreas to bring them back down again. All carbohydrate-containing foods and beverages have a glycemic index (GI) value that’s usually measured relative to that of a solution of pure glucose, which is arbitrarily set at 100. A GI value of 70 or more is considered high; 56 to 69 is medium; and 55 or less is low.*


*For a more extensive discussion of the glycemic index and its important companion concept, the glycemic load, see “Glycemic Control—Best for Weight Loss” in the September 2007 issue. And for the most authoritative and comprehensive source of information on all aspects of the glycemic index, see the book The New Glucose Revolution, 3rd ed. (2007) by Jennie Brand-Miller et al., available from Life Enhancement Products.


The “spike” in glucose levels following a high-GI meal is unhealthy, because it represents an acute (transient) condition of hyperglycemia (excessive glucose in the blood), which, in turn, can cause acute hyperinsulinemia (excessive insulin in the blood). Both of these conditions are harmful in various ways, especially if they occur daily on a chronic basis—as they do with the typical Western diet, which is rich in high-GI foods.

Age-Related Eye Diseases Are Associated with Carbohydrates

One of the consequences of this diet—painfully evident whenever we go out in public and look around—is a tendency toward weight gain, leading to obesity. And obesity opens a Pandora’s box of other disorders and diseases, especially type 2 diabetes and cardiovascular disease. Diabetes is a disease of dysfunctional glucose metabolism, resulting in chronically high blood glucose levels, which can damage many organs throughout the body—including the eyes.

Even nondiabetic people, however, are at risk for age-related eye diseases associated with the carbohydrates in their diet, as a group of American researchers has recently shown.1 Now the same research group has published a follow-up study of the association between dietary glycemic index and both the early and late stages of age-related macular degeneration (AMD), a neurodegenerative disease of the central retina, or macula.2 AMD is the leading cause of irreversible blindness in the developed world.*


*There is no effective therapy for AMD, but various nutritional supplements have been found to help prevent the disease or retard its progression. For more on this subject, see “A Nutrient ‘Cocktail’ to Protect Your Vision” (February 2006), “Can Age-Related Macular Degeneration Be Prevented?” (March 2007), and “Antioxidants Combat Age-Related Macular Degeneration” (April 2007).


How to Classify Age-Related Macular Degeneration

The researchers drew on 4099 participants, aged 55–80 (average 68), in the Age-Related Eye Disease Study (AREDS) of the National Eye Institute of the National Institutes of Health, a long-term study to assess the risk factors, clinical course, prognosis, and prevention strategies for AMD and cataracts. In the study reported here, they examined a total of 8125 eyes for evidence of AMD and classified them according to the degree of severity of the condition:

Group 1: Controls—no drusen, or only nonextensive small drusen. (Drusen are the characteristic lesions seen on the retina in cases of AMD; the more drusen or the larger the drusen, the worse the condition, and the worse the prognosis for further degeneration.)

Group 2: One or more intermediate drusen, extensive small drusen, or pigmentary abnormalities associated with AMD; these are strong predictors of subsequent advanced AMD.

Group 3: One or more large drusen or extensive intermediate drusen.

Group 4: Geographic atrophy, a pattern of well-demarcated atrophy of certain cell layers in the retina, leading to vision loss—an advanced stage of AMD.

Group 5: Neovascularization, or the growth of unwanted blood vessels beneath the macula of the retina—an advanced stage of AMD (called “wet” AMD as opposed to the much more common “dry” AMD).

Don’t Be Old, Uneducated, or a Smoker

The researchers compiled physical and demographic data for each participant, including what they ate, based on a detailed food-frequency questionnaire. This enabled them to calculate the average dietary glycemic index (dGI) value for each participant, based on the weighted average of the GI values for each food item in the diet. Using the calculated values, the researchers then divided the participants into quintiles—five same-sized groups, ranging from the lowest 20% of dGI values to the highest 20%. Most of the values were between 74 and 83, which is in the high range—not unusual for American diets. For a separate analysis, the participants were also divided into quintiles based on their total carbohydrate intake.

The goal of the study was to determine whether or not the participants’ dGI values or their total carbohydrate intakes correlated with the degree of severity of AMD in those participants who had the disease. First, however, using statistical techniques, the researchers had to eliminate the effects of confounding factors, both dietary and nondietary, that are known to affect the risk (either positively or negatively) for AMD, so that only the effects of dGI and carbohydrate intake would emerge from the data.


The central portion of a human retina. The dark region to the left of center is the macula, and the light spots within it are drusen. (The light region on the right is the optic disk.)
They included, in their analysis, all such factors, namely: age, sex, race, educational level, body mass index, alcohol intake, smoking status, sunlight exposure, hypertension history, lens opacity, refractive error, and energy-adjusted dietary variables, including the intakes of total fat, lutein and zeaxanthin, folic acid, niacin, riboflavin, thiamine, beta-carotene, vitamin C, vitamin E, and zinc. The three most important risk factors for the development of AMD in the AREDS population were old age, lower educational level, and smoking.

20% of the AMD Cases Were Preventable

There was no correlation between the quantity of carbohydrate intake and the risk for AMD—good news for those who love their carbs (and who doesn’t?). By contrast, there was a significant correlation between dGI values—which represent the quality of the carbohydrates ingested—and the severity of AMD in those who had the disease: the more severe the AMD, the higher the dGI turned out to be. For participants whose dGI was greater than the median (midpoint) value for the entire group, there was a 49% increase in the risk for advanced AMD than for participants whose dGI was below the median.

Glycemic Control Is Good for
Your Head—Inside and Outside

Didn’t Mom always say that breakfast is the most important meal of the day? Was Mom ever wrong about anything? All right, then. Now comes yet more evidence of her infallibility: a paper just published in the medical journal Appetite, whose title pretty much says it all: “A low glycaemic index breakfast cereal preferentially prevents children’s cognitive performance from declining throughout the morning.”1

We must forgive the British authors their inability to spell glycemic correctly. Aside from that, they did a fine job of describing their study of 64 children (26 boys and 38 girls, aged 6 to 11), whom they tested for cognitive performance both before and after breakfast on two consecutive days. In random order, each kid was given 35 g (1.2 oz) of either a popular, high-GI cereal (Coco Pops; GI 77) or a more healthful, low-GI cereal (All Bran; GI 42), with 125 ml (½ cup) of semi-skimmed milk. (The kids’ opinion of the All Bran was, perhaps wisely, not disclosed.)

Low-GI Breakfast Helps Kids Stay Mentally Sharp

The kids were tested on nine tasks that fell into one or more of five categories of cognitive function: speed of attention, speed of memory, accuracy of attention, secondary memory, and working memory. The data showed that when they ate All Bran for breakfast, they experienced significantly less decline in cognitive performance throughout the morning than when they ate Coco Pops. This effect was observed in two of the five categories: accuracy of attention (the ability to sustain attention) and secondary memory (the ability to store, hold, and retrieve information). There were no statistical differences between boys and girls.

The results were tentatively attributed to the postprandial (after-meal) changes in the children’s blood glucose levels, based on prior evidence that these play an important role in cognitive performance throughout the morning.* There is general scientific agreement that increased blood glucose levels have positive effects on cognitive performance, as demonstrated by studies that focused on having breakfast vs. not having breakfast. (Having it is better, as Mom could have told the scientists.)


*Because the two cereals differed not just in their carbohydrate content and GI values but also in their caloric value and their content of protein, fat, and fiber, it’s possible that one or more of these factors—especially protein, which is known to affect cognitive performance—also played a role in the results. Further studies are needed to sort this out.


What the present study indicates, though, is that it’s not just the presence of glucose in the blood that helps cognitive performance but also the relative constancy of the glucose levels, i.e., the absence of the strong fluctuations that can occur when high-GI foods are ingested (first the glucose spikes up, then it crashes to below optimal levels when the strong insulin response overcompensates for the spike). That’s what glycemic control is about.

Glycemic Control May Reduce Acne

Another area in which glycemic control may be helpful is just inches from the brain. It’s the face, which, in Westernized societies with relatively high-carbohydrate (and often high-GI) diets, is vulnerable to acne. Here the culprit may be repeated (daily) episodes of acute hyperinsulinemia, caused by acute hyperglycemia; the former has been implicated in acne because of its association with hormonal imbalances that are known to exacerbate the condition.

Acne afflicts mainly adolescents and young adults, but it can occur in anyone. Recently, researchers in Australia and Finland investigated the effects of a low-GL (low-glycemic-load) diet vs. conventional (mostly high-GL) diets in 43 males, aged 15 to 25, who had mild to moderate facial acne.2 Over a 12-week period, they observed significant reductions in total and inflammatory acne lesions in the low-GL group compared with the controls.

Also observed were significant weight loss and improved insulin sensitivity; it’s not clear, however, whether the latter was due primarily to the controlled glucose levels or to the weight loss. For that matter, as the authors admitted, it’s also not clear which of these factors was primarily responsible for the reduction in acne lesions. Much work remains to be done.

References

  1. Ingwersen J, Defeyter MA, Kennedy DO, Wesnes KA, Scholey AB. A low glycaemic index breakfast cereal preferentially prevents children’s cognitive performance from declining throughout the morning. Appetite 2007;49:240-4.
  2. Smith RN, Mann NJ, Braue A, Mäkeläinen H, Varigos GA. A low-glycemic-load diet improves symptoms in acne vulgaris patients: a randomized controlled trial. Am J Clin Nutr 2007;86:107-15.

Analysis of the data indicated that 20% of the prevalent cases of AMD would have been prevented if the participants in this study had consumed diets with a dGI value below the recorded median. That’s remarkable—and a tragic reminder of how much chronic disease and suffering could be prevented, if only we would pay more attention to the principles of good nutrition.

The authors concluded,2

In summary, these cross-sectional analyses suggest that poor dietary carbohydrate quality as defined by dGI, a modifiable risk factor, may increase the risk of AMD through several common etiologic factors of diabetes and CVD [cardiovascular disease], including the formation of AGE and increases in oxidative stress, inflammation, and hyperlipidemia. Our results also suggest that the quality, but not the quantity, of dietary carbohydrate influences the risk of AMD in both the early and late stages of the disease.

AGE is an apt acronym for advanced glycation end product, a gunky substance that builds up in many tissues of the body, including the eyes, as people grow older. It’s the result of chemical reactions that occur between glucose and various proteins when glucose levels are excessive.

Glycemic Control—A Cornerstone of Good Nutrition

One of the greatest advances in nutritional science in the past quarter-century has been the emergence of a fundamentally important new principle: glycemic control. This reflects the fact that a high dietary glycemic index has been implicated in the development of obesity, type 2 diabetes, CVD, and various cancers; studies also suggest that a high dGI is associated with several components of the metabolic syndrome, such as low levels of HDL-cholesterol (the “good cholesterol”), and with certain measures of chronic inflammation, such as elevated levels of C-reactive protein, which have been related to CVD and AMD.2

Thus, glycemic control is becoming recognized as a cornerstone of good nutrition. Bear in mind that reducing your dGI is very easy and can be done without compromising the quantity of the carbohydrates in your diet. It requires only that you select foods with a lower GI (better carbs) in preference to foods with a higher GI (poorer carbs).

This is important mainly for those carbohydrate-containing foods that are eaten in relatively large amounts—staples such as cereal, bread, pasta, potatoes, rice, etc.—because they produce the largest amounts of glucose upon digestion. Thus, we want a slow release (low GI) of glucose rather than a fast release (high GI) so as to produce relatively constant glucose levels throughout the day and avoid harmful glucose (and insulin) spikes.

Barley—A Good Way to Achieve Glycemic Control

A good way to achieve this goal is to substitute low-GI barley (average value: 25), as much as possible, for the much higher-GI wheat (average value: 47). Barley in the form of flour, flakes, or nuggets can be used in a wide variety of recipes to make foods that are healthful in more than just the traditional ways—they can also substantially reduce the overall glycemic index of a meal.* Other foods that are valuable in this regard are resistant starch, a relatively indigestible, low-calorie type of starch that helps to reduce postprandial (after-meal) glucose and insulin levels, and erythritol, a natural, low-calorie sweetener that’s safe for diabetics because it does not increase glucose or insulin levels at all.


*For more about the bountiful benefits of barley, see “Lowering Cholesterol with Beta-Glucan-Rich Barley” (July 2007) and “Beta-Glucan-Rich Barley Helps Control Blood Sugar” (August 2007).


If used regularly, foods such as these will thus reduce your overall dietary glycemic index and, probably, protect more aspects of your health than you could have imagined. Do you see?

References

  1. Chiu CJ, Hubbard LD, Armstrong J, Rogers G, Jacques PF, Chylack LT, Hankinson SE, Willett WC, Taylor A. Dietary glycemic index and carbohydrate in relation to early age-related macular degeneration. Am J Clin Nutr 2006;83:880-6.
  2. Chiu CJ, Milton RC, Gensler G, Taylor A. Association between dietary glycemic index and age-related macular degeneration in nondiabetic participants in the Age-Related Eye Disease Study. Am J Clin Nutr 2007; 86:180-8.


Will Block is the publisher and editorial director of Life Enhancement magazine.

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