Low-Glycemic Index Diets: Should They Be Recommended for Diabetics?

Article Outline

• Abstract
• Medical Nutrition Therapy
• The Glycemic Index Concept
• GI and Risk for Diabetes
• LGI Diet for Diabetics: Retrospective, Epidemiological, and Pilot Studies
• LGI Diet for Diabetics: Randomized Controlled Trials
• LGI Diet for Diabetics: Meta-Analyses
• Participant Satisfaction with LGI Diet
• Criticisms of the LGI Diet
• Conclusion
• Implications for Practice
• References
• Copyright

Abstract 

Diabetes continues to be an overwhelmingly prevalent disease worldwide, and its occurrence continues to increase. It is well established that medical nutrition therapy is effective for glycemic control, but the exact diet recommended to diabetics remains unknown. The relatively new glycemic index (GI) and the low-glycemic index (LGI) diets are gaining momentum, but their place in diabetic therapy continues to be controversial. This article presents a review of the current literature on the LGI diet and its relation to glycemic control in diabetics. There is ample evidence that the LGI diet significantly improves glycemic control in diabetics, but there is also much criticism. Nurse practitioners need to educate themselves on the GI concept and the LGI diet to educate patients and accurately identify who will most benefit from the LGI diet.

Keywords:  Diabetes , glycemic index , hemoglobin A1c , low-glycemic index diet , medical nutrition therapy

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According to the American Diabetes Association, the prevalence of diabetes in the United States is 20.8 million or 7% of the population.¹ Diabetes is the 5th leading cause of death in the United States and the death rate from diabetes has increased by 45% since 1987.² Deaths from diabetes are largely related to its resulting chronic complications. Diabetics have a 2 to 4 times higher death rate from heart disease than non-diabetics and also a 2 to 4 times higher risk of stroke.³ Diabetes is the leading cause of new cases of blindness in adults between the ages of 20 and 74, and diabetic retinopathy accounts for 12,000 to 24,000 new cases of blindness every year.³ Diabetes is also the leading cause of kidney failure, resulting in 44% of incident cases in 2002.³ Sixty to seventy percent of diabetics have some form of nervous system damage and more than 60% of nontraumatic lower extremity amputations occur in diabetics.³ Overall, the total annual cost of diabetes in the United States in 2007 was estimated at $174 billion.⁴ Despite these alarming statistics, there is ample research demonstrating that controlling one's blood sugar, blood pressure, and lipid profile can decrease the risk of all these diabetes-related complications. For example, the landmark UKPDS study found that for every one percentage point drop in hemoglobin A1c (HbA1c), the risk of any diabetes-related end point was reduced by 21%, the risk of any diabetes-related death was decreased by 21%, the risk of myocardial infarction was lowered by 14%, the risk of stroke was diminished by 12%, the risk of amputation was decreased by 43%, and the risk of any microvascular complication was reduced by 37%.⁵ Also, every 10-mmHg drop in systolic blood pressure results in a decreased risk of any diabetes-related complication by 12%, and improved lipid panels can decrease the risk of cardiovascular complications by 20% to 50%.³ Thus, controlling blood sugar, blood pressure, and lipid profiles can prevent the deadly complications of diabetes.

Blood glucose control is the primary goal of diabetes management and thus interventions that decrease postprandial blood glucose values are imperative.⁶

Postprandial blood glucose concentration is determined by the appearance of glucose in the circulation, which is chiefly the result of carbohydrates ingested, as well as the rate of glucose removal from the blood stream.⁶ Carbohydrates have several variables that affect postprandial blood glucose levels, including the type of starch, the style of food preparation, ripeness, and the degree of processing of the carbohydrate.⁶ Other variables that affect postprandial glucose response are the preprandial blood glucose level, the macronutrient distribution of the meal, the amount of insulin available, and the degree of insulin resistance.⁶ Monitoring carbohydrate intake via carbohydrate counting or exchanges has been a mainstay in achieving glycemic control, but new research has shown that the choice of carbohydrate consumed, not just the amount, also has an impact on blood glucose levels.⁶ The concept of the low-glycemic index (LGI) diet has created a controversy regarding which specific diet to recommend to diabetics.

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Medical Nutrition Therapy 

An integral component of managing diabetes is medical nutrition therapy (MNT). The goals of MNT are to maintain blood glucose levels and blood pressure in the normal range, to maintain a lipid profile that decreases the risk of cardiovascular complications, to prevent or slow the development of the diabetic complications, to address an individual's cultural and personal dietary preferences, and to maintain the pleasure of eating.⁶ Multiple clinical trials and outcome studies have demonstrated the effectiveness of MNT in diabetics with decreases in hemoglobin A1c (HbA1c) of 1% to 2%, depending on the duration of diabetes.⁷ However, the exact diet recommended remains controversial.

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The Glycemic Index Concept 

The concept of the glycemic index (GI) was introduced by Jenkins et al in 1981 as a means to categorize carbohydrates according to their resulting glycemic response.⁸ Ludwig defines the GI as “the incremental area under the glucose response curve after a standard amount of carbohydrate from a test food relative to that of a control food (either white bread or glucose) is consumed.”^8(p2414) Foods classified as having a low glycemic index (LGI) are non-starchy vegetables, fruits, legumes, milk, yogurt, and traditionally processed grains such as wholegrain bread, pasta, and oats.⁹ Refined, starchy foods, such as white bread, processed cereals, potato, watermelon, and most crackers, on the other hand, are classified as having a high glycemic index (HGI).⁹ The GI ranks foods on a scale from 0 to 100, with LGI foods having a GI less than or equal to 55 and HGI foods having a GI greater than or equal to 70.⁹ Glycemic load is defined by Ludwig as “the weighted average glycemic index of individual foods multiplied by the percentage of dietary energy as carbohydrate”^8(p2415) and thus represents both the quality and quantity of the carbohydrate and is calculated by multiplying the GI value by the grams of carbohydrate consumed and dividing the product by 100.

HGI foods are rapidly absorbed and digested and thus can have deleterious effects on blood glucose control.⁸ Within 2 hours of eating an HGI meal, blood glucose concentration is at least double that of ingesting an LGI meal.⁸ This sharp rise in blood glucose potently stimulates insulin secretion and inhibits glucagon release, which results in a high insulin to glucagon ratio.⁸ Two to four hours after ingestion of a HGI meal, nutrient absorption is decreasing but the effects of the high levels of insulin and low levels of glucagon remain, which results in rapidly falling blood glucose levels, often into the hypoglycemia range.⁸ Four to six hours after eating an HGI meal, the low levels of metabolic fuels in the bloodstream trigger a counter-regulatory hormone response that brings blood glucose levels back to normal, but also increases free fatty acid concentrations to levels above those occurring after ingestion of a LGI meal.⁸ In contrast, ingestion of an LGI meal does not produce the same hypoglycemia and its hormonal sequelae because the absorption of LGI nutrients is much slower and thus postprandial blood glucose levels rise and fall slowly without the peaks and troughs that are associated with HGI foods.⁸

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GI and Risk for Diabetes 

Several studies¹⁰, ¹¹ have found that a HGI diet in healthy individuals is associated with an increased risk of developing type 2 diabetes. The Health Professionals Follow-up Study (1997) was a national longitudinal study that followed 42,759 healthy male health professionals, aged 40 to 75 years, over 6 years, and discovered 523 incident cases of confirmed type 2 diabetes over this time.¹⁰ Men who consumed an HGI diet were at an increased relative risk of developing diabetes, even after adjusting for age and other known risk factors for diabetes.¹⁰ When comparing the highest GI quintile (median GI = 79) with the lowest GI quintile (median GI = 65), the relative risk of developing type 2 diabetes was 1.37, which is statistically significant.¹⁰ The Nurses' Health Study II (2004) followed 91,249 healthy U.S. women, aged 24 to 44 years, over 8 years and found 741 incident cases of confirmed type 2 diabetes over this time.¹¹ The women who consumed an HGI diet were at an increased relative risk of developing type 2 diabetes, even after adjusting for age and other known diabetes risk factors.¹¹ There was a 59% increased risk of developing type 2 diabetes in the highest GI quintile (median GI = 82.1) versus the lowest GI quintile (median GI = 74.1), which is statistically significant.¹¹

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LGI Diet for Diabetics: Retrospective, Epidemiological, and Pilot Studies 

The results from retrospective, epidemiological, and pilot studies, randomized controlled trials, and meta-analyzes regarding the association between an LGI diet and glycemic control in diabetics is summarized in Table 1. Many studies have been conducted in an attempt to determine if an LGI diet is beneficial to diabetics. Burani and Longo¹² performed a retrospective study to evaluate the effects of a LGI diet on glycemic control in 21 diabetic subjects. All subjects were referred for diabetic MNT and all had received LGI dietary counseling.¹² Their results reveal a statistically significant 19.4% drop in HbA1c as a result of MNT counseling about low-GI meals.¹²

Table 1. Summary of Relevant Research on Effectiveness of LGI Diet on Glycemic Control

Buyken et al's¹³ epidemiologic study used the EURODIAB IDDM Complications Study to examine the relationship between GI and glycemic control in 2054 European type 1 diabetics. This study revealed that dietary GI is independently related to HbA1c, which is statistically significant.¹³ There is also a statistically significant difference in the adjusted HbA1c values in the lowest GI quartile (median GI = 75) versus the highest GI quartile (median GI = 89).¹³ The adjusted HbA1c in the lowest GI quartile was 8% lower than that found in the highest GI quartile.¹³

Ma et al¹⁴ performed a pilot study to evaluate the effectiveness of an LGI dietary intervention with personal digital assistant (PDA) support on glycemic control in 13 adults with poorly-controlled type 2 diabetes. This study was conducted over a period of 6 months and consisted of nutritionist-delivered education and counseling about the LGI diet, in which all subjects were involved in an initial 2.5-hour group session, a 1-hour individual session at week 2, a group grocery tour at week 4, and subsequent 30-minute individual phone sessions at months 2, 3, and 5.¹⁴ The results of this study found a statistically significant mean decrease in HbA1c of 0.5% after the LGI dietary intervention.¹⁴

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LGI Diet for Diabetics: Randomized Controlled Trials 

Many randomized controlled trials (RCT) have also compared the effects of an LGI versus an HGI or conventional diet on glycemic control in diabetics. Rizkalla et al¹⁵ performed a study to compare the effects of an HGI versus an LGI diet on glucose control of 12 type 2 diabetic men. All subjects were randomly assigned to two 4-week periods of an LGI or HGI diet separated by a 4-week washout period, in a crossover design.¹⁵ All subjects were educated on the LGI and HGI diets by providing lists of LGI and HGI foods and an individual counseling session with a dietician.¹⁵ The results revealed that their HbA1c was lower at the end of the LGI diet than at the end of the HGI diet, which was statistically significant.¹⁵ In addition, their HbA1c increased during the HGI diet and decreased during the LGI diet, with the change in HbA1c during the LGI diet being significantly greater than during the HGI diet, which was statistically significant.¹⁵

Heilbronn et al¹⁶ completed an RCT involving 45 type 2 diabetics to determine whether an LGI diet versus an HGI diet differentially affected blood glucose levels. All subjects followed the average Australian diet for the first 4 weeks and then were randomly assigned to a HGI or LGI group for the remaining 8 weeks.¹⁶ Key LGI or HGI foods were provided to all subjects and all subjects also received counseling with a dietician every 2 weeks.¹⁶ The results revealed that mean HbA1c levels were significantly reduced in both groups (LGI diet HbA1c fell from 6.65% to 6.04% and HGI diet HbA1c decreased from 6.35% to 6.06%) but there was not a statistically significant difference between the groups.¹⁶

Jimenez-Cruz et al¹⁷ performed an RCT to compare the effects of an LGI versus an HGI Mexican-style diet on glycemic control in 14 overweight Mexican type 2 diabetics. All subjects were randomly assigned to two 6-week periods of an LGI or an HGI diet separated by a 6-week washout period, in a crossover design.¹⁷ Detailed instructions, including a pamphlet, were given to all subjects regarding how to incorporate LGI or HGI foods into their diet.¹⁷ The results showed a statistically significant greater decrease in HbA1c for those in the LGI versus the HGI group.¹⁷

Ma et al¹⁸ performed an RCT comparing the effects of an LGI diet with the ADA diet among 40 type 2 diabetics on blood glucose control. All subjects were randomly assigned to an LGI or the standard ADA diet, and educational sessions were provided by dieticians monthly for the first 6 months and then at months 8 and 10.¹⁸ Their results revealed a statistically significant decrease in HbA1c in both groups, but no statistically significant difference between the groups.¹⁸ However, those consuming the LGI diet had a statistically significant much lower likelihood of adding or increasing their diabetic medication regimen versus those eating the ADA diet.¹⁸ Thus, those in the LGI group were able to achieve equivalent control of blood glucose levels using less diabetic medications.¹⁸

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LGI Diet for Diabetics: Meta-Analyses 

Several meta-analyses have been completed to determine the effectiveness of the LGI diet on glycemic control. Opperman et al¹⁹ performed a meta-analysis to examine the evidence that LGI diets have a beneficial effect on carbohydrate metabolism when compared with HGI diets. This meta-analysis examined 16 randomized controlled trials published in English between January 1981 and April 2003 that were of crossover or parallel design.¹⁹ The 16 randomized controlled trials involved a total of 396 subjects, of which 105 had type 1 diabetes, 228 had type 2 diabetes, 46 had coronary heart disease, and 17 were healthy.¹⁹ Their results reveal a statistically significant decrease in mean fructosamine of 0.1 mmol/L in those receiving the LGI diet compared to the HGI diet, as well as a statistically significant reduction in mean HbA1c of 0.27% in those receiving the LGI diet.¹⁹

Kelly et al²⁰ completed a meta-analysis to examine the evidence from RCTs of the relationship between the LGI diet and coronary heart disease and risk for coronary heart disease. Fifteen RCTs met the strict inclusion criteria and were thus included in the meta-analysis.²⁰ Twelve of the studies included subjects with diabetes, one studied participants with advance coronary heart disease, one examined overweight subjects, and another included participants with impaired glucose tolerance and another risk factor for diabetes, and 14 of the 15 studies compared the effect of the LGI diet with the HGI diet.²⁰ This meta-analysis revealed mean HbA1c levels of those consuming the LGI diet of 0.45% less than the HGI diet, which was statistically significant.²⁰ This effect was observed at 12 weeks, but not at 4 to 5 weeks.²⁰

Brand-Miller et al²¹ conducted a meta-analysis to determine whether LGI diets versus conventional or HGI diets improve glycemic control in diabetics. This meta-analysis examined 14 RCTs of a crossover or parallel design published in English between 1981 and 2001 and included a total of 356 subjects.²¹ Their results revealed that LGI diets reduce HbA1c levels 0.43% points and also reduce fructosamine levels by 0.2 mmol/L above and beyond that produced by conventional or HGI diets, both of which are statistically significant.²¹ A combined meta-analysis, using HbA1c in studies longer than 6 weeks in duration and fructosamine in studies less than or equal to 6 weeks in duration, demonstrated that LGI diets decrease glycated proteins by 7% to 8% above and beyond that with conventional or HGI diets, which is statistically significant and amounts to a decrease of 0.6 HbA1c points if the end point for HbA1c is 8%.²¹

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Participant Satisfaction with LGI Diet 

Several of the above studies included questionnaires to determine subject satisfaction with the LGI diet. Burani and Longo¹² found that 100% of their study subjects felt that choosing LGI foods assisted them in improving their diabetes and that they intend to continue choosing LGI foods and incorporate these choices into their lifestyle. Ma et al¹⁴ discovered that 7 of the 13 subjects found adherence to the LGI diet easy, 3 found it difficult, and 3 were neutral, but 12 of the 13 subjects liked the LGI diet.

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Criticisms of the LGI Diet 

The LGI diet is criticized because the GI values themselves are highly variable. Many foods have been classified as LGI by one lab, medium-GI by another, and HGI by a third.²² The majority of this variation was due to random within-person variability.²² Increasing the number of replications for each subject could reduce this within-person variability and would be necessary to standardize GI measurement.²²

The GI concept is also criticized because the GI measures the blood glucose above the beginning fasting level and only for 2 hours.²³ The blood sugar of type 2 diabetics requires longer than 2 hours to return to normal, so the differences in GI values would be less if calculated over 4 hours, which is a more reasonable postprandial period for a diabetic.²³ In addition, GI is only measured after an overnight fast and there have been several studies that reveal the differences in GI would be much less if the GI were measured after lunch rather than after breakfast.²³

The fact that GI only measures individual foods and not total meals is another criticism of the LGI diet. Summing the individual food GI's cannot reliably predict the total GI of the meal.²² For example, the addition of fats and proteins to a carbohydrate meal is known to decrease the glycemic response of the individual.²² Thus, it is not possible to accurately predict the total GI of a meal from the individual GI components.

Yet another criticism of the LGI diet is that some LGI foods, such as ice cream, have LGI scores, but are not necessarily healthy, as they are high in fat.²² Thus, making healthy food choices should not be based only on GI, as one must also consider fat content, type of fat, energy density, fiber content, and serving size.²²

The fact that postprandial glycemia is based on more than GI classification, including the extent to which food is chewed and swallowed, individual biologic variation in digestion and absorption rates, cooking preparation, and temperature of the food, is another criticism of the LGI diet.²² Thus, to base a diet on GI alone is too simple and may not result in improved postprandial glucose control.

A final criticism of the LGI diet is that the studies that attempt to examine the relationship between LGI diet and glycemic control are not strong. Very few of the studies reviewed mentioned power analysis. In addition, some of them did not include pertinent demographic characteristics such as ethnicity, and those that did mention ethnicity were overwhelmingly representative of a Caucasian population. Kelly et al²⁰ concluded that the RCTs in their meta-analysis were of poor quality, short duration, and small sample size, and that there is a need for adequately powered and well-designed RCTs of longer than 12-weeks duration to adequately reveal the potential effectiveness of the LGI diet.

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Conclusion 

There is obviously considerable controversy surrounding the appropriate use of the LGI diet for diabetics. It is clear that the validity of the GI values needs to be increased via standardization. Decreasing the within-person variability of the GI values could be accomplished by increasing the number of replications of each subject.²² Although this will be expensive, it may be cost-effective, given the financial burden of diabetes and its chronic complications. The meta-analysis by Brand-Miller et al²¹ was particularly strong in support of the LGI diet, as it revealed an overall decrease in HbA1c of 0.43% above and beyond that produced by conventional dietary recommendations. Although this may seem like a small percentage, any decrease in HbA1c can have a beneficial impact on a diabetic's health. The landmark UKPDS study found that for every one percentage point drop in HbA1c, the risk of any diabetes-related endpoint was reduced by 21%, the risk of any diabetes-related death was decreased by 21%, the risk of myocardial infarction was lowered by 14%, the risk of stroke was diminished by 12%, the risk of amputation was decreased by 43%, and the risk of any microvascular complication was reduced by 37%.⁵ The ADA's most recent position, as of January 2008, is that “the use of glycemic index and load may provide a modest additional benefit over that observed when total carbohydrate is considered alone.”^6(pS64) Clearly, further research is needed involving RCTs, with larger sample sizes and of longer duration, to adequately elucidate the relationship between dietary GI and glycemic control in diabetics.

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Implications for Practice 

The potential use of the LGI diet for diabetes management has some implications for nurse practitioner practice. Nurse practitioners will have to educate themselves on the concept of the LGI diet as well as its importance in glycemic control so this information can be provided to diabetic patients. It will also be important for nurse practitioners to be able to identify which diabetic patients could most benefit from the LGI diet. The LGI concept adds an extra step to carbohydrate counting, as now the patient must count the quantity of carbohydrate and also be aware of its GI value. Thus, the LGI diet is probably not the best recommendation to the newly diagnosed diabetic who is overwhelmed with learning the basics of the disease and a healthy diet. However, there have been several studies¹², ¹⁵ that measured subject satisfaction with the LGI diet, and these studies reveal that the majority of diabetics are highly satisfied with the LGI diet and will continue to incorporate the concept into their lifestyle after completion of the study. Thus, these studies show that the LGI concept is not too complex for diabetics and that when provided with education, many diabetics enjoy the LGI diet. The patient most likely to benefit from the LGI diet would be the patient who is knowledgeable about diabetes, has mastered the basic nutrition recommendations, and is motivated to learn more information about improving glycemic control. Nurse practitioners will also need to be knowledgeable about where to locate GI reference values so that this information can be provided to patients. There are many GI reference websites and books available that provide GI reference lists. The International GI Database is housed in the Human Nutrition Unit, School of Molecular and Microbial Biosciences at the University of Sydney, and is headed by Dr. Jennie Brand-Miller.²⁴ Nurse practitioners can refer patients to this database via www.glycemicindex.com, where patients can enter food items and the database will provide the GI and GL of the food.²⁴ Table 2 gives examples of representative foods and their associated GI and GL values found in the International GI Database.²⁴ Finally, nurse practitioners will have to be aware of which nutritionists and diabetic educators in their community are knowledgeable about the LGI diet so that appropriate referrals can be made to enhance patient education. In conclusion, the GI concept is relatively new, so nurse practitioners must educate themselves about the LGI diet as well as its limitations. Overall, the LGI diet may be beneficial, despite its limitations, for some diabetic patients, and the prudent nurse practitioner will need to review the literature on the LGI diet to ensure they are providing evidence-based practice and appropriately selecting patients to whom the LGI diet should be recommended.

Table 2. Glycemic Index and Glycemic Load Values of Foods from International GI Database²⁴

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