Weighing the evidence on low glycemic index diets

Weighing the evidence on low glycemic index diets
Weighing the evidence on low glycemic index diets

Most popular diets are based on a simple idea: eating foods that minimally increase blood sugar, instead of foods that produce a blood sugar spike, can help people lose weight.

The rating system for comparing how much blood glucose the body produces after consuming a certain food is known as the glycemic index (GI). Every food is assigned a number, which represents how much blood sugar rises compared to when glucose or white bread is consumed. People who follow a low GI diet can look up the number assigned to thousands of different food items in a database maintained by the University of Sydney.

Many mechanisms have been proposed for how the low GI diet works, but is it really the most effective way for people with diabetes to lose weight?

Despite efforts to definitively determine the effectiveness of the low GI diet, it remains unclear just how well it works compared with other dieting strategies.

What is the glycemic index and why is it important?

In theory, the low GI diet sounds like the perfect weight-loss strategy for people with diabetes. But in practice, the interaction between the foods a person eats and the body's response is complex and not always easy to predict.

In healthy individuals, the digestive system begins breaking down complex carbohydrates into simple sugars after a meal. In order to maintain a healthy blood glucose level, cells in the pancreas release a hormone called insulin, which signals other cells to absorb and store sugar. If blood glucose levels begin to get too low, another hormone called glucagon signals for sugar to be released.

In people with type 2 diabetes, this process does not run smoothly – cells do not respond correctly to insulin's signal to absorb sugar, and blood glucose levels can rise dangerously high and stay that way after a meal. Eventually the cells that secrete insulin wear out, and less and less of the signal is released.

The theory behind the low GI diet is that if individuals only consume foods that contain low levels of carbohydrates and sugars, the body's blood glucose level will not increase to such extremes. This will put less strain on the cells that produce insulin, which may recover with time.

To determine the GI rating for a particular food, researchers fed people servings of particular foods after a period of fasting and tested blood glucose levels every 15 minutes for two hours. Those results were then compared to the changes in blood glucose when a person consumes the same serving of a reference food. Foods with a ranking of 55 or lower are classified as "low GI;" a score of 56 to 69 is "medium GI" (bananas); and 70 and above is considered "high GI" (potatoes and white rice).

Some examples of low GI foods include dried beans and legumes, such as kidney beans or lentils; non-starchy vegetables; many milk products; most fruits; whole grain breads and cereals; and meats and fats, which do not contain carbohydrates.

Although foods are assigned precise scores, a myriad of factors can change their true GI content. Factors such as how the food is cooked, how ripe it is when it is eaten, the fat content of foods that are eaten with it and more, can influence a food's glycemic index. Although a food's glycemic index can give a dieter an estimate of how it compares to other items, it may not perfectly predict the effect that food will have on blood glucose levels.

Discrepancies in dietary data

In 2002, Obesity Review published two journal articles in the same issue with conflicting findings. Pawlak et al wrote in favor of the low GI diet, explaining potentially beneficial underlying mechanisms.2 In another study, Raben analyzed 31 short-term studies and 20 long-term studies, comparing the outcomes of high and low GI diets. Findings indicated that participants lost almost exactly the same amount of weight on either diet, and that participants were evenly split on whether the low GI diet made them feel more satiated.3

Although additional studies have been published during the last decade, none have produced a definitive answer regarding which diet is best for patients with diabetes.

In a two-year study of obese participants comparing the effects of a low-carbohydrate diet to a low-fat diet, Foster et al found similar outcomes. Both diets were equally effective at helping people initially lose weight and maintain weight loss over two years – there was no significant difference between the outcomes of the two diets.4 The study involved people who were obese, but who did not necessarily have type 2 diabetes.

In 2004, Anderson et al reviewed 10 studies involving patients with diabetes that were assigned to either a low GI diet or high GI diet (5). Although it was unclear whether either diet produced differences in weight loss, participants on the low GI diet did experience slight decreases in triglycerides, LDL cholesterol and other markers of cardiovascular. Findings from another meta-analysis by Willet et al indicated that low GI diets improved glycemic control and reduced the risk of developing type 2 diabetes.7

Another type of diet, a “low-energy diet,” in which low GI foods including lentils and long grained rice are consumed and highly processed foods are avoided – significantly reduced insulin levels and helped participants lose significant amounts of weight in a study by Slabber et al.8 However, the researchers found similar outcomes for participants assigned to a control diet.

In 2005, Liese et al found no differences in glycemic index, insulin sensitivity or secretion, BMI or waist circumference among those that consumed low and high GI foods. The study, which involved almost 1,000 people, was published in Diabetes Care.9

But some have suggested that there are differences in the way that people with high and low insulin secretion respond to low GI diets. Results from a six-month pilot study comparing the effects of high and low GI diets, Pitas et al found that overweight adults with normal glucose tolerance lost more weight on the low GI diet, whereas those with lower insulin secretion did not see the same benefits.10 The researchers hypothesized that high insulin secretion may help people reap the weight loss benefits of the low GI diet.

Different measures of success

Most people gauge a diet's success on how effective it is at achieving weight loss. Despite mixed results on this measure, low GI diets may still offer important benefits for patients with diabetes.

Participants on a low GI diet experienced reductions in risk factors associated with diabetic complications, such as HbA1c levels, and improvements in HDL cholesterol levels, independent of weight loss, results from a 2008 study in the Journal of the American Medical Association by Jenkins et al indicated.11

Additionally, many people might experience improved glucose control before losing significant amounts of weight, as was the case in a 2003 study published in the American Journal of Clinical Nutrition.12

What role does the low GI diet play in diabetes management?

Several diabetes organizations including WHO, the European Association for the Study of Diabetes, Diabetes Australia and the Canadian Diabetes Association  – endorse the use of a low GI diet to control diabetes.5

However, the American Diabetes Association (ADA) is more cautious in its recommendations, concluding in 2008 that there is not enough sufficient and consistent evidence to confirm that a low GI diet helps prevent diabetes.

Instead the ADA recommends people with diabetes follow ADA glycemic index guidelines combined with other weight loss strategies, including carbohydrate counting, to make healthier choices.13 Carbohydrates should account for 45% to 65% of daily food consumption, the organization recommends.

In 2008, Olendzki et al took a different approach. Instead of placing participants on a conscribed diet, the researchers provided nutrition counselling, educating participants about both the ADA guidelines and the low GI diet.

Participants were then allowed to choose their own foods in an effort to simulate real world situations in which people have the opportunity to make choices, and may or may not conform to one particular diet. Interestingly, both the ADA and low-GI dietary guidelines led to better glucose control, but neither resulted in significant weight loss.14

In the end, the most effective diet will be the one that a patient can readily understand, easily follow and stick with over the long term. Individuals with diabetes should speak to health care providers before making major dietary changes and can also seek the help of a nutritionist to develop a strategy that meets their needs.

Haley Bridger is a freelance medical writer living in the greater Boston area.

References

1.   Esfahani A, Wong JM, Mirrahimi A et al. "The Application of the Glycemic Index and Glycemic Load in Weight Loss: A Review of the Clinical Evidence." IUBMB Life. 2011 Jan;63(1):7-13.

2.   Pawlak DB, Ebbeling CB, Ludwig DS. “Should obese patients be counselled to follow a low-glycaemic index diet? Yes.” Obes Rev. 2002 Nov;3(4):235-43.

3.   Raben A. “Should obese patients be counselled to follow a low-glycaemic index diet? No.” Obes Rev. 2002 Nov;3(4):245-56.

4.   Foster GD, Wyatt HR, Hill JO et al. “Weight and metabolic outcomes after 2 years on a low-carbohydrate versus low-fat diet: a randomized trial.” Ann Intern Med. 2010 Aug 3;153(3):147-57.

5.   Anderson JW, Randles KM, Kendall CW et al. “Carbohydrate and fiber recommendations for individuals with diabetes: a quantitative assessment and meta-analysis of the evidence.” J Am Coll Nutr. 2004 Feb;23(1):5-17.

6.   Solomon TP, Haus JM, Kelly KR et al. “A low-glycemic index diet combined with exercise reduces insulin resistance, postprandial hyperinsulinemia, and glucose-dependent insulinotropic polypeptide responses in obese, prediabetic humans." Am J Clin Nutr. 2010 Dec;92(6):1359-68.

7.   Willett W, Manson J, Liu S. “Glycemic index, glycemic load, and risk of type 2 diabetes.” Am J Clin Nutr. 2002 Jul;76(1):274S-80S.

8.   Slabber M, Barnard HC, Kuyl JM et al. “Effects of a low-insulin-response, energy-restricted diet on weight loss and plasma insulin concentrations in hyperinsulinemic obese females.” Am J Clin Nutr. 1994 Jul;60(1):48-53.

9.   Liese AD, Schulz M, Fang F et al. “Dietary glycemic index and glycemic load, carbohydrate and fiber intake, and measures of insulin sensitivity, secretion, and adiposity in the Insulin Resistance Atherosclerosis Study.” Diabetes Care. 2005 Dec;28(12):2832-8.

10. Pittas AG, Das SK, Hajduk CL et al. “A low-glycemic load diet facilitates greater weight loss in overweight adults with high insulin secretion but not in overweight adults with low insulin secretion in the CALERIE Trial.” Diabetes Care. 2005 Dec;28(12):2939-41.

11. Jenkins DJ, Kendall CW, McKeown-Eyssen G et al. “Effect of a low-glycemic index or a high-cereal fiber diet on type 2 diabetes: a randomized trial.” JAMA. 2008 Dec 17;300(23):2742-53.

12. Liese AD, Roach AK, Sparks KC et al. “Whole-grain intake and insulin sensitivity: the Insulin Resistance Atherosclerosis Study.” Am J Clin Nutr. 2003 Nov;78(5):965-71.

13. American Diabetes Association. “Nutrition recommendations and interventions for diabetes: a position statement of the American Diabetes Association.” Diabetes Care. 2008 Jan;31 Suppl 1:S61-78.

14. Ma Y, Olendzki BC, Merriam PA et al. “A randomized clinical trial comparing low-glycemic index versus ADA dietary education among individuals with type 2 diabetes. Nutrition. 2008 Jan;24(1):45-56.

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