Low Glycemic Index Diets
The glycemic index was developed in 1980 by a group of researchers at the Food and Nutrition Science Department at the University of Toronto . It was developed when establishing dietary recommendations for diabetic people. The glycemic index measures how quickly foods cause blood sugar to increase. It uses a 0 to 100 scale, with lower numbers affecting blood sugar less than higher numbers.
Carbohydrates are dietary sugars. Simple carbohydrates, such as glucose and sucrose, can be digested quickly, providing quick energy and a rapid rise in blood sugar. Complex carbohydrates are slower to digest, and foods with complex carbohydrates also generally contain more fiber, which itself leads to slower digestion. Therefore, foods with simple carbohydrates trigger a more significant insulin response, and have higher glycemic indices, than foods with complex carbohydrates. Vegetables, fruits, whole grains, and legumes all contain carbohydrates and fiber.
A meta-analysis examined the relationship between a low-glycemic diet and glycated hemoglobin (HbA1c) . The investigators also examined fructosamine, which is used to measure glycated protein and reflects glucose control over the past 2 to 3 weeks , which can be useful in studies with shorter time durations that may not have enough time to access HbA1c.
The researchers analyzed 14 studies with a total of 356 people who had either type 1 or type 2 diabetes. These were randomized, controlled trials that were at least 12 weeks long and had modified at least two meals per day or 50% of the carbohydrate intake. Participants were considered to be on low-glycemic diets if most of their carbohydrates came from low-glycemic food sources, such as lentils, peas, beans, pasta, pumpernickel bread, oats, bulgar, parboiled rice, and barley. High-glycemic diets were defined as predominantly coming from high-glycemic foods, such as potatoes and high-glycemic varieties of breakfast cereal, rice, and bread.
Their analysis revealed that after approximately 10 weeks, among people with both types of diabetes, HbA1c levels were down approximately 0.4% in the low-glycemic groups compared to the high-glycemic groups. After approximately 4 weeks of the intervention diets, the low glycemic index diet revealed a 0.2 millimole/liter reduction of fructosamine.
A more recent meta-analysis was published in 2019. It reviewed 54 randomized, controlled trials, ranging from one week to 20 months, of children, adolescents, and adults with both types of diabetes . This study attempted to measure how diets based on the glycemix index compare to low-fat, low-carb, and conventional weight loss diets in adjusting blood glucose and body weight.
Low-glycemic diets showed significant and favorable decreases of 0.15 for HbA1c (n=2077) and 1.67 mg/dL for fasting blood glucose (n=2067). The low-glycemic diet lowered fasting blood glucose significantly more than the other diets. Low-glycemic diets were also shown to cause significant decreases in BMI, total cholesterol, and LDL. However, there were no significant changes to insulin requirements, fasting insulin, Homeostatic Model Assessment for Insulin Resistance (HOMA-IR), HDL, or triglycerides.
A randomized, controlled trial from 2014 examined 122 overweight and obese adults following one of three diets for 24 weeks: a moderate-carbohydrate and high-glycemic diet (HGI), a moderate-carbohydrate and low-glycemic diet (LGI), and a low-fat, high-glycemic index diet (LF) . At weeks 16, 20, and 24, the LGI group had a more significant decrease in BMI than the LF group. However, the HGI group’s BMI did not significantly differ from the other two diet groups.
In another randomized, controlled trial, participants were randomized into one of the following diet groups : low protein, low glycemic index (LP/LGI); low protein, high glycemic index, (LP/HGI); high protein, low glycemic index (HP/LGI); high protein, high glycemic index (HP/HGI); and a diet that adhered to the national healthy eating recommendations . For the first 8 weeks, all participants were on a low-calorie diet which, on average, resulted in 24.7 pounds of weight loss, and the average weight regained after one year was 8.6 pounds.
This study had a high dropout rate, as only 139 out of 256 people completed it. The high-protein, low-glycemic diet group had only a 26% dropout rate, while the low-protein, high-glycemic diet group had a 61% dropout rate. Between the low and high glycemic diet groups, there was no significant impact on weight loss. However, the high-protein diets had higher rates of weight loss maintenance.
Another meta-analysis included 14 studies in adults with data that ranged from from 1990 to 2016 . Results from 1097 people eating a low glycemic diet demonstrated a significant reduction in systolic blood pressure and diastolic blood pressure by a mean of 1.1 millimeters of mercury (mmHg) and 1.3 mmHg. Researchers then examined glycemic load, which is a measurement of a specific amount of total carbohydrate content and how rapidly it raises blood sugar. If glycemic load was reduced by 28 points, in pooled estimates for blood pressure, it reduced systolic and diastolic blood pressure by 2 mmHg.
The research on glycemic index-based diets has yielded mixed conclusions. Detailed dietary intervention research has been difficult due to lack of funding, short duration, and potential dietary and lifestyle confounders.
More, better controlled, research is needed to prove if, and to what degree, low-index foods are healthier than high-index foods. However, many low-index foods are higher in fiber and are typically more nutrient dense, so they continue to be recommended by most nutrition and health professionals.
This is just a brief review of selected meta-analyses and randomized, controlled studies examining diets based on the glycemic index. Like for many other diets, more of these trials would be helpful to elucidate other health benefits or discover the populations that might benefit the most. However, the evidence is clear: people who are trying to control their blood sugar or prevent getting diabetes in the first place would be wise to go for foods with lower glycemic indices.
This article is only a very brief summary. It is not intended as an exhaustive guide and is based on the interpretation of research data, which is speculative by nature. This article is not a substitute for consulting your physician and dietitian about which food, beverages, or supplements may or may not be right for you. We do not endorse supplement use or any product or supplement vendor, and all discussion here is for scientific interest.
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