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Following a low glycemic index diet within an overall DASH-style diet doesn't appear to improve insulin sensitivity, lipid levels, or systolic blood pressure in overweight and obese patients, researchers found.

In the randomized, controlled OmniCarb trial, following a low-glycemic index, low-carbohydrate diet, compared with a high-glycemic index, high-carbohydrate diet did not affect any of these parameters, but it did lower triglycerides from 111 to 86 mg/dL (-23%, P≤0.001), according to , of Harvard School of Public Health, and colleagues in the Dec. 17 issue of

"Paradoxically, the low-glycemic index, high-carbohydrate diet decreased insulin sensitivity and increased LDL cholesterol and LDL apolipoprotein B levels while other dietary factors that affect LDL levels such as saturated fat, cholesterol, and fiber were held constant," the researchers wrote. "These findings are contrary to our hypotheses on glycemic index."

The OmniCarb Randomized Clinical trial was a crossover study conducted to better understand the effect of glycemic index (GI) and total dietary carbohydrate on risk factors for cardiovascular disease and diabetes.

Glycemic Index: What's in a Carb?

The health effects of certain carbohydrates have been the subject of much speculation and debate among the general public, health professionals, and policymakers, with the increased recognition that different types of carbohydrate-rich foods have different effects on blood glucose levels, the researchers wrote.

This property, called the glycemic index, is represented by a number reflecting the total rise in a person's blood glucose following consumption of a specific food. The glycemic index is quantified according to the amount 50 grams of its carbohydrate compared with 50 grams of glucose increased blood glucose over the course of 2 hours, the researchers noted.

"For example, a banana increases blood glucose more than an apple that has the same amount of carbohydrate. Boiled sweet potato increases blood glucose more than boiled carrot. Meals or complete diets may be designed using these tables to have a desired overall glycemic index," the researchers wrote.

They further noted that even though some have advocated eating low-glycemic foods as a strategy for improving health, "the independent benefits of glycemic index are uncertain, especially when persons are already consuming a healthful diet rich in whole grains, vegetables, and fruits."

Feeding Study Included 163 Overweight Adults

The OmniCarb trial included 163 overweight adults with baseline systolic blood pressure readings of between 120-159 mmHg who were randomized to four different diets, including all meals, snacks, and calorie-containing beverages, for 5 weeks, with each participant crossing over to a different diet for a second feeding period.

For any pair of the four diets, there were 135 to 150 participants contributing at least one primary outcome measure.

Eligible participants began an 8-day run-in phase, in which each study diet was given for 2 days. After the run-in, they were randomized to a sequence of the 4 study diets.

"For a crossover study with four diets to be administered, there are 24 possible sequences, of which we used eight," the researchers wrote. "We wanted to ensure that the high– and low–glycemic index diet components were each used in the first two periods for all participants."

Each diet was given for 5 weeks separated by a break of at least 2 weeks during which study participants ate foods of their choosing. Calorie intake was adjusted to maintain initial body weight. Participants completed a daily food diary for each day on the controlled diets, recording any foods that they did not eat and any additional items eaten.

On-site, weekday meal attendance was recorded and meal consumption was monitored by trained staff to ensure that the subjects ate the entire meal.

Among the two study diets with a high carbohydrate composition (58% of daily energy), one had a high glycemic index (≥65 on the glucose scale) and the other had a low glycemic index (≤45 on the glucose scale).

The two other diets had a low carbohydrate composition (40% of daily energy), with one having a high (≥65%) and the other having a low (≤45%) glycemic index. These glycemic index cut points corresponded approximately to the first and fifth quintiles of U.S. intake.

Primary outcomes were insulin sensitivity, systolic blood pressure, LDL and HDL cholesterol, and triglyceride levels. Secondary outcomes included diastolic blood pressure, fasting and 2-hour blood glucose and insulin, and other lipoprotein parameters.

Low Carb, Low GI Diet Did Not Lower BP, Cholesterol

Women made up 52% of the study cohort, and 51% of the participants were African American. About one in four (26%) had hypertension and 56% were obese (BMI ≥30). Just over two in three (68%) had LDL cholesterol of 130 mg/dL or greater and 30% had fasting blood glucose of 100 mg/dL or greater.

Among the main study findings:

• At high dietary carbohydrate content, the low- compared with high-glycemic index level decreased insulin sensitivity from 8.9 to 7.1 units (-20%, P=0.002); increased LDL cholesterol from 139 to 147mg/dL (6%, P≤0.001); and did not affect levels of HDL cholesterol, triglycerides, or blood pressure.
• At low carbohydrate content, the low- compared with high-glycemic index level did not affect the outcomes except for decreasing triglycerides from 91 to 86mg/dL (-5%, P=0.02).
• In the primary diet contrast, the low-glycemic index, low-carbohydrate diet, compared with the high-glycemic index, high-carbohydrate diet, did not affect insulin sensitivity, systolic blood pressure, LDL cholesterol, or HDL cholesterol but it did lower triglycerides from 111 to 86mg/dL (-23%, P≤0.001).

The researchers did not study the influence of lowering glycemic index in people with type 2 diabetes. They noted that two previously reported meta-analyses showed benefits for a low glycemic index diet in this population, and they cautioned that their findings could not be extrapolated to type 2 diabetics.

The findings from a recently publishedsuggested that following a low-glycemic index diet has little effect on HDL cholesterol or triglyceride levels, with lower LDL cholesterol levels observed only in the presence of increases in dietary fiber.

In an editorial published with the study, cardiologist Robert H. Eckel, MD, of the University of Colorado Hospital, Denver, wrote that many of the results from the OmniCarb study were not expected.

'Glycemic Index Less Important Than Overall Diet'

He noted that when glycemic index was lower in the high-carbohydrate diet, insulin sensitivity decreased, and levels of LDL cholesterol and apolipoprotein B increased with no changes in HDL cholesterol or triglyceride level or blood pressure. Fasting glucose was also higher with the low-glycemic index, high-carbohydrate diet.

"The unexpected findings of the study by Sacks et al. suggest that the concept of glycemic index is less important than previously thought, especially in the context of an overall healthy diet, as tested in this study," Eckel wrote. "These findings should therefore direct attention back to the importance of maintaining an overall heart-healthy lifestyle, including diet pattern."

Eckel co-chaired the writing committee responsible for the 2013 American College of Cardiology/American Heart Association guidelines on lifestyle management for reducing cardiovascular risk. Those guidelines identified the DASH diet, along with a Mediterranean-style diet, as heart healthy.

Both diets are plant focused, with an emphasis on fruits, vegetables, whole grains and nuts, and the DASH diet also includes low-fat and nonfat diary, lean meats, fish, and poultry.

Eckel wrote that asking patients about their diet and physical activity is an important but often overlooked aspect of the patient interview.

"Unless the patient has diabetes, the study by Sacks et al. suggests that glycemic index or load is less relevant than an emphasis on the overall dietary pattern," he wrote."Coincidentally, if the diet is heart-healthy to begin with, the glycemic index and load would be relatively low."

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