Low-GI Diet for 1-year Suggests Improvements for T2DM?

As I was browsing through the January issue of the American Journal of Clinical Nutrition this weekend, a perspective written by John Miles caught my attention. In his article, A role for the glycemic index in preventing or treating diabetes, he wrote, "Elsewhere in this issue of the Journal, Wolever et al (7) report the results of the Canadian Trial of Carbohydrates in Diabetes (CCD). Patients with well-controlled type 2 diabetes who were treated with diet alone were randomly assigned to receive either a high-GI diet, a low-GI diet, or a low-carbohydrate, high-monounsaturated fat diet for 1 y.

The study was carefully conducted and of longer duration than many earlier trials. The investigators found no weight loss and a small increase in glycated hemoglobin (HbA1c) in all 3 groups. This increase in HbA1c is what one would expect with no intervention (8). The fact that glucose concentrations 2 h after an oral glucose challenge were significantly lower in persons who had followed the low-GI diet for 1 y than in those who followed the other 2 diets for 1 y suggests improvement in either insulin sensitivity or insulin secretion (or improvements in both)."

Sounds like a compelling study, so intrigued, I clicked open the study mentioned, The Canadian Trial of Carbohydrates in Diabetes (CCD), a 1-y controlled trial of low-glycemic-index dietary carbohydrate in type 2 diabetes: no effect on glycated hemoglobin but reduction in C-reactive protein, and read the abstract.

In it, researchers concluded "In subjects with T2DM managed by diet alone with optimal glycemic control, long-term HbA1c was not affected by altering the GI or the amount of dietary carbohydrate. Differences in total:HDL cholesterol among diets had disappeared by 6 mo. However, because of sustained reductions in postprandial glucose and CRP, a low-GI diet may be preferred for the dietary management of T2DM."

Here I was even more intrigued - a study trial comparing three different dietary approaches, and one low-carb for a year?

But I wondered, how was it that the low-carb diet didn't do as well as other studies would suggest it should have?

Wanting to know this, I opened the full-text to understand how it was possible that greater improvement was not found in the "low-carbohydrate" subjects and why HbA1c didn't remain stable or improve in the course of one year with either low-GI or low-carbohydrate diets. Previous study data published by others would suggest that HbA1c would at least remain stable with low-carb, no?

Well, it took no more than five minutes to fully see why things turned out as they did - the "low-carbohydrate" diet was not a low-carbohydrate diet afterall - at baseline the subjects assigned the low-carb diet ate an average of 210g of carbohydrate each day and during the low-carb diet consumed an average of 199g of carbohydrate each day.

Hello? In whose fantasy world is 199g of carbohydrate each day a low-carb diet?

Ah, but I digress...

While the researchers took pains to measure many risk factors, at the end of the year, the subjects in every group experienced progression of their diabetes risk factors - there simply was no improvement to laud in this trial, no matter how you twist the data.

What's absolutely disappointing in how the findings are presented is that the researchers honed in on two measures of improvement - CRP and post-prandial glucose - to the exclusion of significant declines in other measures that are critically important for those with type II diabetes.

Where do I even begin?

Weight remained fairly stable in all three groups, with only the low-GI group actually gaining some weight, despite no meaningful difference in calorie intake from baseline through one year on the low-GI diet.

Worse though is the lack of critical thought around the marked and significant increase in waist circumference in all three diet groups.

• The high-GI group started with a waist circumference of 99.1cm - it increased over the year to 103.1cm (+1.6 inches) - this despite stable weight on the scale (84.4kg at baseline; 84.3kg after 1-year on the diet).
• The low-GI group started with a waist circumference of 98.3cm - it increase over the course of the year to 104.9cm (+2.6 inches). They also experienced a weight gain, going from 81.1kg at baseline to 83.9kg at the end of the study (+6.2-pounds gained).
• The supposed "low-carbohydrate" group - eating 199g carbohydrate on average - started with a waist circumference of 98.6cm - it increased over the study period to 103.1cm (+1.8-inches) - like the high-GI group, this increase was despite a stable scale weight...they started at 84.7kg and ended the year at 84.3kg.

If that wasn't bad enough - all three groups experienced increases in their HbA1c too.

Those consuming a high-GI diet saw a rise from 6.2 to 6.34; low-GI saw a rise from 6.2 to 6.34; and those on the supposed low-carb diet rose from 6.1 to 6.35.

Over time the researchers reported that this rise was statistically significant - and I'd say clinically significant too!

Now with just these risk measures, you'd think there was enough to maybe, just maybe, inspire the researchers to challenge the efficacy of any of the above diets for those with type II diabetes. Maybe even say that perhaps the level of carbohydrate - despite quality or glycemic index or load improvements - matters; that simply modifying the type of carbohydrate in the diet does NOTHING for glycemic control and if carbohydrate is consumed habitually at levels seen in this study, perhaps someone with type II diabetes should expect a continued progression of their disease?

But, ya know what? They didn't even consider that. And the above problems were not all they reported either.

Let's see what else was reported in the data:

Total cholesterol didn't do much in any of the groups; LDL didn't change significantly in any group....HDL however declined in the low-GI group, but rose in the high-GI and supposed low-carb groups.

Triglycerides fell slightly in the high-GI and supposed low-carb groups, but rose in the low-GI group.

Two more markers of potential health risks found to be problematic in the low-GI diet over a year - and the researchers even noted it in the full-text - "With the low-GI diet, mean triacylglycerol was 12% higher, HDL was 4% lower, and the ratio of apoB to apoA was 4% higher than with the low-CHO diet

But a 12% increase in triglycerides and a 4% drop in HDL didn't set off any alarm bells either.

Neither did the fact the low-GI group saw an increase in their fasting plasma glucose over the year, which was also noted and basically disregarded.

With regards to the main focus in the abstract, C-Reactive Protein, the researchers did find a significant difference between the low-GI diet and the high-GI diet, but also noted that between the low-GI diet and the supposed "low-carb" diet there was no significant difference.

Yet, they chose to focus on the low-GI diet as better for a type II diabetic, despite the fact it led to

• weight gain
• waist circumference increase
• increase in HbA1c
• increase in fasting plasma glucose
• a marked rise in triglycerides
• and a decline in HDL

The conclusion here speaks volumes when taken in context to the carbohydrate intake in each group, "In subjects with T2DM managed by diet alone with optimal glycemic control, long-term HbA1c was not affected by altering the GI or the amount of dietary carbohydrate."

Better stated might be, with no meaningful change in absolute carbohydrate consumption, even with improvement in quality of carbohydrate - it is likely a type II diabetic will experience progressive decline in glycemic control along with other declines in risk factors over a year.

The data is published right in the full-text - the glycemic index as a means to reverse or prevent diabetes is no solution.

In this study, those who followed the low-GI diet had the worst overall outcome - they gained weight, increased waist circumference, saw triglycerides rise while HDL fell, and experienced a decline in glycemic control as evidenced by their increased HbA1c.

Yet you wouldn't know it from the abstract which focuses instead on "sustained reductions in postprandial glucose and CRP" and then concludes that "a low-GI diet may be preferred for the dietary management of T2DM."

And then back to the article from John Miles, who said this study "suggests improvements" in those who followed the low-GI diet for one year. Who's he kidding?