What kind of diet helps prevent type 2 diabetes?

The Canadian Diabetes Association reports that nine million Canadians live with diabetes or prediabetes and 20 new cases are diagnosed every hour. For this second instalment in a series on diabetes and the work of U of T researchers, writer Jenny Hall sat down with Thomas Wolever, professor in the Department of Nutritional Sciences at U of T and scientist at the Keenan Research Centre of St. Michael’s Hospital. His research focuses on the role of dietary carbohydrates in health and disease.

To what extent is type 2 diabetes caused by diet and is it possible to reverse it through diet?

Several large research trials have tested the effect of intensive exercise and weight loss programs in people with impaired glucose tolerance, which is borderline diabetes. If you compare them to people who just get normal advice, they reduce their conversion to diabetes by about 60 per cent—quite an impressive reduction. These studies go on for four years or so, so whether this would last forever is not known. Generally, there’s a kind of inexorable deterioration of insulin secretion with time and people tend to become more and more diabetic and they get older.

Aside from that, I’ve seen, and I’m sure other doctors have too, people who’ve lost a lot of weight and cured themselves from diabetes.

So yes, we know that diet and exercise is an effective treatment to prevent diabetes. But it’s easier said than done. Most people can’t do much about it with food because most weight loss efforts are not successful.

If people are interested in trying to prevent or cure diabetes with diet, what would you tell them?

Reduce saturated fat intake, eat moderately high levels of carbohydrate, eat more fibre. There’s extremely good evidence for a low GI diet.

“GI” means “glycemic index,” right? Foods with a low glycemic index or “GI” are not going to raise your blood glucose level because they’re going to be digested more slowly?

That’s right. Low GI is not the same as low carb, though. We’re talking about a high carbohydrate diet that elicits a certain response in the body.

A high carbohydrate diet isn’t what most of us think we should be eating, whether we have diabetes or not.
There’s a big move toward low carb diets now and they can certainly help people lose weight. There are some good studies on this topic, but I think we’re really quite confused about this. I believe the recommendation should be to have high carbohydrate intake, by which I mean 50 to 60 per cent of your energy intake.

But the type of carbohydrate you’re eating matters, doesn’t it?

Yes. Carbohydrates that are high in fibre and have low GIs are associated with a reduced risk of diabetes.
We’ve done a number of studies where we’ve compared two groups of eaters. One group eats foods that are high in carbohydrate but have low GIs. The other eats less overall carbohydrate but achieves the same GI level as the first group. The high carb/low GI diet is more beneficial, at least for diabetes, in terms of beta cell function—your beta cells are the cells in your pancreas that make insulin, and they’re the cells that become damaged in type 2 diabetes.

The problem with most studies is they tend to be short term: a few weeks or a few months. We’ve done studies that last a year. What you find at the end of the year is the opposite of what you find at three months. At three months, people with diabetes on the overall low carb diet have better beta cell function than people with diabetes on the high carb/low GI diet. But after a year people on the high carb/low GI diet increased beta cell function by about 25 per cent, whereas people on the overall low carb diet had reduced beta cell function.

That’s surprising. Why do you think that is?

It may be related to colonic fermentation, which is how the fibre is broken down by the bacteria in our colon. This is a very complicated ecosystem which would take months and months to change. There are a lot of gut hormones involved.

We did a study where we fed people all-bran wheat fibre for a year. It took a year for the GLP-1 levels to go up. GLP-1 is a gut hormone. It increases insulin secretion and insulin sensitivity. There are several diabetes treatments based on it. The rationale was that there are GLP-1 secreting cells in the colon. When we feed the colon fibre, the short chain fatty acid products of fermentation gradually modify. It took 9 months for those to stabilize. What we’re seeing is that the environment in the colon changes slowly.

So it’s much more complex than we thought.

Yes, there aren’t really any easy fixes for diabetes. There are some good drugs and lots of new ones are coming out. But as far as I know none of these drugs are able to actually halt the progression of the disease. People think that some of the newer ones, which are based on GLP-1, might do that, but we don’t know that yet. You put someone on one drug and their glucose goes down, but then it starts to rise gradually with time, and it’s rising at the same rate as if you hadn’t done anything at all. Eventually another drug needs to be added, and then another.

If you had to recommend a diet—for anyone—what would you tell them to eat?

Follow Canada’s Food Guide. Fibre is probably the area where the fewest people meet the recommendations. The recommendations are 26 grams for women and 38 for men, on average. The average intake in Canada is 20 grams or less. Seventy-five per cent of people aren’t getting the recommended intake.

How can the average person get fibre?

Whole grains and legumes are the big sources. People talk about fruits and vegetables—they’re great but they don’t contain very much fibre per serving. Brown rice is OK, but doesn’t really contain too much fibre. Other good sources are whole wheat bread, whole oats and barley.