老司机直播

Carbs that are high in fibre with a low glycemic index can help prevent type 2 diabetes (Photo by David Smith, sxc.hu)

What kind of diet helps prevent type 2 diabetes?

Second in a series on 老司机直播 research into 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 老司机直播 researchers, writer Jenny Hall sat down with Thomas Wolever, professor in the Department of Nutritional Sciences at 老司机直播 and scientist at the Keenan Research Centre of St. Michael鈥檚 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鈥攓uite an impressive reduction. These studies go on for four years or so, so whether this would last forever is not known. Generally, there鈥檚 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鈥檝e seen, and I鈥檓 sure other doctors have too, people who鈥檝e 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鈥檚 easier said than done. Most people can鈥檛 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鈥檚 extremely good evidence for a low GI diet.

鈥淕I鈥 means 鈥済lycemic index,鈥 right? Foods with a low glycemic index or 鈥淕I鈥 are not going to raise your blood glucose level because they鈥檙e going to be digested more slowly?

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

A high carbohydrate diet isn鈥檛 what most of us think we should be eating, whether we have diabetes or not.
There鈥檚 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鈥檙e 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鈥檙e eating matters, doesn鈥檛 it?

Yes. Carbohydrates that are high in fibre and have low GIs are associated with a reduced risk of diabetes.
We鈥檝e done a number of studies where we鈥檝e 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鈥攜our beta cells are the cells in your pancreas that make insulin, and they鈥檙e 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鈥檝e 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鈥檚 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鈥檙e seeing is that the environment in the colon changes slowly.

So it鈥檚 much more complex than we thought.

Yes, there aren鈥檛 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鈥檛 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鈥檚 rising at the same rate as if you hadn鈥檛 done anything at all. Eventually another drug needs to be added, and then another.

If you had to recommend a diet鈥攆or anyone鈥攚hat would you tell them to eat?

Follow Canada鈥檚 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鈥檛 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鈥攖hey鈥檙e great but they don鈥檛 contain very much fibre per serving. Brown rice is OK, but doesn鈥檛 really contain too much fibre. Other good sources are whole wheat bread, whole oats and barley.

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