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Lowering the glycemic index of potatoes

A team of Canadian plant and nutritional scientists is working on new varieties that might make health-conscious consumers give potatoes a second look. Despite being rich in nutrients, vitamins and minerals, potatoes are often the subject of negative publicity. The Atkins Diet that had people shunning carbohydrates is a well-known example. Another is the low glycemic index, or low GI diet. People who are interested in low GI use a rating system to choose foods to help them lose weight and manage health issues, including diabetes. While freshly cooked potatoes tend to have high GI numbers, the Canadian research is targeted at improving their rating.

The glycemic index is not a diet, but a system that classifies carbohydrate foods based on how rapidly they are broken down and absorbed into the bloodstream. Certain carbohydrate foods, like freshly cooked potatoes, elevate blood sugar levels more quickly than other foods. Dr. J. Alan Sullivan, professor in the Department of Plant Agriculture at the University of Guelph, is one of the researchers trying to develop low GI potatoes. He says the key to achieving the result is the type of starch in the potato. “When humans digest it, the starch in the potato is broken down very quickly to sugar and absorbed into the bloodstream, and it creates a spike in blood sugar levels,” says Sullivan, explaining why the starch in potatoes is called a rapidly digestible or non-
resistant starch.

The ‘where’ of starch digestion is key
Sullivan notes that many foods have resistant starch, meaning it is not digested in the small intestine. When it gets into the large intestine, it is fermented by colonic bacteria to produce short-chain fatty acids, which are available to the human host as energy. The slower digestion of the starch reduces the glycemic index. “If the amount of resistant versus non-resistant starch can be altered in a particular variety, then the glycemic index may also be changed. So the team is working on changing the relative proportions of non-resistant and resistant starch,” says Sullivan, referring to his work to alter the starch profile of potatoes.

The research team is examining how the starch is accumulated in the tuber and how genomics and plant breeding can be used to create new varieties. With the assistance of graduate student Stephanie Bach, Sullivan says they are looking at the impact of the environment and genotypes on starch profiles, and breeding behaviour of genotypes with different starch profiles. The goal is to create potato varieties with altered starch profiles that result in a lower glycemic index. “We’re specifically looking for genotypes that have a higher level of resistant starch and a lower level of non-resistant starch,” says Sullivan. “In other words, genotypes with a high level of slowly digestible starch and a low level of rapidly
digestible starch.”

The study began with genotypes obtained from the plant breeders at the Potato Research Centre of Agriculture and Agri-Food Canada in Fredericton, New Brunswick. The genotypes were then measured for different starch profiles and the number was narrowed down for further study. “Some of them do look promising in terms of their starch profile. We’ll know more in a few months once we analyze the samples,” says Sullivan. “It is one of the most exciting projects we have with potatoes because of the potential payoff for the industry and human health. We hope to alter the starch profile but I’m not sure how much is possible.”

A lengthy, involved process
Altering the starch is just one step. Sullivan says the success of the effort will be determined by the ability to reduce the glycemic index number. He adds that the only way to know for sure is to actually test the blood sugar response in humans. For that part of the study, plant researchers are enlisting the help of nutritional scientist Dr. Tom Wolever of the University of Toronto.

Wolever was involved in the original development of the GI concept, supervised by Dr. David Jenkins at the university. “My PhD thesis involved validating the GI and developing the methodology so that it’s accurate and precise,” says Wolever. “I think we have made an impact on the methodology and we have done studies internationally with 28 research laboratories. We can show this measurement works the same on food around the world.”

Wolever says this potato study is exciting because the researchers are going to learn more about how the starch structure and starch chemistry interact with the physiology in people. “I believe it’s critical to what I am interested in, which is how carbohydrate foods influence our health. The opportunity to work with chemists, who understand the plants, doesn’t happen very often,” he explains.

The early testing so far has revealed that the preparation method of the potatoes is an important factor. Wolever says the GI values were much higher in freshly cooked potatoes when compared to cold. In a study by Tracy Moreira, one of Woelver’s graduate students, the variety that had the highest GI when it was hot, saw a GI reduction of 50 percent when it was allowed to cool. Meanwhile, other varieties that showed lower GIs when they were hot, maintained the same number when cooled. Cautioning that it is too early to draw conclusions because the research is in its infancy, Wolever says there are implications for how processing impacts the GI. For example, he has had conversations with a group that is trying to develop an instant mashed potato product that is rich in antioxidants. “When they heard our presentation, they thought the product might have a high GI, which might undo the good work they were doing (with antioxidants). So it opens up a lot of dialogue between different groups,” says Wolever.

Looking ahead for consumers
As for when consumers might be able to buy “low GI” potato products, Sullivan says the team must first develop a suite of varieties that can be adapted to different growing regions of Canada and the northeastern part of the US. He says one of his tasks is to study the adaptability and environmental influence on the lines, not only for the agronomic characteristics, but for the starch profiles, as well. “One of the things that this research has done has identified genotypes with different starch profiles. They’re fed back into the breeding program and will be used to create improved lines. That is going to be a long-term process,” says Sullivan. “If we have something in the trials now that has good agronomic characteristics and a much improved starch profile, then I would say (it would be commercially available) in less than five years.”

While it remains to be seen if the research will achieve its goal of producing a better potato, there is confidence that the study marks a new era of crop research. Instead of focusing on agronomics, processing consistency or even taste, more studies are delving into characteristics that would be of interest to the consumer in terms of health. “I’m excited about being able to do some really good science,” says Wolever. “We’re going to be leading the world on potato research because there is nothing else out there with this kind of combination working together.”

November 30, 1999  By Blair Andrews