In the two decades I've been writing about food and health, one piece of diet advice has remained consistent: Eat more whole plant foods. More vegetables and fruits, more legumes and grains, more tubers and roots. There has been, that I can recall, only one notable exception, and it is the beleaguered potato. Eat more plants! Just not potatoes.
Why? One word: starch.
Starch is made up of molecules of glucose, a simple sugar, which our cells can use as fuel with very little processing from our bodies. It goes right to the bloodstream, and the blood sugar spike prompts the pancreas to release insulin, which enables our body to either use or store that sugar. When that's done, we're hungry again. The quicker it happens, the sooner we start casing the kitchen, looking for our next meal, and the fatter we get.
That's the theory, at any rate, but there's no potato consensus in the nutrition community.
Spearheading the anti-potato side is Walter Willett, professor of epidemiology and nutrition at Harvard's T.H. Chan School of Public Health. "Potatoes don't behave like most other vegetables," he said when I spoke with him. "In study after study, potatoes do not seem to have the benefit of reducing cardiovascular disease, and they are related to a higher risk of Type 2 diabetes." They're also associated with weight gain and hypertension, he noted.
But the key word there is "associated," and Marion Nestle, professor of nutrition, food studies and public health at New York University, is unwilling to finger potatoes as the cause of that litany of health problems. "This is using one particular food or nutrient as a reductive explanation for diseases and problems that are very complicated and have multiple causes," she says. "It's nutritionism."
Because the association between potatoes and disease derives from research on people who are asked what they eat and then tracked until something bad either happens or doesn't, it's hard to conclude that potatoes cause the disease. For starters, accurate self-reported diet data is hard to get.
To see just how hard, try filling out one of the questionnaires used by the researchers at Harvard. At Nestle's suggestion, I did, and found it nigh-on impossible. I do most of the shopping and cooking at my house and seldom eat out, but I still hadn't the foggiest idea how often I ate a half-cup serving of cabbage over the past year.
Willett fully acknowledges the imperfections and says the surveys are most useful to compare people who seldom eat a particular food with people who eat it several times a week. (If you fall into one of those categories, you're likely to be able to answer accurately.) Even so, a person who eats a lot of potatoes may be different from a person who eats no potatoes – and different in many non-potato-related ways – so it's impossible to definitely blame that heart attack on those French fries.
Is the association between potatoes and bad health outcomes a result of how people eat potatoes (often, fried, or with salt and plenty of sour cream)? Or is it because potato eating is part of a dietary or lifestyle pattern that could include, say, cheeseburgers and "Survivor" reruns, and it's the pattern, not the potatoes, that does the damage? Or is it just because the data are unreliable? We don't really know.
"Potatoes have calories," says Nestle, and she mentions that pesky word "moderation." But she's sure not giving them up, and she doesn't think we have to, either.
Alice H. Lichtenstein, professor of nutrition science and policy at Tufts University's Friedman School of Nutrition Science and Policy, also finds reason to be sceptical about the potato's bad reputation - in part, and a bit counterintuitively, because consumption has been tied to so many bad outcomes. "When it's associated with everything, you have to suspect that there's something else that is . . . accounting for it," she says. "Rarely in the science of nutrition do we have any dietary factors that span all outcomes."
When you move on from population data to research in which people are fed potatoes in a lab, the picture changes a bit. The knock on potatoes is that the quick spike in blood sugar and subsequent insulin response leave people hungry, but when you feed people potatoes and then ask them how full they are a couple hours later, and track what they eat at the next meal, potatoes seem to be quite satiating.
Back in 1995, a group of Australian researchers gave 240 calories' worth of food to subjects who each ate one of 38 specific foods. They tracked how hungry the subjects got and developed a Satiety Index (using white bread as a benchmark, with a score of 100). The hands-down winner, with a Satiety Index of 323, was potatoes. In second place was fish (225), and oatmeal took third (209).
It's an imperfect study, as it tracks hunger for only two hours, and few additional studies have compared satiety of potatoes vs. satiety of other foods (and some have been funded by the potato industry). Generally, there is enough disagreement over whether the speed of insulin response correlates with satiety that we shouldn't be so hard on the potato. A food is undoubtedly more than its contribution to blood sugar, and it's not unreasonable to believe that potatoes have other qualities (fibre, water, resistant starch) that could contribute to satiety.
Part of the potato's problem is simply its classification. When you call it a vegetable, you ask it to fight above its weight class. Compare potatoes with green vegetables, and you get more calories and less nutrition. But compare potatoes with whole grains, and you find surprising similarities, and even a case that potatoes are more nutritious. Compare 100 calories of baked potato to 100 calories of oatmeal, and you find a bit less protein (3 grams vs. 4), a bit more starch (18 grams vs. 16) and a similar mineral profile (potatoes have more potassium, but oats have more selenium). But potatoes beat out oats in just about every vitamin, as well as fibre.
Both Willett and Lichtenstein say they think nutrition guidelines should classify potatoes with grains; Willett would group them with refined grains and Lichtenstein would position them between whole and refined grains.
OK, so maybe potatoes should have a place at the table (although both frying and sour cream clearly have to be deployed with care). But if we're going to eat responsibly, we have to look beyond our own health and try to assess the environmental impact of the choices we make. From that point of view, the potato is a contender.
Because all crops confer calories, I like calories-per-acre as a starting point for environmental impact. When I use it, I hear from a few (sometimes quite a few) people suggesting (or insisting) that I need to take nutrients into account. Which is absolutely true; we need both calories and nutrients.
So, let's look at the potato's per-acre potential to deliver those nutrients vs. the potential of a nutrient powerhouse, broccoli. Sure, potatoes produce about 15 million calories per acre to broccoli's 2 million, but how about individual vitamins and minerals? The potato still scores more wins than losses on nutrients. It yields about half the calcium and vitamin C of broccoli per acre and none of the vitamin A, but it has three times the iron, phosphorus and potassium.
Here's what it boils down to: Broccoli delivers nutrients without attendant starch calories, and potatoes deliver nutrients with them. If you're a privileged American with a weight problem, broccoli's a great choice. Green vegetables are, calorie for calorie, the most nutrient-rich foods we can put on our plate. But if we're trying to feed a planet, we have to look at how to maximise both the calories and the nutrients we can grow on the land we have, and potatoes do that very well.
Let me be clear: I am very pro-green-vegetable. I eat a lot of them, and I employ various strategies to get my husband to eat them, too. (If I hear "This is the food my food eats" one more time…) Americans' health clearly would benefit if we all ate more of them. But the problem isn't just us. The problem is feeding the world, and we have to avoid crafting solutions in our own dinner's image. Let's hear it for the potato.
Written by Tamar Haspel. First appeared on Stuff.co.nz.