It’s More Nuanced Than Just “Eat More Fruits and Veggies”

We hear the advice from everywhere. The key to a healthy diet is when we fill at least half our plate with fruits and veggies (see choosemyplate.gov). Or, taking the language of the 2015 report of the Dietary Guidelines Advisory Committee, a healthy and more environmentally-friendly diet is one that’s predominantly plant-based.

But, as I’ve written about before, sometimes easy-to-remember dietary advice that can fit in 140 characters on Twitter doesn’t always reflect prevailing science on the topic. For example, by combining “fruits” and “vegetables” into a single phrase/piece of advice, do we unintentionally portray them as nutritionally equal in the eyes of consumers? Of course this isn’t true, fruits and vegetables often have very different nutritional profiles (take sugar content for example).

Taking this one step further, by generally recommending people consume more fruits and vegetables, do we overlook key differences within these two food groups? The basic question is this: do all vegetables influence health outcomes in the same way? Or taken another way, are there certain fruits or vegetables that might be less healthy than others?

This is the precise question asked by a team of researchers from Harvard University. In particular, they hypothesized, “greater consumption of fruits and vegetables with a higher fiber content or lower glycemic load would be more strongly associated with a healthy weight.”

The study analyzed data from three large, prospective cohort studies of more than 133,000 men and women in the US between 1986 and 2010 to see if intake of specific fruits and vegetables influenced weight gain. Researchers looked at data within 4-year time intervals and took into account a number of other lifestyle factors known to influence weight gain and obesity risk, such as other aspects of the diet, smoking status, and physical activity.

Let’s take a look at what they found.

The first thing that stands out is the variability in how different kinds of fruits and vegetables were associated with varying degrees of weight change.

 fruit and veggies
When subdivided into high- and low-glycemic fruits and vegetables, they found some not so surprising results. Low GI foods promoted the greatest weight reductions. High GI vegetables were the only category that was associated with weight gain.
glycemic indexNow if we dive a bit more into this category of high GI vegetables, there were considerable differences within this group. Corn consumption was associated with the greatest amount of weight gain per 4-year interval, at 2 pounds.
starchy veggies
Now if we look at more crocifurous and lower GI vegetables, their consumption was associated with weight loss of between 0-1.5 pounds. Interestingly, tofu and soy consumption was associated with the largest amount of weight loss, at 2.5 pounds per 4-year interval.
nonstarchy veggiesTurning to the fruit side of things, you’ll see that consumption of many was associated with weight loss, though the confidence interval for several stretched into the positive range. Berries stand out as associated with the greatest amount of weight loss.
fruit

So what’s the take-away?

Well, the main message is that like so many things in nutrition, not all foods within a particular food group are similar, just like not all macronutrients (such as fat) are similar. Not all vegetables induce the same physiological response. Same with fruit. There are important differences in the nutritional profile within these groups. We shouldn’t discount these nuances in favor of simple messages. They matter for the health outcomes we care about.
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