A curious listener, who may or may not have been my scientist sister (Hi, Pam!) recently forwarded a headline that caught her attention:
“Eating vegetables does not protect against cardiovascular disease, finds large-scale study.”
“I thought your readers might come across this and would want to hear your take,” she wrote. “At least, I do!”
Just for context, this story was not on some clickbait site but on the blog of Frontiers, a respected science technology platform and publisher. And the headline does faithfully reflect the conclusion of the authors, who found that higher vegetable consumption was not associated with a reduced risk of heart disease or overall mortality. This analysis was done on dietary and health records collected as part of the “UK Biobank” study, which involves almost 400,000 people. So, these findings are correlations only; they do not prove cause and effect. However, the more expensive and difficult research needed to prove cause and effect often starts with this type of observational finding.
Interestingly, the authors looked at the effects of cooked and raw vegetables independently. Is one form more protective than the other? They found that cooked vegetable consumption was not associated at all with CVD or mortality, but people who ate more raw vegetables were somewhat less likely to develop heart disease or die.
Are raw vegetables better for you?
Proponents of a raw diet might be tempted to seize on this as evidence that cooking destroys the healthful properties of foods and that raw foods are more nourishing. I don’t agree with this view. For one thing, raw vegetables can lose up to half of their original nutritional value simply by sitting on your counter for two days—or in your refrigerator for two weeks. Although cooking does involve some nutrient losses, a vegetable that’s cooked the day it’s harvested could end up retaining more nutrients than a raw vegetable that’s been sitting around.
Furthermore, cooking actually makes some nutrients more absorbable. For example, the lycopene in cooked tomatoes is up to 4 times more bioavailable than that of fresh tomatoes.
Nutrients are also lost when foods are dehydrated, frozen, soaked, or juiced. So, when it comes to nutrient losses, unless you can arrange to eat every meal in the field where it was grown, it’s all sort of relative. And even though nutrients are lost, don’t worry—there are still plenty left!
In my view, the biggest nutritional advantage of a raw food diet isn’t the enzymes or the extra nutrients you glean by not cooking your vegetables. Rather, it’s the fact that a raw food diet contains no fried foods and no baked goods. No partially hydrogenated fats, refined flour, Twinkies, or potato chips. A raw food diet is rich in minimally processed fruits, vegetables, nuts and seeds and devoid of virtually all junk and processed foods. But you can achieve that without eschewing cooking.
Besides, after finding a modest association between raw vegetable consumption and improved health outcomes, the researchers of this latest study took into account confounding factors such as physical activity, body weight, age, smoking, drinking, use of supplements, and other dietary factors. Once they adjust for all of that, the raw food advantage pretty much disappeared. Residual confounding, they concluded, is likely to account for much, if not all, of the observed associations.
Bottom Line: Neither raw, cooked, nor total vegetable consumption appeared to reduce the risk of heart disease in any meaningful way.
But before you cross “eating vegetables” off your to-do list, let’s take a closer look at just how many vegetables these participants were eating.
How many vegetables is enough?
Remember that the goal is to be eating 5 servings, or 2 1/2 cups, of vegetables a day. The consumption of vegetables among participants in this study, however, is so low that it is not measured in cups or even fractions of cups. It’s measured in tablespoons. (For our metric-minded listeners, a tablespoon is equal to 15 ml.) The average daily vegetable consumption among the Biobank subjects is 2.8 T of cooked vegetables and 2.3 T of raw vegetables. That adds up to less than a single serving of vegetables a day.
For the purposes of their analysis, the researchers divided the subjects into four categories of vegetable consumption. The lowest vegetable consumers ate 0-1 tablespoons of vegetables a day. The highest vegetable consumers at ate least 8 tablespoons a day. They then compared the highest group to the lowest group and saw little to no difference.
I shared these details with my sister (who designs statistical analyses for a living), and she wrote back that an equally accurate headline would have been “Eating a negligible amount of vegetables is not detectably better than eating none at all.”
Would eating 5 servings of vegetables a day (or even 2 or 3) make a difference in your heart health? This study can’t answer that question, but other studies have. As the authors write, “There exists a large body of research evidence to suggest that a high vegetable intake may protect against a wide range of health outcomes, including cardiovascular disease.”
Given the very low vegetable consumption in this population, I’m not even sure that this study succeeds in answering the question of whether raw vegetables are more or less heart-protective than cooked vegetables.
Finally, it’s worth pointing out that the putative benefits of vegetable consumption are far broader than their effect on cardiovascular disease. Higher vegetable consumption (and by “higher”, I mean something closer to the recommended 5 servings a day) is associated with high overall diet quality, and reduced risk of cancer, dementia, overweight and obesity, and all-cause mortality. That’s one of the reasons that eating 5 servings of vegetables is one of the key health habits tracked in the Nutrition GPA app.
If you haven’t checked it out yet, the Nutrition GPA is a free smartphone app that I created to help my own clients track and upgrade their nutrition. And, while they may not get 5 servings every single day, Nutrition GPA users eat about 5 times more vegetables than the average American adult.
I want to thank my sister Pam for forwarding this study and giving me an opportunity to do two of my favorite things: 1) Dig a little deeper into sensational health headlines and 2) Nag you to eat more vegetables.
If you have a study or a headline you’d like me to unpack in a future episode, email it to me at email@example.com, and don’t forget to include a link to your headline or study so that I can track it down.