We could all use more fruits and vegetables in our diets, and it certainly helps when they're in season and taste wonderful. If they aren't appetizing, it's hard to eat enough of these nutritious foods every day.
So what's the best way to make them tasty while maximizing nutrients? Some fruits and vegetables taste better cooked, and the process can make them easier to digest. But cooking can change their nutritional content.
Interestingly, some foods become healthier with they're cooked while others lose a lot of their nutritional value, so there's no black-and-white answer.
The advantages of raw
There's a lot of buzz about raw food diets. The belief is that heating food above a certain temperature (often 115 degrees Fahrenheit) destroys natural enzymes and other nutrients.
For example, as U.S. News & World Report points out, vitamin C is easily destroyed by heat. Many vegetables — like broccoli, peppers and leafy greens — are filled with vitamin C, which ebbs away when they're cooked. The same thing happens with B vitamins such as niacin, thiamin, riboflavin and biotin. These aren't as prevalent in vegetables as vitamin C, but they are also diminished when exposed to heat.
Advocates of raw diets believe that eating raw foods can help slow aging, prevent disease and boost well-being.
Eating fruits and vegetables raw may also help ease symptoms of depression, according to a study conducted by the University of Otago. Researchers surveyed 422 young adults in New Zealand and the U.S. and found that those who ate raw produce reported fewer symptoms of depression and a higher life satisfaction and more positive outlook compared to those who ate more cooked, canned or otherwise processed fruits and vegetables.
The advantages of cooked
But cooking food doesn't always take away its nutrients. That's what researchers first recognized in 2002, in a study published in the Journal of Agriculture and Food Chemistry when they found that cooking boosts the amount of the antioxidant lycopene in tomatoes. Lycopene has been linked to a lower risk of heart disease and some cancers, according to Harvard Health.
Cooking vegetables can also increase the amount of calcium some vegetables provide, offering more of the mineral for your body to absorb.
In addition to increasing the amount of nutrients, cooking can have other benefits. A Harvard University study found that cooking also makes food easier to chew, easier to digest, and improves its net energy value.
One thing experts agree on is that if you're going to cook your produce, stay away from boiling. That method makes many nutrients leach into the water. One study published in the Journal of Food Science found that on average about 14% of antioxidants were lost in 20 vegetables when they were boiled.
Which foods you should cook and which you should eat raw
Here's a look at some popular fruits and vegetables and the healthiest way to prepare them.
Tomatoes — In the earlier-mentioned tomato lycopene study, researchers found that after 30 minutes of cooking tomatoes at 190.4 degrees Fahrenheit (88 degrees Celsius), the amount of lycopene increased by 35%. However, cooking reduced the levels of vitamin C in raw tomatoes. But as you break out your favorite spaghetti recipe, keep this in mind: Some of lycopene's cancer-fighting benefits are diminished when tomatoes are eaten with iron-rich foods.
Spinach — Leafy greens like spinach have more calcium cooked than raw. Nutrients like iron, magnesium and zinc are also more readily absorbed when spinach is cooked, says Healthline. In addition, spinach shrinks dramatically when cooked, so you eat much more of it when it's prepared that way versus when it's served raw.
Kale — Kale is a tricky one. It's packed with vitamin C, which is destroyed when cooked. But it also has calcium and cooking kale makes it easier for your body to absorb this important nutrient. Raw kale can be difficult for some people to digest, so try steaming it. That softens the fiber and still keeps the nutrients.
Broccoli — Eat your broccoli and cauliflower raw. A study published in the Journal of Agricultural and Food Chemistry found that raw broccoli contains three times the amount of the cancer-fighting compound sulforaphane. If you don't like the taste or crunch of raw cruciferous vegetables, then consider lightly steaming them. That will soften them without removing many nutrients. Unlike boiling or stir-frying in oil, steaming keeps most of broccoli's glucosinolate, WebMD suggests. That's a healthy compound that may prevent certain cancers.
Carrots — You don't have to crunch on raw carrots if you don't want to. Instead, you can steam or lightly roast them. These gentle cooking methods will preserve antioxidants and vitamin C, according to a study in the Journal of Agriculture and Food Chemistry. Cooked carrots also have higher levels of beta-carotene, an antioxidant that's converted into vitamin A and helps with eye and reproductive health, as well as bone growth and immunity.
Onions — Although onions are healthy when eaten raw or cooked, raw onions have antiplatelet agents, which help ward off heart disease.
Garlic — Similarly, garlic is healthier raw rather than cooked. Raw garlic has sulfur compounds with anti-cancer properties. Both raw and cooked garlic is rich in selenium, an antioxidant that also may lower the risk of some cancers, as well as control high blood pressure.
Asparagus — A study in the International Journal of Food Science and Technology found that cooking asparagus increases its antioxidants and its cancer-fighting abilities. Cooking asparagus also breaks down its fibrous cell walls, says Healthline, making folate and vitamins A, C and E more readily absorbed.
Bell peppers — Try to eat your peppers raw. One study found that bell peppers of every hue lose up to 75% of their antioxidants when they're cooked.
Mushrooms — (Technically not a vegetable, but they're in the produce department, so we're mentioning them here.) Cooking can increase the amount of antioxidants in some mushrooms, like shiitakes. Some mushrooms, specifically Agaricus or almond mushrooms, also contain a potentially carcinogen substance called agaritine. Cooking them helps get rid of those toxins.
Fruit — Raw fruits are packed with vitamins and minerals and often fiber, as well as antioxidants and other nutrients. Some people like to cook fruit because it concentrates the natural sugar, which makes the fruit taste even sweeter. As long as you don't add extra sugar during the cooking process, the liquid used to cook fruit is healthy, says HuffPost, just as it is with cooked vegetables.
The key takeaway here, however, isn't to stress so much about how you eat your fruits and vegetables. Just make sure you eat them.
As Dr. Michael Greger says on NutritionFacts.org: "The best way to eat your veggies is really whichever way will get you to eat the most of them ... with the exception of frying which adds way too many empty calories."
Editor's note: This story has been updated since it was published in August 2019.