Raw or cooked? The healthiest way to eat 14 vegetables

We know we should eat lots of plants, but how should we eat them to get the most nutrients?

We know we should eat lots of plants, but how should we eat them to get the most nutrients?

Munching crudites or tossing crisp, uncooked florets into a super-salad is lauded as the best way to maximise nutrient intake from vegetables. But while some vegetables are best served raw, in others the vitamins, minerals and beneficial plant compounds are more effectively absorbed into our system when we cook them. To cook, or not to cook? - that is the question.

---

Carrots

Best eaten: Cooked

Eating a raw carrot stick might seem virtuous, but you’d be better off cooking them, according to a study published in the Journal of Agricultural and Food Chemistry by food scientists at the University of Arkansas. They found that cooked then pureed carrots had up to 34.3 per cent higher levels of beta carotene, the carotenoid antioxidant that gives them their orange colour, than raw carrots.

Beta-carotene is converted by the body into vitamin A, which plays an important role in healthy vision, bone strength and regulating the immune system. Cooking carrots without peeling them first bumped up the beneficial nutrients even more. How you cook them matters, and boiling until slightly tender has been shown to best preserve carotenoids while steaming for a maximum of ten minutes was shown to enhance the potassium content of carrots. “There is a trade-off if you boil carrots as you get more carotenoids but lose a lot of the beneficial polyphenols,” says Alex Ruani, researcher in nutrition science at University College London and chief science educator at the Health Sciences Academy. “Steaming until tender will cut polyphenol losses by 43 per cent while unlocking the carotenoids you’re after.”

Beetroot

Best eaten: Raw

Beetroot contains vitamin C, folate and betalains - pigment compounds that give the root vegetable its purple hue and have powerful antioxidant properties. Boiling or roasting beetroot for lengthy periods can cause a drop in all of these. It is nitrates, however, that are beetroot’s USP. These compounds are converted by the body to nitrite, which not only relaxes and widens blood vessels but influences how efficiently our cells use oxygen. Numerous studies have shown nitrates to play a role in healthy blood flow, immune function and the cardiovascular system, as well as potentially enhancing exercise performance and recovery. But they too can be destroyed through cooking.

“Nitrates are water-soluble so boiling beetroot is not recommended,” says Andrew Jones, professor of applied physiology and a beetroot researcher at the University of Exeter. “Unless you consume the cooking liquid you would lose a lot of the nitrate so it wouldn’t be a good thing to do for maximum beetroot benefits.” Juicing raw beetroot or grating it into a salad are the best means of nutrient preservation.

Green beans

Best eaten: Cooked

They are sometimes added raw to salads but cooking green beans can increase the availability of nutrients in the vegetable. Indeed, food technologists at the University of Murcia and the University of Complutense in Spain who studied the effects of six cooking methods - boiling, pressure cooking, baking, microwaving, griddling and frying - on nutrient levels of different vegetables found green beans to be one of the few to have increased antioxidants after being cooked by any of these methods except boiling. A subsequent study placed microwaving at the top for amplifying antioxidant availability in green beans. “Raw green beans also contain high levels of lectins, which may cause digestive issues in certain individuals,” says the nutritionist Eli Brecher. “Cooking them will ease the digestive burden.”

Kale

Best eaten: Raw

Kale is labelled a super-veg with reason. It provides fibre and vitamins A, K, B6 and C, plus calcium, potassium, copper and manganese for an all-round health boost. What makes it a star, though, is the presence of glucosinolates, which are converted into metabolites with anti-inflammatory and antioxidant properties when it is chopped or chewed. While it might be tough to swallow, cooking kale can destroy an enzyme called myrosinase, which helps to convert glucosinates into these beneficial metabolites. What’s more, cooking kale by any one of five methods - boiling, steaming, microwaving, pressure cooking and vacuum cooking - will significantly reduce its total antioxidant and mineral content, with reductions in levels of calcium, potassium, iron, zinc, and magnesium.

If you can bear it, raw kale can be chopped or shredded with lettuce for a salad or tossed into pasta - massaging a little olive oil into the leaves for a few minutes will soften them. If not, minimal cooking - flash stir-frying - is required.

Celery

Best eaten: Cooked

It’s not the most inspiring vegetable, but crunchy celery stalks do contain fibre, antioxidants that may have anti-inflammatory properties, potassium and vitamin K, which is important for healthy bones. Before you think of tossing it into a salad, bear in mind that the Spanish food technologists found that pressure cooking, baking, griddling or frying celery increased its antioxidant levels. By steaming it for five minutes, roasting for ten minutes or adding to a stew or bolognese sauce, you will lose fibre but gain beneficial nutrients.

Tomatoes

Best eaten: Cooked

All tomatoes and tomato-based products - ketchup included - contain the powerful antioxidant lycopene, consumption of which has been linked to reduced risk of some cancers, cardiovascular disease and neurodegeneration. Plenty of research has shown that lycopene bioavailability is higher when tomatoes have been heated or cooked, and especially if prepared with a dash of oil. Preparing them in this way helps to separate the lycopene from the carrier proteins so that it is more easily absorbed by the body.

In 2020 research funded by the World Cancer Research Fund, Gary Fraser, professor of medicine and epidemiology at Loma Linda University Health, found that men who consumed canned and cooked tomatoes five to six times a week had a 28 per cent decreased risk of prostate cancer compared with men who never consumed cooked tomatoes.

“Interestingly, we found no significant association between prostate cancer and consumption of raw tomatoes, tomato soup, and tomato-based vegetable juice,” Fraser says. Any cooking method will enhance lycopene levels, although Brazilian researchers suggested lightly steaming tomatoes brushed with oil was the best approach.

Potatoes

Best eaten: Cooked, but cooled

The humble spud is an important supplier of vitamin C, vitamin B6, folate and iron, potassium and even fibre in the diet. It’s unthinkable to serve them raw, not just because they taste appalling but because uncooked potatoes contain resistant starch, which is difficult to digest and can cause gastrointestinal issues. Cooking will reduce levels of some water-soluble nutrients, such as vitamin C, and a little fibre but potatoes retain enough to remain beneficial.

Last year a study of 16,000 nine to 18-year-olds found that those who regularly ate potatoes in any cooked form - mashed, baked or fried - had higher intakes of potassium and selenium, as well as a better range of nutrients overall. They were the only vegetable of those studied by the Spanish food technologists to retain their high antioxidant levels with any cooking method.

A downside is that they do cause a blood sugar spike. “Mashing with some butter or oil will reduce their glycaemic load (GL) although the best way to serve them is cooked but cooled - or served cold in a vinaigrette dressing - which can cut the GL from 90 to 56 per cent,” Ruani says.

Mushrooms

Best eaten: Cooked

Mushrooms contain fibre and most varieties have an array of B vitamins and are a source of potassium, chloride and copper, as well as being high in selenium - nutrients that are lacking in many UK diets. But they also have tough cell walls and some find raw mushrooms difficult to digest.

“Even common edible mushrooms can contain potentially hazardous toxins with most reported issues occurring when mushrooms are eaten raw or undercooked or after they have been stored for too long,” Ruani says.

You are better off cooking them, but how? Spanish researchers assessed the effect of various methods - boiling, microwaving, grilling and frying - on white button, shiitake, oyster and king oyster mushrooms. Their findings, published in the International Journal of Food Sciences, showed boiling and frying both resulted in lower antioxidant levels of mushrooms, with frying in a pan also producing a significant drop in protein and carbohydrate content, and an increase in fat levels. The best way to preserve nutrients? Under the grill or in a microwave.

Spinach

Best eaten: Cooked

An excellent source of vitamin C, vitamin B6, magnesium, calcium and iron, spinach is also packed with oxalate, a substance that blocks the absorption of iron and calcium by the body. Boiling and steaming for even a minute or so have been shown to reduce oxalate by up to 87 per cent, meaning there is more calcium available for the body to absorb - although cooking in water will result in the loss of some vitamin C. “If it is mainly a shot of vitamin C that you are after, then try microwaving spinach for 2-3 minutes without water to cut losses of the vitamin by 90 per cent,” Ruani says.

Garlic/onions

Best eaten: Raw

The benefits of garlic and onions are most potent in their uncooked form, which is bad for your breath but good for your body. Both contain allicin - although garlic is by far the most concentrated source - an active compound responsible for the strong odour that is released when they are chopped, sliced or smashed. Allicin acts like an antioxidant and has been implicated in healthy cardiovascular function, immune function and protection against disease, but is deactivated when cooked. If you do cook them, chopping or slicing both onions and garlic beforehand will maximise allicin preservation.

Asparagus

Best eaten: Cooked

Thinly sliced or marinated raw asparagus spears are a superfood salad staple. But you’d be better off cooking it to boost its nutrient value. Researchers have shown that lightly cooking asparagus increases its total antioxidant activity by 16-25 per cent with levels of two super-antioxidants - beta carotene (important for eye health and a strong immune system) and quercetin (a natural pigment that helps to control blood sugar and inflammation) - raised by 24 per cent and 98 per cent respectively. It’s a similar story with white asparagus, as cooking was found to produce three times more disease-fighting antioxidants per spear than when served raw. Don’t overcook or nutrients will be leeched thin asparagus can be cooked in one minute while thick spears take no more than 4-5 minutes.

Peppers

Best eaten: Raw

Peppers provide vitamin C, carotenoids and beneficial polyphenols, but lose up to 75 per cent of their nutrients when cooked according to the Spanish scientists. If you do cook them, stick to dry-heat methods such as stir-frying or roasting. A study in the journal Preventive Nutrition and Food Science found that boiling red peppers reduced their vitamin C content by up to 66.5 per cent and antioxidant activity by 60 per cent, whereas losses when stir-frying or roasting were reduced, with vitamin C falling by 25.9 per cent and antioxidant activity by 17.9 per cent. Of all cooking methods, fast stir-frying came out on top.

Broccoli

Best eaten: Raw

With its high levels of the beneficial glucosinolates - natural substances that break down during chopping and digestion into biologically active compounds - broccoli is a star of the vegetable world. However, researchers have shown that heat, particularly boiling and microwaving, damages the beneficial enzyme myrosinase, which converts the glucosinolates in broccoli into sulforaphane, shown to help with blood sugar control, detoxification and disease prevention.

The best way to get maximum sulforaphane is from munching on raw broccoli florets. If you do cook it, here’s a tip: chop it into small pieces to enhance myrosinase activity, then leave for at least 30 minutes (but preferably 90 minutes) to allow time for the helpful sulphuric compounds to develop before flash stir-frying for four minutes. This was shown in a study at Zhejiang University of Science and Technology in China to produce 2.8 times more sulforaphane than broccoli cooked in the same way but immediately after chopping, although the same scientists showed that steaming retained more vitamin C and glucosinolates.

Cauliflower

Best eaten: Raw

In the Spanish study, the highest antioxidant losses of all vegetables were observed in cauliflower after boiling and microwaving. Griddling - described as cooking on a flat metal surface with no oil or stir-frying were the best options if cooking. Avoid boiling, which destroys the beneficial compounds of cruciferous veg.

“If you can’t stand raw cauliflower, then a tip to get the most sulforaphane possible is to make cauliflower rice by blending or chopping it into tiny pieces from fresh,” Ruani says. Sulforaphane will be further activated by refrigeration, so storing in the fridge will allow it to develop.