Every whole plant food is a complex mix of hundreds of chemical compounds. Some are nutrients like vitamins and minerals, which are essential for health. Others are phytochemicals — compounds that are not essential nutrients, but may have important health benefits.
Food scientists estimate that there are tens of thousands of these phytochemicals in foods. Our knowledge about them is still in its infancy since researchers have only just begun to study them.
Food preparation has a big effect on the levels of nutrients and phytochemicals in food and also on how well they are absorbed. Cooking, chopping, blending and juicing have both positive and negative effects making it hard to determine the single best way to prepare each and every food.
Raw versus Cooked
Cooking reduces some of the nutrients and phytochemicals in fruits and vegetables. Vitamin C is especially vulnerable to heat and even boiling foods for a few minutes can greatly reduce the vitamin C content of foods.
Heat also reduces the absorption of some of the phytochemicals found in the cruciferous vegetables — cabbage, broccoli, Brussels sprouts, kale, cauliflower, bok choy, turnips and radishes. It can also interfere with activation of protective phytochemicals in garlic and onions.
However, cooking has positive effects on foods as well. For example, it makes both protein and carbohydrate more digestible. Heat also softens plant cell walls which releases some of the phytochemicals and makes them available for absorption.
Lycopene — which is a phytochemical associated with reduced risk for prostate cancer — is absorbed at much higher rates from cooked tomatoes than from fresh raw tomatoes. People who eat exclusively raw food diets have lower levels of lycopene in their blood. The vitamin A precursor beta-carotene is also absorbed better from cooked vegetables than from raw.
Cooking also destroys antinutrients in foods including compounds that interfere with protein digestion. Boiling vegetables can reduce levels of oxalate, a compound that binds calcium and makes it unavailable for absorption. It should be noted, however, that it’s still not clear how this might affect the levels of calcium in the food since calcium could be lost along with the oxalate.
Many of the phytochemicals in fruits and vegetables are antioxidants. They neutralize free radicals which are naturally-formed compounds in the body that can damage cells and raise risk for chronic diseases such as cancer and heart disease. The antioxidant potential of fruits and vegetables is one reason why these foods are thought to lower disease risk.
Some antioxidants are destroyed by heat but others are actually enhanced by it. Cooking often reduces the total antioxidant activity of foods, but in some cases, these levels are increased in cooked foods.
Juicing, Mashing, and Pureeing
Manipulating the food “matrix” has an even greater effect than cooking on the bioavailability of healthful compounds in foods. For example, juicing foods eliminates some of the fiber and liberates certain compounds for absorption. Juicing can also increase the availability of phytochemicals.
And simply mashing or pureeing a food can break down cell walls and improve the absorption of compounds like lycopene and beta-carotene. One study showed that beta-carortene is absorbed at a much higher rate from pureed spinach than from whole spinach leaves. Likewise, crushing garlic results in activation of some of its anti-cancer compounds.
However, pureeing, blending and juicing foods, or even cutting them into small pieces exposes the nutrients and phytochemicals to air which can be destructive to certain compounds. Levels of vitamin C in particular are quickly reduced by exposure to air.
Finally, how we serve food affects absorption of nutrients and phytochemicals. Some, like beta-carotene, depend on fat for absorption. As a result, eating very low-fat meals can interfere with absorption. Simply tossing an oil-rich avocado into a blended raw or cooked soup can greatly enhance absorption of some phytochemicals.
Food Preparation is a Balancing Act
When we consider the competing effects of processing on the thousands of potentially beneficial compounds in fruits and vegetables, it becomes difficult to make specific decisions about how to prepare different foods.
Here are just a few examples of how complex the situation is:
– Cooking tomatoes liberates lycopene, but reduces levels of vitamin C.
– Cooking broccoli boosts its overall antioxidant activity and frees up beta-carotene for absorption, but can greatly reduce some specific phytochemicals that may have anticancer activity.
– Juicing foods breaks down plant cell walls making some phytochemicals more readily absorbed, but it can eliminate healthful fiber and greatly reduce vitamin C levels.
– Boiling spinach might reduce its oxalate content and make calcium more available, but many vitamins and phytochemicals are leached out into the cooking water along with the oxalate.
Getting the Most From Fruits and Vegetables: Some Simple Guidelines
The fact is, there is no perfect way to prepare each and every food. No matter how you choose to prepare any particular fruit and vegetable, there will be some losses and gains. Yet, while it’s not possible to come up with specific recommendations for most foods, you can follow some simple guidelines to make certain you’re getting the most from your intake of fruits and vegetables.
– Choose raw fruit as often as possible. There isn’t much science showing benefits of cooking fruits so it makes sense to go with the most convenient and obvious way to enjoy these foods.
– Treat vitamin C-rich foods like oranges, grapefruit, papaya, kiwi, and cantaloupe with care. That is, eat them whole rather than juiced or blended into smoothies. Cut them up right before serving. This keeps the vitamin C protected from the destructive effects of oxygen.
– Eat blended or juiced foods as soon as possible after preparing them to get the most of their nutrient content.
– Don’t count on high-oxalate vegetables to meet calcium needs. While these foods — spinach, beet greens and Swiss chard — are super-nutritious and should play an important role in your diet, regardless of how you cook them, they shouldn’t be relied on for calcium. Instead, focus on low-oxalate choices like kale, collards, and bok choy for boosting your calcium intake.
– When you cook vegetables, be gentle. Opt for steaming or baking over boiling and microwaving. Cook foods just to the tender-crisp stage. That gives you the best of all worlds—a little heat to soften cell walls, without cooking the compounds right out of the vegetables.
– Aim for the best of all worlds by eating about half of your vegetables raw and half in the gently-cooked state.
– Include some raw cruciferous vegetables and onions in your diet to get the most of their healthful phytochemicals.
– For men especially, eating cooked tomato sauces — even prepared ones from a jar — may be a good choice for reducing risk for prostate cancer.
– Include some healthful plant fats in your meals — from naturally high fat foods like nuts, seeds, and avocado or from very small amounts of oils.
– Both roasted and raw nuts have been shown to be highly protective against chronic disease. However, roasting nuts does decrease nutrient and antioxidant levels and there doesn’t seem to be any nutritional advantage over raw. It’s fine to quickly toast nuts to improve their flavor, but for the most part, raw nuts are a better choice.
– Eat lots and lots of fruits and vegetables. Whether you eat them raw or cooked, chopped, blended or whole, simply by consuming generous amounts of these foods, you’ll get the benefits of their thousands of health-promoting compounds. If fresh produce isn’t always available or affordable, frozen options are often just as nutritious and beneficial. In fact, in the winter months, they can be even better. The amount of fruits and vegetables you eat is actually more important than how you prepare them.
References: Vermeulen M, van den Berg R, Freidig AP, van Bladeren PJ, Vaes WH. Association between consumption of cruciferous vegetables and condiments and excretion in urine of isothiocyanate mercapturic acids. J Agric Food Chem 2006;54(15):5350-8.  Fujisawa H, Suma K, Origuchi K, Seki T, Ariga T. Thermostability of allicin determined by chemical and biological assays. Biosci Biotechnol Biochem 2008;72(11):2877-83.  Reboul E, Richelle M, Perrot E, Desmoulins-Malezet C, Pirisi V, Borel P. Bioaccessibility of carotenoids and vitamin E from their main dietary sources. J Agric Food Chem 2006;54(23):8749-55.  Dahan K, Fennal M, Kumar NB. Lycopene in the prevention of prostate cancer. J Soc Integr Oncol 2008;6(1):29-36.  Gartner C, Stahl W, Sies H. Lycopene is more bioavailable from tomato paste than from fresh tomatoes. Am J Clin Nutr 1997;66(1):116-22.  Garcia AL, Koebnick C, Dagnelie PC, et al. Long-term strict raw food diet is associated with favourable plasma beta-carotene and low plasma lycopene concentrations in Germans. Br J Nutr 2008;99(6):1293-300.  Rock CL, Lovalvo JL, Emenhiser C, Ruffin MT, Flatt SW, Schwartz SJ. Bioavailability of beta-carotene is lower in raw than in processed carrots and spinach in women. J Nutr 1998;128(5):913-6.  Kiran KS, Padmaja G. Inactivation of trypsin inhibitors in sweet potato and taro tubers during processing. Plant Foods Hum Nutr 2003;58(2):153-63.  Chai W, Liebman M. Effect of different cooking methods on vegetable oxalate content. J Agric Food Chem 2005;53(8):3027-30.  Wu X, Beecher GR, Holden JM, Haytowitz DB, Gebhardt SE, Prior RL. Lipophilic and hydrophilic antioxidant capacities of common foods in the United States. J Agric Food Chem 2004;52(12):4026-37.  Gibson RS, Perlas L, Hotz C. Improving the bioavailability of nutrients in plant foods at the household level. Proc Nutr Soc 2006;65(2):160-8.  McEligot AJ, Rock CL, Shanks TG, et al. Comparison of serum carotenoid responses between women consuming vegetable juice and women consuming raw or cooked vegetables. Cancer Epidemiol Biomarkers Prev 1999;8(3):227-31.  van Het Hof KH, West CE, Weststrate JA, Hautvast JG. Dietary factors that affect the bioavailability of carotenoids. J Nutr 2000;130(3):503-6.  Castenmiller JJ, West CE, Linssen JP, van het Hof KH, Voragen AG. The food matrix of spinach is a limiting factor in determining the bioavailability of beta-carotene and to a lesser extent of lutein in humans. J Nutr 1999;129(2):349-55.  Song K, Milner JA. The influence of heating on the anticancer properties of garlic. J Nutr 2001;131(3s):1054S-7S.  Brown MJ, Ferruzzi MG, Nguyen ML, et al. Carotenoid bioavailability is higher from salads ingested with full-fat than with fat-reduced salad dressings as measured with electrochemical detection. Am J Clin Nutr 2004;80(2):396-403.  Rickman JC BD, Bruhn CM. Nutritional comparison of fresh, frozen and canned fruits and vegetables. Part 1. Vitamins C and B and phenolic compounds. J Sci Food Agric 2007;87:930-44