Does cooking destroy nutrients in vegetables?
In my last article I wrote about the nutrient density of vegetables . Today I want to address how cooking affects the nutritive value of vegetables.
As you know by now I am a big proponent of eating raw vegetables. I love vegetables. A meal without raw vegetables seems incomplete and imbalanced. I consider leafy greens and cruciferous and colorful vegetables an essential part of healthy eating. We eat vegetables for their numerous health benefits and also to enhance meals with their delicious flavors and colors. But do we need to eat them all raw? How does cooking affect the nutritive value of cooked vegetables?
The main disadvantage to cooking vegetables is that it reduces several of the protective factors such as vitamins, minerals, phytochemicals, antioxidants and enzymes.
It is beyond doubt that cooking induces significant changes in chemical composition and vitamin content, although it depends of the type of cooking. The greater the heat and the longer the time of exposure, the greater the nutrient destruction.
Let’s explore what cooking does to vegetables.
Heating foods above 118°F starts the process of denaturing the enzymes. However, plant enzymes are being denatured by stomach acids and are then digested as proteins. They are of little benefit for digestion contrary to popular beliefs in the raw food world. Also, vegetables do not require a large amount of digestive enzymes to be digested as they are low in protein, fat and starch – for non-starchy vegetables.
The minute a vegetable is cut or plucked from the earth it starts losing some vitamins, mainly vitamin C and B-complex vitamins. To slow the degradation of vitamins, most vegetables should be kept refrigerated until used.
B-complex vitamins and vitamin C are water-soluble vitamins and are not stored in the body. They must be replaced each day. Water-soluble vitamins are the most unstable when cooked or stored improperly. Vitamins C, thiamin (B1), pantothenic acid (B5), pyridoxine (B6), and folate (B9) tend to be the most vulnerable nutrients when subjected to heat and oxygen. Some vitamins are easily destroyed by oxygen, so cut vegetables or juiced vegetables should be stored in airtight containers in the refrigerator. Water-soluble vitamins dissolve easily in the broth when we boil vegetables so the broth should be saved for other uses such as adding to your smoothies or soups.
Fat-soluble vitamins E and K are minimally affected by cooking. High temperatures produced during deep-frying can destroy vitamin E. Vitamin K is a resilient nutrient and is fairly well retained in most cooked or stored foods. Fat-soluble vitamins are stored in body fat and the liver.
Pro-vitamin A includes a broad group of carotenoids such as beta-carotene. They are transformed into vitamin A in the body. See phytochemicals below for carotenoids.
Virtually all minerals are unaffected by heat although they can get easily washed away in the cooking water. If you throw away the water after steaming or boiling vegetables then you lose them. Again, save the water and use it in your smoothies or soups.
Essential fatty acids are sensitive to heat and are also damaged by cooking and by exposure to light and air. Leafy greens and other cruciferous vegetables contain a fair amount of omega 3. Eating vegetables raw will prevent oxidation and the formation of free radicals of essential fatty acid omega 3.
There is controversy about absorption of phytochemicals in their raw, cooked or blended form. Some phytochemicals are better absorbed when raw and others are better absorbed when cooked and most of them are better absorbed when foods are blended, pureed or thoroughly chewed.
Vegetables contain thousands of phytochemicals, nutrients that serve as antioxidants and cancer protection. Many of these phytochemicals are useful to the body only when not heated with the exception of the carotenoid family.
A study published in the Journal of Agricultural and Food Chemistry (October 25, 2008) entitled “Bioavailability and Kinetics of Sulforaphane in Humans after Consumption of Cooked versus Raw Broccoli,” it was found that the anti-cancer compound sulforaphane was bioavailable at the rate of 37% if the broccoli was eaten raw, but drops to 3.4% if eaten cooked.
Another study was done at the Roswell Park Cancer Institute in Buffalo. They found that eating raw broccoli and cabbage slashed bladder cancer risk by 40%. However, the researchers did not find such a protective benefit from eating the same quantity of cooked cruciferous vegetables. It is evident that the anti-cancer properties of vegetables are destroyed by cooking.
Exceptions are the carotenoids lycopene and beta-carotene associated with reduced prostate cancer risk and lower rates of heart disease. Lycopene is found in tomatoes and beta-carotene in carrots.
Let’s take the case of beta-carotene. A study looked at the actual absorption of cooked, pureed carrots versus raw and pureed carrots. It showed that 75% of beta-carotene was absorbed from cooked, pureed carrots versus 47% percent from the raw, pureed carrots. Raw foodists in general eat more vegetables and fruit than the average mainstream population and in fact get more beta-carotene in their diet, even if there is more absorption of beta-carotene in cooked carrots.
A study published in the British Journal of Nutrition in 2008 found that a group of 198 subjects who followed a strict raw food diet had normal levels of vitamin A and high levels of beta-carotene but low levels of the antioxidant lycopene.
As for lycopene, studies indicate that it is more readily released and bioavailable if the tomatoes are cooked. This is mostly because the plant cell walls break down and release the phytochemicals more easily during cooking and also because the lycopene concentration is higher when cooked due to loss of water.
Whether or not cooking tomatoes modifies the availability of lycopene, raw tomatoes do contain a beneficial amount of lycopene. In addition to this, raw tomatoes are packed full of nutrition. They provide an excellent source of vitamin C, vitamin K, biotin, and carotenes. They are also a very good source for several B vitamins and fiber. Since tomatoes also contain vitamins, potassium, and other carotenoids and antioxidants, it may be that other compounds in tomatoes may account for some of the protective effects first thought to be due to lycopene.
In another study, scientists from the US Agricultural Research Service have found that the watermelon contains more lycopene than tomatoes. The ARS also assessed the bioavailability of lycopene in watermelons in a study that involved 23 healthy adults. The scientists used tomato juice as the known benchmark for judging the relative bioavailability of lycopene. The researchers had expected lycopene availability to be greater from tomato juice because it had received heat treatment, which is believed to improve lycopene bioavailability. However, they found out that lycopene concentration was similar regardless of whether subjects consumed 20 milligrams of lycopene from tomato juice or from watermelon juice, which was not heat-processed.
Lycopene is not an essential nutrient. It is not transformed into vitamin A as is beta-carotene. You can certainly get a lot of lycopene by eating red pigmented raw vegetables and fruits. Blending releases more lycopene than eating due to the breaking down of the cell walls that occurs during blending. The presence of fat in a meal, such as one teaspoon of seeds or nuts, enhances absorption of fat-soluble carotenoids such as lycopene and beta-carotene found in greens and orange vegetables.
The bottom line is when you eat a raw food diet with lots of vegetables you are getting other nutrients besides lycopene and they all work synergistically to optimize nutrition and function of the cells. It is likely that the preventive effect of diets high in fruits and vegetables cannot be explained by just one single phytochemical of the diet. As a general rule, protective phytochemicals are higher in raw vegetables than in cooked vegetables.
Blending your vegetables:
To release the nutrients from vegetables they need to be thoroughly chewed; otherwise, they will stay locked in the cell walls and will not be absorbed. Anywhere from 70 to 90% of the valuable nutrients may never make their way to the bloodstream if not well chewed. To get the maximum nutrients from your vegetables, the plant walls must be split up to release the nutrients inside the cells. Blending your vegetables in a smoothie or soup is a great way to unlock the nutrients and make them more absorbable in the bloodstream. When you blend your greens, the protein will be easily broken down into amino acids and the phytochemicals will be more absorbable. Rarely do we chew our vegetables well enough to release all their nutrients, so blending our vegetables is a great option.
For those who are not used to a lot of fiber or are having digestive problems, blending or juicing is a great way to get easily absorbable nutrients from raw vegetables.
There is no denying that some nutrients are lost when foods are cooked. Steaming 1-5 minutes (1 minute for greens and 4-5 for cruciferous) results in minimal loss of nutrients. The results of a study done with various methods of cooking broccoli found that all cooking treatments, except steaming, caused significant losses of chlorophyll and vitamin C and significant decreases of total soluble proteins and soluble sugars. Total phytochemical glucosinolates were significantly modified by all cooking treatments but not by steaming. In general, the steaming led to the lowest loss of phytochemical glucosinolates, while stir-frying and boiling presented the highest loss. Steaming appears to be the best in retention of the nutrients in cooking broccoli.
Boiling as in soup is the second best method, as you eat the broth with the soup. However, in soups vegetables are more cooked unless you make a miso soup with half-cooked vegetables.
Steaming and boiling are best if you save the water and these methods also minimize the formation of toxic by-products.
Cooking vegetables is a good addition to raw vegetables when we want to increase vegetable consumption and when we want warm and comforting foods. I always save the small amount of water I used for steaming, putting it in the fridge to use in my green smoothies or soups.
Superior nutrition is accomplished when you get all the nutrients you need to function optimally. Raw vegetables contain an array of precious nutrients, and eating a good portion of raw vegetables at every meal is essential for optimal health. Adding some lightly steamed vegetables is also beneficial as it make us eat even more vegetables. Vegetables are nature’s most perfect foods and are also the most abundant foods on earth. They are low in calories, alkaline-forming and rich with the vitamins, minerals and phytochemicals needed to heal your body. There is little doubt that one of the best ways to improve your health is to make sure you are eating plenty of fresh, minimally processed high quality vegetables, ideally locally-grown and organic, with a majority of them consumed raw. Eating more raw vegetables will boost your energy and vitality and get you lean.