I am convinced that fresh/raw/minimally cooked fruits and veggies are more nutritious than, say, a can of sweet peas or a pot of collards simmered on the stove for 4 hours. I am trying to convince a friend that raw, whole foods are much better for you than overcooked mush. She is a hardcore conservative who thinks that any books with the words “organic, raw, vegetarian” in the title are written by left-wing wackos. Can I get the straight dope on this, with cites that even this person could accept?
Did a quick search on the BBC website, which might prove neutral enough for your friend, and the answer seems to be “it depends”…
Cooking vegetables ‘improves benefits’
High-veg diet ‘wards off cancer’
Getting the best out of your food - scroll down about halfway for the Raw vs Cooked section.
Grim
I don’t have any cites, but both positions that you have given are extremes and therefore problematic.
There are some kinds of veggies that are better eaten un-cooked, because the vitamins are lost with heat. Eg salads. Also fruits like apples.
There are some kinds of veggies that contain substances that are activated when cooked - tomatoes are famous for this. In this case, the health benefit isn’t the vitamin, but the colour-carrying compound (I think flavonid lyco… something), which prevents cancer or similar. Thus, tomato soup is better than uncooked tomato juice.
And there are some kind of veggies that definitely need to be cooked (or otherwise prepared enzymatically) - the most known of this group are beans. (There is the famous story of a vegetarian congress a few years back, where the cook had no knowledge or experience in cooking for vegetarians, so he made raw beans without other treatment - and everybody got sick.)
As for books or cites not written by left-wing wackos - almost every serious scientist in the field of health/ nutrition that I’ve heard of will talk about the benefits of the Mediterraen diet - small meals 5 times a day, meat only 1-2 times a week, lots of fish, lots of veggies, olive oil etc.
Any serious nutrioutnist will also tell you that the benefit from veggies - whether uncooked or not - is not only the elusive vitamins, but the flavonids and fiber and lots of other stuff. So even if the vitamins have boiled away, apple juice is still better than lemonade with artifical flavouring. (And eating vitamin tablets doesn’t improve your health as much).
The Master has also spoken about vegetarianism.
It’s true that the usual quotes about how vegetarianism is better for your health come from the vegetarians themselves, but from what I’ve heard, the studies themselves were done by scientists, so the claims that eating veggies and no meat reduces the risk of colon cancer, arterisclerois and other heart diseases, and other ailments related to too much animal fat and proteins are pretty well-proven. (That applies to people who eat responsible, not those who skip the meat, but still eat only french fries.)
The truth is that SOME raw veggies are better for you than their cooked to mush canned counterparts and SOME veggies are best frozen and SOME veggies are best canned. It all has to do with how quickly something can get to market in its fresh state and if it’s faster than the nutrients can degrade. Obviously, if you’re picking them out of your garden and eating them that night, that’s a different situation.
Often, frozen veggies are frozen within hours of picking, and they’re picked when ripe, so they have all their nutrients locked in. I don’t know of any nutrients which are damaged by freezing. When you thaw them, they are the most identical thing to out-of-the-field you can get.
Tomatoes are apparently best canned, as their lycopene breaks down soon after picking and doesn’t make it to the market. In addition, market tomatoes are bred for durability, not nutrition or flavor. A canned tomato, on the other hand, can be a flavorful, nutritious but easily bruised variety, because looks aren’t important.
Fresh raw or minimally cooked leafies are generally speaking better than cooked to mush ones, because they have thin cell walls which release their nutrients fairly easily, and some nutrients do degrade with heat exposure. In addition, sulphur compounds are released with long cooking, and make the dish less appealing, and you may not eat as much. However, a moderate cooking time with consumption of the pot liquor (the liquid at the bottom of the pot, into which some nutrients have leached) is not a great loss of nutrients.
Finally, some nutrients, particularly in root vegetables, are just not very digestible unless the cell walls are broken down by heat or mechanical action (like mashing). People aren’t very good at digesting cellulose. You’ll notice most of lightly cooked corn comes out the same way it went it. Good fiber, but not terribly nutrient dense.
If you’re into organics and alternative therapies, you might be interested to know that raw foods are not at all a good thing in Traditional Chinese Medicine. They are generally very hard on the digestive system, and lead to an excess of “Cold”. Women, especially (as we tend to Cold disorders) are encouraged to at least warm up any food before consuming, if not cook it thoroughly. (I know TCM is considered quackery by most on this board, and I’m not willing to go to great lengths defending it, but if you’re into raw foods, you may have respect for it as an alternative modality. Or not.)
And, on preview, I see **grimpixie **has given you some excellent cites, which saves me the time and energy of getting them. Thanks, grimpixie! 
Rather than cooking everything to mush for 4 hours, the best preperation with veggies is often blanching - small amount for a few minutes in boiling water - or steam-cooking it (for 20 to 30 minutes in a steam cooker). This is the healthiest way to keep the veggies fresh.
Cooking tip: while these preparations are generally best for maintaining the quality of the vegetable, color can be lost as the veggies cool down after boiling or steaming. So after removing from heat, immediately “shock” the veggies by plunging them into very cold (iced) water for a minute or so. This arrests the cooking and keeps the vegetables fresh-looking instead of limp and muddy-colored. Best of both worlds.
It is my life’s purpose to serve you… 
Grim