Does cooking onions destroy the nutrients and therefore reduce their health benefits? Well take a look at the above ORAC value which measures sauteed onions and compare it to when they're fresh. The cooked version measures out at 1,220 while the raw version is 913. You may be thinking how is it possible that the amount of antioxidants went up after being sauteed?!
There's a good reason for that. As you are aware, you can fill your entire frying pan full of peeled onions and after 10 minutes, they will shrivel up into a tiny pile. The ORAC measurement uses 100 grams of the food. Of course, 100 grams of cooked onions is a lot more concentrated than 100 grams of fresh. This explains the difference. It's not that the antioxidants increased, rather that the amount of food being measured on the sauteed version is more, since much of its water has evaporated out. For this same reason, you increase the calories when cooking, not in actuality, but in the sense that say, 1/4 of cooked will have much more calories than 1/4 of raw.
Even though the ORAC increased it does not mean all the nutrients and their benefits were preserved in the process. In fact, the difference suggests that a great deal were lost. Think about it... the size of cooked may be 1/5 or 1/7 that of fresh. With such a decrease in size, we would expect the ORAC to go up 500 to 700%. Instead, we only see it go up around 25% which suggests that indeed, a high amount of nutrients are being destroyed by the heat. One in particular we can be sure is destroyed is the vitamin C, since it's so sensitive to heat.
So if nutrients are being reduced, does it also mean cooking reduces the acidity in onions? That's a great question. When you heat acids it can cause them to degrade. Though the flip side of that coin is you're also making them more concentrated, since you're reducing the ratio of water vs. everything else in the onion. This makes it a catch-22 if you are trying to cook them to help with heartburn, acid reflux, or indigestion. It may or may not help, depending on how you prepare them and what other ingredients are with them.
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010