Calories in cooked versus raw foods

Earlier today a friend of mine posted this on facebook:

“This may sound stupid but I cooked some mushrooms in a dry pan (no oil, butter or water) and they were nearly half the weight they were before. So do I calculate the calories and carbs by the fresh weight or cooked? “

At first I thought the answer was obvious. Dry frying will dehydrate mushrooms, so they’ll lose water. If you calculate the calories by the fresh weight, because you haven’t added any fats or other ingredients, they should be the same as the calories in the (lower) cooked weight. Simple!

But then I did a little research. The first thing I discovered was that research suggests that eating cooked food generally provides more energy than eating the same food raw. There is a scientific basis for this; starches (eg wheat or potatoes) are largely composed of sugar-based molecules which are tightly packed and inaccessible to digestive enzymes in their raw state. Cooking releases these molecules and exposes them to digestive enzymes, so while the amount of food is the same, you can get more energy from eating a cooked potato than a raw one (not that I would ever recommend eating a raw potato!) A similar mechanism means that more energy is obtained from cooked meat than when in it’s raw state.

That all seems fair enough. Cooking most foods reduces their water content (with the exception of things like rice and lentils), and after all, 100g of sundried tomatoes will have a much higher calorie content than fresh tomatoes.

However, when we consider nutrients in food, it becomes a whole different story. Vitamin C, folate and potassium are all easily affected by cooking methods, whereas minerals such as calcium are less affected. For example, a baked potato will lose about 60% of the vitamin C it contained in its raw state but retain almost all its calcium, but if you boil the same potato you’ll lose half the vitamin C and up to 75% of the calcium.

So how can we combat this and obtain maximum nutritional value from our foods? We certainly aren’t recommending everyone switch to a raw food diet, though eating raw fruit and steamed vegetables will retain the maximum possible vitamin content. However, if you’ve boiled your vegetables, you can then use that liquid to make rice or couscous, so the nutrients leached out of them will be eaten another way.