One argument against a raw food diet is that some people's stomachs may have more difficulty digesting raw foods. While I think this is rare, I do believe that alternative diets can be recommended for such people. That being said, I would also like to emphasize that, compared to our ancestors, humans have more access to instant fire and other cooking technologies today. As a result, we probably depend on cooking too much. Here are some suggestions if you are having stomach pains on a raw food diet:
1) Make more smoothies. Famous raw foodist Markus Rothkranz lives almost entirely on green smoothies (half greens, half fruit plus water) (Link 1) (Link 2). Smoothies break down the fiber in your food, making it less work on the stomach to digest.
2) Drink more fresh Juice. Fresh juice is easily digested and extremely nutritious. Some super simple recipes include carrot/celery juice and cucumber/apple juice.
3) Consider a Whole Foods Vegan Diet. A whole foods vegan diet still includes fresh fruits and veggies. But it also includes foods such as beans, rice, potatoes, oats, soups, grilled and steamed vegetables, etc. Balance your raw foods and cooked foods with what makes you feel the best.
4) Exercise. Often digestion problems come from anxiety. One proven way to release anxiety is through exercise.
5) Eat slowly and chew thoroughly. No matter which diet you are on, chewing thoroughly is essential for proper digestion.
6) Determine the Source: Figure out which raw foods are causing stomach pain and temporarily eliminate that food from your diet (rather than eliminating all raw foods)