Improper Food Combining
The first cause is eating improperly combined meals. There are many different food combining rules and in my experience several of them can be disregarded. However, there is one rule that is more important than all the others. That is - do not combine fat and sugar. On a raw food diet, your main source of carbohydrates will be fruit and your main sources of fat include avocado, nuts, and seeds. This basically means that you should not combine fruit with any of those sources of fat. This is because fruit digests much faster than fat. While the fruit is waiting for the fat to be digested it begins to ferment which causes gas. Even though this is the most important rule it too can be broken occasionally. If you are experiencing bloating I recommend that you follow all of the food combining rules and break one at a time to see which combinations do and don't work for you. All of that being said, if you are eating a standard american diet (SAD) and incorporating more fruits into your diet it is best to eat them alone first thing in the morning before eating any other foods. Also, do no eat any fruits after eating other foods to ensure optimum digestion.
Consuming Too Many Hard to Digest Veggies
Some veggies like broccoli, cabbage, and cauliflower, are known to cause gas and bloating. These vegetables are hard for humans to digest and can cause excessive gas when consumed in large quantities. I usually eat these veggies only occasionally and not in large amounts when I do.
Consuming Too Much Fat
Fat produces foul smelling gas. In The 80/10/10 Diet Dr. Doug Graham recommends consuming 10% of calories or less from fat. On a 2000 calorie diet that would mean your fat intake is limited to 1/2 of a large avocado, 1/4 c nuts or seeds, or 2 Tbsp nut or seed butter. It is best to eat fat in the evening with your last meal. Since fat is slow to digest it is best to eat it last so that it can be properly digested.
Previous Diet History
If you have decided to dive head first into a raw food diet from a diet high in fatty and/or processed foods, some gas and bloating is almost unavoidable. Even if you follow all food combining rules perfectly it can take a few days to a few weeks for the gas and bloating to subside. This is because when eating a high fat dense diet digestion moves very slowly. When fresh fruits and vegetables come into your slow moving digestive system they sit around for too long and begin to ferment and cause gas. Dr. Doug Graham states that, "Initially, gas problems may arise from an impaired digestive tract, which holds foods in little pockets of the intestines, where they may ferment or putrefy and cause gas. Over time, this will stop, as the intestines heal". In order to reduce the amount of gas you experience in the beginning be sure to eat no more than 4 meals, avoid drinking water with meals or directly after meals, and get plenty of exercise. These are things that improve digestion and help reduce gas and bloating.
Just remember one thing, it's not the fault of the healthy fruits and veggies! Keep eating healthfully and paying attention to what works for you and what doesn't.