The term "food combining" can be used in several different ways. Most commonly, it is used to describe the planned consumption of different foods at different times in order to optimize the process of digestion. While it is true that different foods digest in different ways and at different speeds, there's no good research evidence to support the practice of food combining when it is understood in this way. However, many people have found food combining for improved digestion to be essential for their overall health, and many healthcare practitioners continue to support this practice despite the absence of research evidence.
Another common use of the term "food combining" involves avoidance of certain macronutrient combinations. For example, you may have heard the food combining statement, "never eat proteins with carbohydrates." This goal is essentially impossible, since most vegetables, grains, nuts, seeds, legumes, and dairy products contain both proteins and carbs. You'd have to eliminate all foods from these foods groups if you wanted to strictly separate your intake of proteins and carbohydrates, and that step would leave you with a very unhealthy way of eating.
Sometimes the idea of food combining is simply used as a way of reminding people to avoid extremes when it comes to macronutrient intake. Excessive intake of macronutrients can indeed pose problems for our health. For example, large amounts of protein (like the 80+ grams of protein that would be found in a 12-ounce steak) together with large amounts of carbohydrates (like the 40+ grams of sugar found in an 8-ounce glass of grape juice) might indeed be a taxing combination for your digestive tract. The combination of these two foods might be harder on your digestive system than either food alone. In this sense, these two foods (one high-protein and one high-carb) might be better off eaten separately than in combination. However, I see the basic problem here as one of going to extremes (too much protein at once and/or too much sugar at once) rather than food combining.
Advocates of food combining also focus upon not including fruit as part of a meal. For example, a common recommendation is to wait at least an hour after consumption of any non-fruit foods before eating a piece of fruit. Once again, I have not seen any research evidence to justify this recommendation. Yet, many people do report better overall digestion when they follow this procedure and a large number of healthcare practitioners advocate fruit consumption separate from meals.
There are a number of theories used to support this practice. One is that the fermentation of the fruit might take place at the expense of other digestive events. But once again, I cannot find indexed journal research to support these claims. Separate fruit consumption may be helpful for some individuals, but I also believe that many individuals can comfortably consume reasonable amounts of fruit together with other foods. As you can see from our website and book, we do include fruit as an ingredient in our recipes and find it to be delicious!
Our digestive tracts are clearly capable of complicated and interwoven digestive processes. If we are fully healthy, we have the right enzymes, digestive fluids (including bile and pancreatic fluid), intestinal bacteria, and muscle tone to digest not only the World's Healthiest Foods but other foods as well. This marvelous combination of digestive abilities would suggest that we are designed, in principle, to consume many different kinds of foods either separately or in combination. However, our digestive tracts—like the rest of our body systems—are also biochemically individual. We have our own unique approach to digestion, in the same way that we have our own unique fingerprint. From this standpoint of biochemical individuality, it would make sense to me that different individuals might truly benefit from different approaches to food combining—including approaches that may not even have been listed in any of the examples described above.