Ayurveda maintains a detailed list of foods that should not be combined with other foods. Of these foods one of the most important is milk, which is a complete food in and of itself, and does not combine well with other foods. While it is true that Indian cookery sometimes calls for milk to be mixed with foods, this is not a recommendation from Ayurveda. In particular, milk should never be mixed with fish, meat, radish, garlic, basil, honey and sour-tasting foods. Similar to milk, honey has a number of contraindications in Ayurveda, and should never be mixed with equal parts ghee, with hot drinks and spicy foods. Similar admonitions are also found in China as part of a folkloric tradition rather than a formal teaching. Some of these incompatible foods include peanut and cucumber; taro and bananas; beef with chestnuts or miso; lamb and cheese; crab with pork or persimmon; rabbit meat with celery or mustard; and honey with onions, garlic, tofu or fish. Although it is difficult to rationalize all these relationships, as a folkloric tradition they are based on real-time observations over thousands of years, and are at least worthy of consideration.

The primary approach to food combining I follow in my clinical practice is based upon the chemical properties of the three macronutrient groups, i.e. carbohydrates, proteins and fats, and how they are digested in different parts of the GI tract. Carbohydrates such as bread, potatoes and rice undergo preliminary digestion in the mouth with the secretion of salivary amylase, and by mechanical digestion (chewing). They then pass through the stomach unaffected by the secretion of gastric juices, and on to the small intestine where the secretion of pancreatic enzymes completes carbohydrate digestion.

Proteins undergo no digestion in the mouth, other than mechanical digestion by chewing, and then pass into the stomach where they are acted upon by powerful gastric juices that include hydrochloric acid and pepsin. The acid and enzymes secreted by the stomach help break down large chains of proteins into smaller peptide chains. These peptides are later acted upon by pancreatic enzymes in the small intestine, and are reduced to amino acids that are then absorbed across the intestinal wall in the blood.

Fats first undergo under the influence of the enzyme lipase, which breaks down dietary fats (triglycerides) into glycerol and free fatty acids. This process begins in the mouth due to the release of lingual lipase, a fat-digesting enzyme released by the tongue. Up to 30% of the dietary fat we eat is hydrolyzed by lingual lipase, which further suggests the importance of chewing. This process continues in the stomach with the release of gastric lipase, and as it enters the small intestine, bile salts released by the liver and gall bladder and pancreatic enzymes complete the process of fat digestion. Another small but important source of fats are from bacteria that inhabit the large intestine, which ferment non-digestible materials such as cellulose to produce short-chain fatty acids, biotin, vitamin B12, and vitamin K2 that are absorbed by the colonic mucosa.

When digestion is strong food combinations rarely cause much of an issue, but if digestion is weak, or if poor combinations are eaten continuously, eventually some kind of notable interaction will occur. The biggest problem I see among my patients is the result of combining animal proteins and carbohydrates in the diet, such as meat and potatoes, which require different mediums in order to be properly digested. Enzymes that break down carbohydrates require an alkaline medium, whereas proteins require an acid medium. When carbohydrates are consumed with proteins they inhibit the secretion of stomach acid, or inhibit the ability of the stomach juices to properly act upon the proteins, which are mixed up with carbohydrate. In some people this may delay stomach emptying and prolong intestinal transit time, resulting in the bacterial fermentation of the carbohydrates, and the release of gas. This leads to stomach bloating, burping, nausea and heartburn, causing the stomach and esophagus to become distended. If this becomes a chronic event, this distension may eventually cause the outside of the stomach to adhere to other tissues in the abdomen, such as the diaphragm, leading to hiatus hernia. Poor gastric motility and bloating may also compromise the function of the lower esophageal sphincter, allowing stomach juices to rise up into the esophagus, resulting in acid reflux and heartburn.

When carbohydrates impair the process of protein digestion improperly denatured proteins pass into the small intestine. Enzymes released by the pancreas that are designed to break down smaller peptides cannot efficiently break these larger proteins down, inducing bacterial protein putrefaction. In the process of digesting improperly denatured proteins, the putrefactive bacteria release toxic metabolites including indole and skatole that damage the intestinal wall, enter into the blood and tax the detoxification systems of the body. Both carbohydrate fermentation in the stomach and protein putrefaction in the intestine place undue stress upon the digestive tract, and set the stage for chronic insult that leads to epithelial damage and intestinal permeability. To avoid this problem, protein and starch combinations should be watched closely. If they are consumed together take them in a way which facilitates better digestion, such as soupy meat dishes taken with boiled whole grains rather than something like a grilled steak and baked potato, or a roast beef sandwich.

Fats also influence the digestion of proteins and carbohydrates by slowing down how quickly the stomach empties. Whenever we eat a fat the body delays stomach emptying so it can begin to synthesize all the bile it will need to properly emulsify the fats. The ideal combination is to eat proteins and fats together, as this delay in stomach emptying helps to ensure proper protein digestion. Mixed with starches, however, fat delays carbohydrate digestion, causing it to sit in the stomach longer. For most complex carbohydrates this isn’t a problem and it can even be beneficial, slowing how quickly the starchy food gets turned into blood sugar. But when fat is mixed with particularly gluey and starchy foods such as bread, pastries and pasta, the glue-like consistency impairs the activity of bile, leading to poor fat digestion and gall bladder problems.

Poor food combinations also affect the large intestine and the creation of feces. Most people are unaware that upwards of 60% of the volume of a bowel movement are bacteria. Since bacteria are dependent upon whatever substrate is available, changes to what we eat results in different types of microbial ecologies, and hence affects the nature and quality of the bowel movement. Improper protein digestion results in protein putrefaction in the colon, leading to foul-smelling gas and bowel movements. A whitish discoloration or feces that float is indicative of poor fat digestion. Both constipation and diarrhea can be caused by a lack of fiber in the diet, from poor food combinations, and in constipation especially, refined carbohydrates such as flour, pasta and bread.