Our digestive system has evolved over millions of years to allow us to eat almost anything and to be able to digest it without difficulty. Homo sapiens has always been an omnivorous animal, and we still are. However, our current way of eating differs greatly from the way our ancestors ate, and to which our physiology is more adapted. Throughout evolution we have been mainly hunter-gatherers for millions of years in prehistoric times, and then farmers and herders since the Neolithic period some 8,000 to 10,000 years ago, but we have not systematically mixed different food groups on the same plate as we tend to do now. In other words, a prehistoric human ate many days of green plants, nuts, cereals or wild fruits that he gathered, and from time to time, if he was lucky and managed to catch a fish or hunt an animal, or steal eggs from a nest, he ate them without rice or fried potatoes as we do now. In this way, the body, which is able to digest different macronutrients (carbohydrates, fats and proteins) by adapting the acidity of the stomach and the release of different digestive enzymes by the stomach and pancreas according to what we eat, was "focused" on the digestion of a single, simpler food. Thus, digestion took place in an exemplary manner, and the macro- and micronutrients in the food were very well utilised.
Macronutrients are not digested in the same way in the digestive tract. Digestion of carbohydrates begins in the mouth, thanks to an enzyme in the saliva called salivary amylase. This enzyme begins to "cut" into small pieces the starch molecules contained in carbohydrate-rich foods (cereals or flours derived from cereals, tubers such as potatoes or sweet potatoes, legumes such as chickpeas or beans, which are complex to digest as they also contain a lot of protein). Salivary amylase requires a neutral or relatively alkaline pH (above 7). It works best at a pH of between 6.5 and 8. If the pH is between 6.5 and 5, its activity decreases considerably, and it is completely inactivated if the medium is very acidic, below a pH of 5. In other words, if we have eaten a meal that is very rich in carbohydrates and low in protein and fat, the stomach will produce little acid. However, if we have eaten foods high in protein and/or fat, our stomach will give priority to these, and will produce a lot of hydrochloric acid, which is stomach acid, to lower the pH. In this case, the digestion of carbohydrates will slow down as salivary amylase will be inactivated.
As for proteins and fats, they do not begin to be digested in the mouth, but directly in the stomach, because acid is needed to digest them, unlike carbohydrates. The first enzyme involved in protein digestion is pepsin, which "cuts" large proteins into smaller pieces so that they can be further digested by pancreatic enzymes as the food leaves the stomach and passes into the duodenum. Pepsinogen, which is the inactive form of pepsin, is secreted by the cells of the gastric wall, and is activated into pepsin only when there is a lot of acid (when the pH is between 1.8 and 4.4). Stomach acid also serves to break down proteins and expose them more easily to the action of pepsin. Gastric lipase, another enzyme produced by the stomach that serves to digest fats, also needs an acidic pH to be able to act, although it is more flexible than pepsin, as it is stable in a pH range between 2 and 8, with optimal functioning between 4.5 and 6.
Thus, if in the same meal we eat a lot of carbohydrates and at the same time a lot of protein and fat (for example, grilled salmon accompanied by rice, which might seem a very healthy dish), we are forcing our stomach to decide whether to give priority to the digestion of protein and fat, lowering the pH a lot and completely inactivating salivary amylase, or carbohydrate, leaving the pH less acidic, which will prevent pepsinogen from being correctly transformed into pepsin and gastric lipase from working properly, so that it can start a good digestion of proteins and fats. The same is true if we take drugs or substances against acidity (bicarbonate, antacids, proton pump inhibitors such as omeprazole) or if we drink too much water during meals, as the digestive enzymes are diluted or less effective. In general, in such a situation, the in-between usually occurs, with the result that neither protein and fat nor carbohydrate are digested properly.
The food then passes into the duodenum, which is the first portion of the small intestine, where the pancreas and liver, via the gallbladder, secrete pancreatic juices and bile. Pancreatic juices are loaded with bicarbonate to counteract the acidity of the stomach and enzymes that help digest carbohydrates (pancreatic amylase), proteins (trypsin, chymotrypsin, elastase and carboxypeptidase) and lipids (pancreatic lipase). All these enzymes act at an alkaline pH (between 7 and 9). Bile salts, which are also alkaline and help to reduce the acidity of the food bolus, are like a kind of soap that serves to make fats soluble, so that they are more easily absorbed by our digestive tract. The duodenum then "takes over" and digestion continues. However, if the food leaves the stomach poorly digested because the pH has not been well regulated and salivary amylase and pepsin have not been able to act as they should, the pancreas and liver will have to make an extra effort to finish digesting the food. In addition, as larger pieces of protein or carbohydrate reach the duodenum, the enzymes in the pancreas will find it more difficult to penetrate the food and cut the molecules into smaller pieces. This is particularly a problem with proteins, as it is the stomach's pepsin that starts to digest larger proteins, whereas pancreatic trypsin, chymotrypsin and aminopeptidase work best if the protein is already partially digested. In addition, small pieces of digested protein in the stomach, passing into the duodenum, promote the release of cholecystokinin, a hormone that stimulates the secretion of pancreatic and bile juices, promoting digestion. The ideal situation would therefore be for proteins to be pre-digested in the stomach, so that pancreatic and bile juices are more abundant in the duodenum and the action of pancreatic enzymes is more effective. This is why a meal with a large mixture of macronutrients will always be more difficult to digest (although not impossible), and all the more so the older we are, as the working capacity of our digestive organs decreases with age.
The consequence of this will be that part of the proteins will not have been digested properly, as well as part of the carbohydrates, and these substances will advance through the digestive tract without being absorbed correctly, as the last stage of digestion, which is produced by the enzymes of the enterocyte wall, which only digest small molecules, requires the previous stages to take place correctly. In addition to not feeding us, these substances will serve as food for certain bacteria and yeasts in the intestine that will ferment the carbohydrates, generating toxic products that will pass into our body. The CandidaYeast, which are part of our commensal intestinal flora in small numbers and feed exclusively on sugars, can become very overgrown if the intestine receives a large amount of poorly digested carbohydrates. These yeasts are particularly toxic if they grow too large, and can promote the development of autoimmune diseases and even psychological problems. On the other hand, other micro-organisms called proteolytic micro-organisms will digest the remains of the protein and cause it to putrefy, generating more toxic products that will enter our body. Among other things, these putrefaction and fermentation mechanisms (fermentation above all) will cause the emission of certain gases that will generate bloating, abdominal discomfort and flatulence. Among these gases, methane will also have the capacity to slow down intestinal transit, causing constipation, causing toxic products to remain longer in the intestine, increasing the probability of their absorption and favouring the overgrowth of these micro-organisms in a sort of vicious circle. On the other hand, the release of these toxic products, as well as the overgrowth of bacteria and other micro-organisms that feed on the remains of poorly digested food, will lead to an activation of the immune system of the intestinal wall, resulting in inflammation. Thus, a simple bad combination of foods can easily lead to poor digestion, poor assimilation of macro- and micronutrients, intestinal inflammation, absorption of toxic substances and altered intestinal transit at the same time. It is therefore important to know how to combine foods.
Bibliography:
Perez R (2020). Les combinaisons alimentaires. Lanore.
