The mucus layer and additives

Healthy state

Mucus sounds unappetising but is one of the body's most important protective layers. In the gut it is made by goblet cells in the lining. Its main building block is a huge, heavily sugar-coated protein called mucin 2, which swells in water and forms a gel, similar to gelatine. For comparison: the stomach and duodenum have their own mucus layers with bicarbonate that protect the wall from acid. Here we are talking about the mucus layer Mucus layer A gel layer of mucins on the gut wall that keeps bacteria at a distance. In the colon, the inner layer is almost free of microbes. of the large intestine, where most bacteria live.

In the large intestine, the mucus lies on the gut wall in two layers. The inner one is firm, attached and, in a healthy gut, almost free of bacteria; it keeps bacteria at a distance from the wall. At the same time it is a reservoir: enzymes, mostly from bacteria, loosen it at the outer edge so that it turns into the outer layer. The outer layer is loose, densely populated with bacteria and home and food for many of them. The mucus is constantly renewed, and the goblet cells are steered by nerves, mast cells Mast cells Immune cells beneath the skin and mucous membranes, filled with histamine and other messengers. They raise the alarm in allergies, infections and stress. More in the chapter “Mast cells: alarm cells in the gut, histamine and the disputed MCAS” and messengers from the bacteria. In danger, for example from a pathogen, they suddenly release more mucus and “flush” it away.

The mucus holds further defence substances, above all secretory IgA IgA (secretory IgA) The antibody of the mucous membranes. It is released into the gut in large amounts, where it holds germs in the mucus without causing inflammation. More in the chapter “IgA: the antibody that keeps the gut clean, and what vitamin A, D and omega-3 have to do with it” and germ-killing proteins (see the chapters on the gut barrier and IgA). The mucus is therefore less a wall than a filter: nutrients, gases and signals get through, bacteria do not.

The bacteria are not opponents here but housemates. Some species have enzymes with which they break down the sugar chains of mucin and live on them; this applies to some Bacteroides species and to Akkermansia Akkermansia A gut bacterium that lives on the mucus of the gut wall and stimulates its renewal. In studies, high levels often go along with good metabolic health. . In small amounts this is intended and keeps the mucus moving, and the new supply compensates for it. The balance only tips when the bacteria use too much of the mucus or too little is supplied.

Disrupted state: causes and effects

What matters is the ratio of supply to consumption. If bacteria get too little fibre, in mouse experiments some species switch to the mucus as food and wear it down. The protective layer becomes thinner, bacteria get closer to the wall, and there were more signs of inflammation and more susceptibility to a pathogen. This is an animal experiment with extremely low-fibre feed; in humans the link is plausible but not shown with this clarity.

A lack of fibre also comes in different forms. Typical is a diet with lots of ready-made food, few plants and plenty of meat and sugar. In forums, very low-fibre diets and fasting periods are also discussed. Whether they thin the mucus layer in humans has not been shown; the mouse data only suggest that “little plant food in the long term” could be a risk.

Emulsifiers Emulsifiers Additives that mix fat and water, such as carboxymethylcellulose or polysorbate 80. Some damage the mucus layer in studies. in processed foods are suspected of bringing bacteria closer to the gut wall. Emulsifiers are substances that make fat and water mix, for example in ice cream, sauces and baked goods; they act a little like washing-up liquid. In a small controlled study of 16 healthy people over 11 days, carboxymethylcellulose (E466) lowered the diversity of the gut microbiota Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity”, and in two of the seven people taking E466, bacteria penetrated the inner mucus layer. In mice given polysorbate 80 or carboxymethylcellulose for a long time, more inflammation and more body fat were observed. These results are indications, not proof of harm with a usual diet: the human study was small, short and used a high dose, and mice differ from humans.

A similar thought applies to other additives: thickeners, sweeteners and preservatives are being studied for effects on the flora and the lining (there is a separate chapter on sweeteners). Reviews on this note that the data are thin and mostly come from cell and animal experiments. There is also a practical problem: people who eat a lot of ready-made products often eat not only emulsifiers but also less fibre. The effect of individual substances is therefore hard to separate out in everyday life.

In inflammation, the mucus layer itself can become a problem. Mice lacking mucin 2 develop gut inflammation spontaneously, and in people with ulcerative colitis a thinner, permeable mucus layer has been described. What comes first is open: inflammation can damage the mucus, and weak mucus can make inflammation easier. In everyday life this means: the mucus layer is protected less by “repairing” it than by feeding the bacteria and avoiding as many irritants as possible.