Digestive health and Immunity

We all think of digestive health as just being related to how a child’s body absorbs nutrients, and their bowel movements. There is actually much more going on in between, and a lot of it is related to the activity of the immune system.

In simple terms, the digestive tract is like a long, flexible tube that is wrapped up in a small space. Food  travels in one direction (although sometimes it goes the other way!) and in fact, when a child turns 5, its intestines alone will be about 5 m in length ^[1].

Even though food goes “into” the body, the gut wall only lets in nutrients in the first few sections after food leaves the stomach. Most of the bulk material that goes in also goes out, but along the way, it dries out through the absorption of water, to form feces. So, it still can be considered as remaining “outside” the body.

With this in mind, it makes more sense to consider that bacteria and viruses that get into the gut via the mouth from outside, will stay in the gut. This means they are still “outside” the body and most of the problems caused by these foreign factors remain in the gut.

The gut is actually forms the largest part of the immune system. The job of the immune system is to protect what happens inside a child’s body from what goes on outside, especially at entry points. In the gut, this means germs entering through the mouth or nose that end up in the stomach and intestines. The digestive system has three main ways of shielding the body from these nasties:

• The gut wall: is very good at absorbing nutrients, but it is also great at blocking bacteria and viruses that are trying to enter the body ^[1].
• The intestine immune cells: inside the gut wall are special immune organs that capture and destroy bacteria that get into the gut ^[2].
• The gut microbiome: living further down in the intestine and made of billions of different varieties of friendly bacteria. They compete with foreign bacteria and block them from getting in as well. The form another barrier on the gut wall and make products that feed it too.

Digestion has many ways of supporting immunity. The first is by providing nutrients that help maintain  the immune system organs all over the body, and feeding the gut wall directly. The second is by feeding the bacteria of gut microbiome.

During times of illness, when the gut wall becomes leaky, loses nutrients and fluids at a much higher rate, and doesn’t work as well to absorb nutrients anymore ^[4]. It also sheds more dead cells. So nutrient demand is slightly higher during these challenging periods.

This is what happens during diarrhea for example, and why children require electrolytes like potassium, calcium, magnesium, and soluble vitamins like B-vitamins and vitamin C. It is also why infants and children lose weight and growth can slow down during times of prolonged infections ^[5].

Prolonged undernutrition in developing countries is a major cause of stunting and is associated with increased levels of gut parasites ^[6].

This can happen in several ways. When bad bacteria take over, they shift the balance away from the good bacteria. Alternatively, the over-use of antibiotics can kill off some of the good bacteria. The end result is a problem called dysbiosis ^[7], or an imbalanced microbiome. One of the effects of this is bloating and wind, caused by the build-up of gas that is made by the bad bacteria.

Nutrients such as oligosaccharide prebiotics (e.g. GOS) can help the microbiome to re-establish itself after these kinds of impacts. Other prebiotic factors, including lactoferrin and lactose, also enter the infant gut naturally from the breast milk, to give the infant microbiome and immune system a jump-start after birth.

Although we often ask which nutrients are required for healthy immunity, it is not such as simple answer. The working immune system requires a working digestive system, and this means that all nutrients are potentially important.