Nutrient deficiency in gut problems: when food no longer arrives

Healthy state

Nutrients are not absorbed equally everywhere in the gut. Iron and folic acid are taken up mainly by the upper small intestine, vitamin B12 only by the last section, the ileum Ileum The last section of the small intestine. It recovers bile acids, absorbs vitamin B12 and joins the colon via the ileocaecal valve. More in the chapter “Where the small intestine meets the colon: ileum, valve and caecum”. Fat and the fat-soluble vitamins A, D, E and K need bile and pancreatic enzymes to reach an absorbable form at all.

Absorption begins before the gut. Through chewing and stomach acid, iron and vitamin B12 are first released from food, and for B12 the stomach also makes a carrier protein, intrinsic factor, without which it cannot be absorbed at the end of the small intestine. After that, folds, villi and microvilli enlarge the surface of the small intestine to around 30 square metres, roughly the floor area of a small flat; there sit the transporters for sugars, protein building blocks, minerals and vitamins.

Deficiencies show up late because the body has stores. The liver’s B12 stores often last for years, iron stores more for months, as do those of the fat-soluble vitamins. Tiredness, hair loss, tingling, concentration problems or pale skin are therefore late, non-specific signs that also have many other causes.

A deficiency therefore rarely arises from poor absorption alone. Four things work together: how much you eat, how well digestion breaks the food down, how much the gut absorbs and how much is lost through diarrhoea or bleeding. On top of that comes an increased need, for example with inflammation.

Iron shows well that the body controls the store itself. The hormone hepcidin, made in the liver, determines how much iron passes from the gut into the blood. With inflammation hepcidin rises, iron is held in the store, and it is lacking in the blood even though the stores are full. Experts call this a functional shortage in inflammation.

Fat-soluble vitamins are especially vulnerable. If fat or bile is poorly digested, they are the first to be lacking. Zinc and magnesium are lost in the stool with persistent diarrhoea.

Needs are not the same for everyone. Women before the menopause lose iron with menstruation, pregnant women need more iron and folic acid, older people make less stomach acid and more often take medications that interfere with absorption, and anyone on a purely plant-based diet has to supplement vitamin B12 anyway. Some vitamins, such as vitamin K and some B vitamins, are also made by gut bacteria; how much of this really reaches the human body is limited and hard to say.

Disrupted state: causes and effects

If the pancreas works too little (exocrine pancreatic insufficiency) or bile is missing, fats stay undigested. Typical are fatty, pale stools, weight loss and a deficiency of the fat-soluble vitamins. With a large bile acid Bile acids Substances made by the liver that make fat digestible. About 95 percent are reabsorbed in the last part of the small intestine; gut bacteria transform the rest. More in the chapter “Bile acids: when bile causes diarrhoea” loss, for example after removal of a longer piece of ileum, fat digestion works less well. After gallbladder removal, bile is enough for most people; only very fatty meals cause problems for some.

In coeliac disease Coeliac disease An autoimmune disease in which gluten damages the lining of the small intestine. It is diagnosed with a blood test and usually a biopsy while you are still eating gluten. More in the chapter “Lactose, fructose, gluten: making sense of intolerances” the gluten reaction destroys the villi in the small intestine, the surface shrinks, and iron, folic acid, calcium and fat-soluble vitamins are absorbed less well. Iron deficiency can be the only sign for a long time. Parasites such as Giardia, bacteria in small intestinal overgrowth (they consume B12) and diarrhoeal illness also reduce absorption.

If the last part of the small intestine is inflamed or removed, for example in Crohn’s disease, vitamin B12 is poorly absorbed, because it only enters the blood there. The taking back of bile acids also suffers, which makes diarrhoea worse. After extensive removal of the ileum or with fat malabsorption, more oxalate Oxalate Plant compound found e.g. in spinach, rhubarb and nuts. Binds to calcium in the gut and is excreted. Too much in the urine can form kidney stones. More in the chapter “Oxalate, salicylate, lectins: hidden culprits or internet myth?” can also be absorbed through the gut, which favours kidney stones.

Little stomach acid, for example from long-term acid blockers or inflammation of the stomach lining, lowers the availability of vitamin B12, iron, calcium and magnesium. In long-term users of acid blockers, vitamin B12 deficiency is more common in studies, and magnesium levels can fall, although studies disagree on how often.

With inflammation, iron shifts into the inside of the cells. The value for stored iron, ferritin, can then be normal or even high, even though iron is missing for blood formation. A ferritin on its own is therefore misleading; it must be read together with a marker of inflammation such as CRP. Vitamin D is also often low in gut inflammation; whether that favours the inflammation or is a consequence of it has not been clarified.

Medications intervene at several points. Metformin lowers the B12 level in the long term, acid blockers reduce the absorption of B12, iron and magnesium, and agents that bind bile acids and are used against diarrhoea or cholesterol can inhibit the absorption of fat-soluble vitamins. After operations on the stomach or small intestine, for example for weight loss, deficiencies often develop years later, and many people need supplements permanently.

On top of that come losses and misreadings. With persistent diarrhoea, zinc, magnesium and electrolytes are lost in the stool, and anyone who eats strictly all the time for fear of symptoms often eats too little. It also matters to read blood results correctly: a “leaky gut” as an explanation for a general nutrient deficiency is not proven, the zonulin value often measured is not a reliable marker, a hair analysis does not show a reliable nutrient status, and the zinc and calcium values in serum reflect the stores poorly.

Deficiency and tiredness are closely linked, yet not every tiredness is a deficiency. Anyone who takes mixed supplements without blood values may be treating the wrong thing and overlooking the cause. Iron deficiency can exist without anaemia, and a normal blood count does not reliably rule out a B12 deficiency.