Why do I lose weight or run low on vitamins (B12, iron, vitamin D), and how is that corrected?
IBD can cause weight loss and nutrient shortfalls because inflammation, eating less during flares, and poor absorption all pull in the same direction. Iron, vitamin B12, and vitamin D are the most common to run low. The fix is not guesswork: your team checks your levels with blood tests and corrects them through diet, supplements, iron infusions, or B12 injections when the part of the gut that absorbs B12 is affected. Regaining weight once you are in remission matters too.
Listen to this article · AI-generated narration
Losing weight or being told your iron is low is one of the more common parts of living with IBD, and there is a clear reason for it.
Why it happens
Several things push in the same direction. The Crohn's & Colitis Foundation describes malnutrition as common in IBD, driven by eating less, absorbing nutrients poorly, and the extra energy demands of ongoing inflammation (Crohn's & Colitis Foundation). During a flare, appetite often drops just as the body needs more, which is how weight comes off.
The common shortfalls
Three deficiencies come up most often:
- Iron. Around 1 in 3 people with Crohn's or Colitis have low iron, which can lead to anaemia (Crohn's & Colitis UK).
- Vitamin B12. B12 is mainly absorbed in the last part of the small bowel, the ileum, so deficiency is more likely when that area is inflamed or has been removed (Crohn's & Colitis Foundation).
- Vitamin D. Levels dip easily, and in winter most people do not get enough sunshine to make their own (Crohn's & Colitis UK).
How it is corrected
The key is that this is measured, not guessed. Your team checks levels with blood tests and then corrects them in the right way: better nutrition, oral supplements, iron given by infusion when needed, or B12 by injection when the gut cannot absorb it well. Regaining weight once you reach remission is part of the same picture. It is worth avoiding high-dose supplements bought on your own, since the dose and the form matter.