A lack of this vital nutrient is one of the world's leading causes of disability – but exactly when it becomes a problem, and the best way to treat it, remains unclear. When Megan Ryan first noticed her fatigue, she assumed it was normal. After all, she was a single parent to a three-year-old. She also worked full-time. Almost every day, when she picked her son up from daycare, she'd fall asleep with him during his afternoon nap. She didn't think much about it. "I just thought, 'Oh, this i s motherhood'," says Ryan, who lives in upstate New York, US. At a routine medical check-up in June 2023, her doctor asked if she felt exhausted. Blood test results showed that Ryan had iron deficiency anaemia.(BBC News)