Hair is not an essential organ. When your body is choosing where to allocate nutrients under conditions of deficiency, hair comes last. This means that nutritional insufficiencies often show up in your hair before they show up in bloodwork or other symptoms. The hair is, in a way, a canary. What you're seeing is not always a hair care problem. Sometimes it's a nutritional one.
Iron Deficiency
Iron deficiency, even at levels that don't yet qualify as anemia, is one of the most common nutritional causes of increased hair shedding in women. Iron is essential for the production of hemoglobin, which carries oxygen to hair follicle cells. When iron levels drop, follicle activity slows.
Signs it might be iron: diffuse shedding all over the scalp, fatigue, feeling cold frequently, pale inner eyelids. Heavy periods put women at particular risk. A ferritin blood test (not just a hemoglobin test) is the most accurate measure of iron stores for hair purposes. Ask your doctor for a ferritin panel specifically.
Vitamin D Deficiency
Vitamin D receptors play a role in hair follicle cycling. Deficiency is extremely common. Studies suggest that over 40% of the US population has insufficient vitamin D, and it's been associated with hair loss, particularly the type that resembles alopecia areata (patchy loss) or diffuse thinning. Many women don't find out their vitamin D is low until they test for it specifically.
Protein Insufficiency
Your hair shaft is made of keratin, which is protein. When dietary protein is consistently insufficient, the body redirects protein toward essential functions and hair growth slows. Hair that's noticeably thinner in strand diameter, grows more slowly than usual, or sheds more with increased brittleness can sometimes be traced to chronically low protein intake. This is particularly relevant for women following very restrictive diets.
What Actually Helps
Get bloodwork done before buying supplements. Supplementing nutrients you don't actually lack is at best a waste of money and at worst counterproductive. If your iron and vitamin D come back within normal range, your hair loss is more likely stress-related, hormonal, or care-related than nutritional.
Biotin supplements are widely marketed for hair growth. The evidence supporting biotin supplementation for hair growth is very limited and primarily applies to people with an actual biotin deficiency, which is rare. If you're not deficient, biotin supplements are unlikely to make a meaningful difference.
Omega-3 fatty acids (from fish, flaxseed, or chia) have some evidence for supporting scalp health and reducing inflammation. A diet already rich in these through food is preferable to supplementation.