Giving babies in Bangladesh a daily zinc pill for 6 months raises the zinc in their blood, but it doesn't change their immune system's gene activity or signaling molecules — so if zinc helps them, it's probably working in ways we can't see with standard blood tests.

Giving babies in Bangladesh a daily zinc pill for six months doesn't change their immune system markers in the blood, so zinc probably helps them in other ways, not by tweaking these specific immune signals.

Giving babies in Bangladesh a daily zinc pill for six months doesn't reliably change a specific immune protein in their blood, even if there's a tiny rise — so this protein can't be trusted to show whether zinc is working in their bodies.

Even when toddlers eat enough zinc-rich foods, their blood tests might still show low zinc — which means the usual way doctors check for zinc deficiency might not be working right.

Even though these little kids in New Zealand are eating enough zinc-rich food, almost 4 out of 10 of them still have low zinc levels in their blood — which means what they eat might not be turning into enough zinc in their bodies.

Giving toddlers in New Zealand more zinc through meat or special milk doesn’t seem to raise the zinc levels in their blood or hair, so just eating more zinc might not fix low zinc levels in their bodies.