It is unabashedly summer. We’re coming out of a heatwave here in France and after a long winter, I’m getting real sun on my skin whether I like it or not.

Hopefully, that also means that my vitamin D deficiency problems are behind me. At least for the next few months.

In January, we couldn’t stop talking about why so many women are low in vitamin D. Around a third of us are vitamin D deficient, and, surprise, surprise, I am among them.

I take the vials every six months or so to keep my levels up, and vitamin D is easy that way: you supplement it or go outside in summer, and your levels go up. You can only imagine my optimism when I found that vitamin D also has a relationship to iron.

I'm perpetually low in both vitamin D and iron. Are they connected? Could a simple-to-fix vitamin D deficiency also help me fix my iron?

That's the question I'm diving into this week.

Before getting to the deep dive, here’s what caught my eye this week:

📄 Vitamin C with meals, put to the test: across 15 trials, vitamin-C-rich fruit at meals raised ferritin and hemoglobin, even though the few single-meal “how much got absorbed” tests didn’t move, suggesting the payoff builds over time. (American Journal of Clinical Nutrition)

📄 Why vitamin D doesn’t move the same for everyone: in a pregnancy trial, the same high dose raised blood levels less as BMI went up, suggesting “one dose fits all” doesn’t account for body size. (The Journal of Nutrition)

📄 More polyphenols, less diabetes risk: in a 7.6-year study of ~8,800 adults, those eating the most polyphenols (i.e., the compounds in coffee, tea, berries, and dark chocolate) had a 19% lower risk of developing type-2 diabetes, with slower-rising insulin resistance. Observational, so a strong link rather than proof. (The Journal of Nutrition)

Just sharing here what's new this week. No affiliates, no sponsorships, no freebies. A mention here isn't a recommendation.

Low vitamin D and low iron do tend to show up together, but taking vitamin D won’t reliably raise your iron.

In pregnant women, being low in vitamin D comes with about 60% higher odds of anemia.

So while they often come together, that’s not the same as one causing the other. The same habits (like less time in the sun and less red meat) can pull both down at once.

Still, it’s not so clear cut. Among South Asians, researchers actually observed lower iron-blocking signals alongside high vitamin D levels.

Vitamin D does have a direct line to iron, through hepcidin. When hepcidin is high, it traps iron storage and blocks you from absorbing more.

Vitamin D switches off the gene that makes hepcidin and lets cells release iron. Inflammation increases hepcidin (via IL-6). So when long-term inflammation is a real problem, calming it can free up the iron you already have in your body.

But unlocking iron for a day doesn’t refill iron you don’t have. A single big dose of vitamin D drops hepcidin within a day, but it’s a brief blip.

When people take vitamin D for weeks or months and then get their iron checked, we don’t see increased iron. Not in older adults, not in pregnant women, and not in kidney disease patients. (Their vitamin D does go up though.)

Honestly, the fundamental scientific research doesn’t yet exist to explain this mechanism very clearly and satisfactorily. We continue to see mixed evidence depending on the group of people someone falls into.

So, there are plenty of good reasons to get enough vitamin D. But don’t count on it to be the solution to your iron troubles.

☀️ If you already know you run low on vitamin D (or take it on a clinician's advice) keep it up for your bones, muscles, and immune system, but don't push the dose higher hoping it'll lift your iron. Correcting a real shortfall is worth it. Once you're in the normal range, extra vitamin D does nothing more, for your iron or anything else.

🔁 If you take an iron supplement, take it once every other day as a single dose (not daily). One dose raises hepcidin for about a day, which blocks the dose that comes too soon after. So spacing them out gets more iron in, usually with fewer side effects.

🔥 If you train hard most days, build in regular rest days. Heavy training drives up inflammation and spikes hepcidin. Calming the inflammation is what frees stored iron, so recovery can be key to maintaining healthy iron levels.