Nutrient blood tests (vitamin D, B12, folate, iron, calcium)

What a nutrient blood test is

A health-screening blood panel is mostly chemistry: glucose, lipids, liver and kidney values. A few of its lines are about nutrients, and those are the ones people search for after the report arrives. This page covers the lines that appear on panels in Taiwan — hemoglobin and calcium on most, vitamin D, vitamin B12, folate and homocysteine on some — and what each one does and does not tell you.

Two things first. No guideline recommends testing healthy adults for these on a schedule: the US Preventive Services Task Force finds the evidence insufficient for vitamin D screening, and the Endocrine Society suggests against routine 25(OH)D testing in healthy adults. And a low value is a reason to eat differently or to treat a cause, not a reason to buy a multivitamin.

What each line measures

What a low value means, and the cut-offs

The cut-offs below are the NIH Office of Dietary Supplements' own, from the fact sheets listed under Sources. Reference ranges are set by the laboratory, so read your report against the range printed next to the value, not against a number you found online.

• Vitamin B12. Most laboratories call under 200–250 pg/mL (148–185 pmol/L) subnormal. Between 150 and 399 pg/mL, an MMA above 0.271 micromol/L or a homocysteine above 15 micromol/L confirms a deficiency; both also rise with reduced kidney function.
• Folate. A serum folate above 3 ng/mL, or a red-cell folate above 140 ng/mL, means adequate status. The NIH gives adequacy cut-offs, not a deficiency line.
• Iron. A ferritin under 30 mcg/L suggests iron deficiency and under 10 mcg/L iron-deficiency anemia; a hemoglobin under 12 g/dL from age 10 suggests anemia. Inflammation raises ferritin, so a normal ferritin with symptoms is not the end of the question.
• Calcium. A serum calcium of 8.8–10.4 mg/dL is normal in healthy people. Under 8.5 mg/dL (hypocalcemia) or over 10.5 mg/dL (hypercalcemia) is a medical question about the parathyroid glands, the kidneys, medicines or vitamin D excess — not a diet question.
• Homocysteine. 5–15 micromol/L is the usual normal range. Above 15 is a prompt to check B12 and folate; it is not on its own a reason to take anything.

How much you need at your age and sex

The Recommended Dietary Allowances below are the NIH Office of Dietary Supplements' values. They change at 19, at 51 for women, at 71 for everyone, and in pregnancy; the age pages print the line for one decade, this table has them all.

Vegetarians need 1.8 times the iron RDA, because plant iron is absorbed less well. The upper limits for adults are 4,000 IU of vitamin D, 1,000 mcg of folic acid from supplements and fortified foods, 2,500 mg of calcium at 19–50 and 2,000 mg from 51, and 45 mg of iron; vitamin B12 has no upper limit.

Food first: where each nutrient comes from

The NIH fact sheets list the foods below with their amounts per serving; a normal diet reaches the RDA for most people without a supplement, which is why every guideline says food first.

• Vitamin D: cod liver oil (1,360 IU per tablespoon), farmed rainbow trout (645 IU per 3 ounces), sockeye salmon (570 IU per 3 ounces), UV-exposed white mushrooms (366 IU per half cup), fortified 2% milk (120 IU per cup), fortified soy, almond and oat milks (100–144 IU per cup).
• Vitamin B12: beef liver (70.7 mcg per 3 ounces), clams (17 mcg), oysters (14.9 mcg), fortified nutritional yeast (8.3–24 mcg per quarter cup), Atlantic salmon (2.6 mcg), canned light tuna (2.5 mcg). Plant foods have none unless fortified.
• Folate: beef liver (215 mcg DFE per 3 ounces), boiled spinach (131 mcg per half cup), black-eyed peas (105 mcg), fortified breakfast cereal (100 mcg), white rice (90 mcg per half cup), asparagus (89 mcg per 4 spears).
• Calcium: plain low-fat yogurt (415 mg per 8 ounces), calcium-fortified orange juice (349 mg per cup), part-skim mozzarella (333 mg per 1.5 ounces), canned sardines with bones (325 mg per 3 ounces), nonfat milk (299 mg per cup).
• Iron: fortified breakfast cereal (18 mg per serving), oysters (8 mg per 3 ounces), canned white beans (8 mg per cup), beef liver (5 mg per 3 ounces), lentils and boiled spinach (3 mg per half cup each).

The Dietary Guidelines 2025–2030 add a life-stage note: young adulthood is when peak bone mass is built, with iron and folate the priorities for women; older adults need equal or greater amounts of protein, vitamin B12, vitamin D and calcium even as they need fewer calories; and vegetarians and vegans should monitor iron, B12, vitamin D, calcium and iodine.

When a supplement is the right answer

Four situations have a guideline behind them. Everything else is a food question, or a measured low value treated by a doctor.

1. 1
   Folic acid, before and in early pregnancy

   Anyone planning to or able to become pregnant: 400–800 mcg of folic acid a day, starting at least one month before conception and through the first 2–3 months (USPSTF 2023, grade A). The CDC says 400 mcg of folic acid, not 5-MTHF, even with an MTHFR variant.
2. 2
   Vitamin D, in three groups

   The Endocrine Society (2024) suggests routine vitamin D supplementation for adults 75 and over, in pregnancy and in adults with prediabetes — and not for healthy adults 19–74, who should meet the RDA from food and sun. It does not set a dose beyond the RDA.
3. 3
   Vitamin B12, for a measured low level or a reason

   Older adults, people on a vegan diet, people with pernicious anemia or gut disease, and people who have had stomach or bowel surgery are the NIH risk groups. A level under 200–250 pg/mL, or a grey-zone level with a high MMA, is treated with a supplement or injections at a dose a doctor sets. No birthday triggers it.
4. 4
   Iron, for a confirmed deficiency or in pregnancy

   A low ferritin or an iron-deficiency anemia is treated, and the cause is found: heavy periods, blood donation, gut disease — and in a man or a postmenopausal woman, bleeding in the gut until proven otherwise. ACOG recommends low-dose iron from the first trimester of pregnancy. Do not take iron for tiredness without a test; the upper limit is 45 mg.

What the panel does not measure

• Ferritin is the iron store, and most Taiwan screening panels do not include it. Ask your coordinator whether the clinic can add it as a custom blood item.
• Zinc, magnesium and the omega-3 index are not on standard panels, and no screening guideline asks for them in healthy adults.
• Vitamin D on most panels sits inside the package's extended blood work rather than as a listed item; check the Items list of the package you are considering, or ask.
• Bone density is the calcium result that matters, and it is a DEXA scan, not a blood test.

Results and follow-up

• A report listing each value next to the laboratory's reference range. Read it against that range; which lines are included, the report language and turnaround are confirmed when you book.
• A low vitamin D, B12 or iron is a question with a usual answer: food or a supplement up to the RDA for a borderline value, a doctor-set dose and a look for the cause for a deficient one, and a recheck in about 3 months.
• A blood calcium outside its range is a medical question, not a diet one; a normal blood calcium says nothing about your bones.
• Keep the report. The age-by-age screening pages print the RDA for your decade, and a trend across years says more than one draw.