Biochemistry and values: test first, then consider supplements

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Start with your values, through a doctor
Omega-3 (EPA and DHA)
Vitamin D3
Vitamin K2
Vitamin B12
Folate (folic acid)
Test, don't guess: a simple routine
FAQ
Full disclaimer

Health disclaimer: This page is general information, not medical advice. It does not diagnose, treat or replace your doctor. Talk to a doctor or pharmacist before you change medicines, start supplements or begin a new training programme, especially if you have a health condition, take medication, or are pregnant.

Supplement shelves promise a lot. The chemistry is simpler: your body needs a certain amount of each nutrient, too little causes problems, and for some nutrients too much causes problems too. The only way to know where you are is to measure. That is why this page starts with tests and ends with supplements – not the other way around.

Start with your values, through a doctor

Blood tests should be ordered and interpreted by a doctor, who knows your history, medicines and symptoms. Values worth asking about if they are relevant to you include:

Vitamin D (25-hydroxyvitamin D). The NIH Office of Dietary Supplements (ODS) says a blood test is one way to know if you get enough vitamin D. ODS describes 50 nmol/L (20 ng/mL) or above as adequate for most people, below 30 nmol/L (12 ng/mL) as too low, and above 125 nmol/L (50 ng/mL) as too high and possibly harmful.
Vitamin B12. ODS states that B12 deficiency affects between 3% and 43% of older adults, and that your doctor can test your level. People over 50, vegetarians and vegans, people who have had stomach or bowel surgery, and people taking metformin or acid-reducing medicines are among those at higher risk.
Folate. Often checked together with B12, because – as ODS explains – large amounts of folate can hide a B12 deficiency.
Blood lipids, including triglycerides. ODS notes that getting more EPA and DHA from foods or supplements lowers triglyceride levels. Knowing your baseline lets your doctor see whether a change makes a difference.

Your doctor may suggest other tests, or none. That is the point: the decision is based on you, not on a label.

Omega-3 (EPA and DHA)

Who might benefit: ODS says people who eat fish and other seafood have a lower risk of several chronic diseases, but it is not clear whether the benefit comes from the foods or the omega-3s in them. The American Heart Association, as summarised by ODS, recommends one to two servings of seafood a week, and about 1 g a day of EPA plus DHA for people with heart disease, preferably from oily fish – with supplements as an option under a health care provider's guidance. The AHA does not recommend omega-3 supplements for people who are not at high risk of cardiovascular disease.

Risks and interactions: The US FDA recommends no more than 5 g a day of EPA and DHA combined from supplements. Side effects are usually mild (bad taste, heartburn, stomach upset). High doses may cause bleeding problems when taken with warfarin or other anticoagulants.

Common-sense step: eat oily fish such as salmon, herring, mackerel or sardines before you reach for capsules.

Vitamin D3

Who might benefit: ODS lists older adults, people who rarely expose their skin to sun, people with dark skin, people with conditions that limit fat absorption, and people with obesity or after gastric bypass as more likely to have trouble getting enough. D3 might raise blood levels higher and for longer than D2, and vitamin D is best absorbed with a meal that contains some fat.

Evidence: Vitamin D and calcium support bone health, and supplements slightly increase bone strength in older adults. But ODS also reports that clinical trials find vitamin D supplements do not reduce the risk of developing heart disease, do not help weight loss and do not prevent depression.

Risks: Too much vitamin D is harmful. The upper limit for adults is 100 mcg (4,000 IU) a day from all sources, unless a doctor prescribes more for a period. Very high blood levels can cause nausea, muscle weakness, confusion, kidney stones and, at extremes, kidney failure and irregular heartbeat. Vitamin D can interact with statins, steroids such as prednisone, orlistat and thiazide diuretics.

Vitamin K2

What it is: Vitamin K is needed for blood clotting and healthy bones. Menaquinone-7 (MK-7) is a common K2 form in supplements.

Evidence: ODS says vitamin K deficiency is very rare, and that some studies found K supplements improve bone strength while others did not. Whether they help prevent heart disease is still being researched.

The big interaction – warfarin: If you take warfarin (Coumadin), ODS says your vitamin K intake from food and supplements should be about the same every day. A sudden change can cause dangerous bleeding (if you consume less) or blood clots (if you consume more). Never add or stop K2 on your own if you take a blood thinner. Antibiotics, bile acid sequestrants and orlistat can also affect vitamin K.

Vitamin B12

Who might benefit: older adults (who often have less stomach acid), vegetarians and vegans, people with pernicious anaemia, coeliac or Crohn's disease, and people who take metformin or acid-reducing medicines such as omeprazole.

Evidence: B12 prevents a type of anaemia and protects nerves. ODS says B12 does not boost energy or endurance in people who already get enough, and lowering homocysteine with B vitamins has not been shown to reduce heart disease or stroke.

Risks: ODS says B12 has not been shown to cause harm, even at high doses. The bigger risk is an untreated deficiency, which can damage the nervous system.

Folate (folic acid)

Who might benefit: All women who could become pregnant should get 400 mcg of folic acid daily, according to ODS. People with alcohol use disorder or absorption problems are also at higher risk of low folate.

Risks and interactions: The adult upper limit from supplements and fortified foods is 1,000 mcg a day. Large doses can hide a B12 deficiency while nerve damage continues, which is another reason to test both. Folate interacts with methotrexate, some anti-seizure medicines and sulfasalazine.

Test, don't guess: a simple routine

Write a list of every medicine and supplement you take, including doses.
Bring it to your doctor and ask which values make sense to check.
If you start something, change one thing at a time and agree on when to re-test.
Keep your results in one place so you can see trends.

Curious about creatine, taurine, GlyNAC, vitamin C or NMN? Read our honest review of longevity supplements – what the human evidence shows, and what is still mostly animal research.

This is the human side of chemistry: not chasing perfect numbers, but making calm decisions with your doctor. Pair it with regular training – see Training – and check your fitness baseline with the biological age calculator.

FAQ

Can I order blood tests and interpret them myself? We recommend doing it through a doctor, who can interpret results in light of your health and medicines.

Is more vitamin D better? No. ODS describes levels above 125 nmol/L (50 ng/mL) as too high, and the adult upper limit is 4,000 IU a day unless a doctor advises otherwise.

Can I take K2 if I'm on warfarin? Only after talking to the doctor managing your warfarin. Changes in vitamin K intake can cause bleeding or clots.

Does B12 give more energy? Not if you already get enough, according to ODS.

Should I take omega-3 capsules? The AHA, as summarised by ODS, does not recommend them for people without high cardiovascular risk. Eating fish is the first step.

Full disclaimer

This page is general information only and is not medical, nutritional or pharmaceutical advice. It does not replace a consultation, diagnosis or treatment by a qualified health professional. Do not start, stop or change medicines or supplements without your doctor or pharmacist. Reference values differ between laboratories and countries; your doctor interprets your results. Supplements can interact with medicines and can be harmful in high doses. Sources were checked in October 2026.