Basic supplements: Vitamin D, vitamin B12 and omega-3: for whom and how much

What it is

Vitamin D, vitamin B12 and omega-3 are the three supplements people ask about most at the counter. All three have groups in whom supplementing makes sense, and none is 'for everyone'.

Low vitamin D is common in winter and in older people or those who get little sun, but guidelines do not recommend routinely supplementing the healthy population. B12 matters in older people, in vegan diets and with some medicines (metformin, long-term omeprazole). Omega-3 supplements have not been shown to prevent heart attacks in the general population; their clear indication is high triglycerides, at doses the doctor prescribes.

Three supplements, three roles

They work in very different ways:

VITAMIN D: regulates calcium and bone. Deficiency causes osteomalacia and increases fracture risk in older people; effects on mood or infections are not proven.
VITAMIN B12: needed for blood and nerves. Deficiency causes anaemia, tingling and memory problems; it is more common in older people, vegans and with metformin.
OMEGA-3 (EPA + DHA): components of cell membranes. At high doses they lower triglycerides; to prevent heart attacks in the general population, supplements have shown no clear benefit.

At the counter

When to recommend it

Vitamin D: older people, people who rarely go out or get little sun, and anyone with a measured deficiency
Oral B12 in older people, vegans or people on metformin: at high doses the oral route works
Omega-3: prioritise oily fish; high-dose supplements are for triglycerides and with the doctor
If there are symptoms (tiredness, tingling), refer for tests before supplementing

When not to

Vitamin D: hypercalcaemia, calcium kidney stones or sarcoidosis, unless the doctor advises it
Do not add up products containing vitamin D without checking the total dose: excess is toxic
Do not promise that a supplement cures an established disease

What to say at the counter. The question that frames the recommendation: 'why do you think you need it?'. If there are symptoms, test first. And for B12 in older people: the oral route at high doses works; most do not need injections.

Warnings

Important VITAMIN D: often low in winter and in older people, but supplementing the whole healthy population is not recommended. More is not better: excess causes hypercalcaemia.
Important ORAL B12: at high doses (1,000–2,000 µg a day) it is as effective as intramuscular B12 for most people, even with absorption problems. The doctor decides the route.
Caution OMEGA-3: it oxidises with light and heat; a rancid smell means it has oxidised. Keep it closed and somewhere cool.
Caution DOSE: for vitamin D, the usual supplement doses are 800–2,000 IU a day; high weekly or monthly doses are medical treatment.
Worth knowing SLOW EFFECT: levels change over weeks or months; symptoms sometimes later.

Pharmacokinetics

Absorption: Vitamin D: better with a meal containing fat. B12: at high doses (1,000 µg) a small part is absorbed without intrinsic factor, enough to correct a deficiency. Omega-3: well absorbed with food.
Distribution: Vitamin D: stored in fat tissue. B12: stored in the liver for years. Omega-3: incorporated into membranes.
Metabolism: Vitamin D: activated in the liver and kidney. B12: stores for years, which is why deficiency takes time to appear. Omega-3: oxidises if stored badly.
Elimination: Vitamin D: biliary. B12: excess is eliminated in the urine. Omega-3: used for energy.

Supplement vs treating a deficiency

Aspect	As a supplement	With a diagnosed deficiency
Vitamin D	800–2,000 IU a day in at-risk groups	Dose and schedule set by the doctor according to blood tests
B12	Low doses in vegans or older people	1,000–2,000 µg oral a day (or intramuscular if the doctor advises)
Omega-3	Better: oily fish twice a week	High doses for triglycerides, on prescription
Duration	As long as the risk factor lasts	According to the blood test results
Who decides	Pharmacist advice	The doctor

Supplementing makes sense in specific groups; treating a deficiency is the doctor's job. And in both cases, a higher dose is not better.

Self-assessment

Three questions. When you check your answers you will see the explanation for each one.

Do guidelines recommend vitamin D for the whole healthy population? Yes, almost everyone should take it No: it makes sense in specific groups (older people, little sun exposure, measured deficiency) Only in summer
An older patient takes oral B12, 1,000 µg a day. Is that enough? No, in older people only intramuscular works Yes, for most people: at high doses the oral route is as effective as intramuscular It depends on the brand
How long does supplementation take to show? 24 hours 1–2 weeks Weeks or months (6–12 weeks for levels)

Training content. It does not replace the summary of product characteristics or clinical judgement.

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