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Do Multivitamins Actually Work?

Quick answer: For the big promise, preventing heart disease, cancer and early death, the large trials mostly found little or nothing. The US Preventive Services Task Force says the evidence is insufficient to weigh benefits against harms (USPSTF 2022). Using one to cover gaps in a restricted diet, like keto, is a different and smaller question, and that's the way KetoDick has talked about them (video lbRPlGZ3Rvw).

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"Work" for what?

Half the confusion about multivitamins comes from asking "do they work?" without saying for what. There are really two questions.

  1. Prevention: does a daily multivitamin keep a reasonably fed adult from getting heart disease, cancer or dementia, or from dying early?
  2. Gap-filling: if your diet is short on certain nutrients, does a multivitamin help close the gap?

The research on the first is big and mostly disappointing. The second is more modest and makes more sense, especially on a diet that cuts out whole food groups. Keep them apart. That's the same move KetoDick makes when he separates "good for health" from "needed for keto" (video DWI8qQl5BbY).

Heart disease and death

The biggest long-running trial is the Physicians' Health Study II: 14,641 male doctors aged 50 and up, followed for a median of 11.2 years. A daily multivitamin did not reduce major cardiovascular events; the hazard ratio was 1.01, which is to say no difference (Sesso 2012).

Meta-analyses point the same way. One found multivitamins, vitamin D, calcium and vitamin C showed no consistent benefit for cardiovascular disease or death (Jenkins 2018). An umbrella review of 277 trials found most supplements, multivitamins included, had no significant effect on mortality or cardiovascular outcomes (Khan 2019).

An observational study of 38,772 older women even found multivitamin use associated with slightly higher total mortality, a hazard ratio of 1.06 (Mursu 2011). That's association, not cause; people who take vitamins differ from people who don't in lots of ways. See Observational vs RCT.

Cancer

Here there's a small signal. In the same Physicians' Health Study II, a daily multivitamin gave a modest reduction in total cancer, a hazard ratio of 0.92 (Gaziano 2012).

Back-of-envelope: 0.92 means roughly 8% fewer cancers, relative to placebo. That's a relative number. Without the base rate, it doesn't tell you how many people per thousand that is, which is the number that matters to you. See Relative vs Absolute Risk.

The review behind the 2022 Task Force statement found multivitamins associated with a small reduction in cancer incidence and no clear effect on cardiovascular disease or mortality (O'Connor 2022). And note who was studied in PHS II: male doctors aged 50 and up. That's not the women 30 to 60 reading this.

Memory and thinking

Mixed. A Physicians' Health Study II sub-study of 5,947 men aged 65 and up, followed about 12 years, found no difference in cognitive performance or decline versus placebo (Grodstein 2013).

Then COSMOS-Mind, 2,262 older adults over 3 years, found a daily multivitamin improved global cognition compared with placebo (Baker 2023). The two trials share some investigators. One positive trial against an older null one is a reason to keep watching, not a reason to promise anything. More in The Multivitamin Memory Study.

Covering gaps: the smaller question

Now the question that actually applies to keto.

Restricted diets can come up short. In a trial of women on different diets, the Atkins group shifted toward inadequate intake of thiamine, folate, vitamin C, iron and magnesium at eight weeks; the other diets had their own shortfalls (Gardner 2010). A menu-modelling study of four popular plans, one of them Atkins, found none met the reference intake for all 27 micronutrients from food alone (Calton 2010).

That's the case for a multivitamin as a backstop. It isn't proof one fixes the gap; those studies measured diets, not pills. The reference bodies put food first: the US Dietary Guidelines say needs should be met mainly from foods and drinks, with supplements useful where needs are otherwise hard to meet (Dietary Guidelines). Which shortfalls to watch is covered in Nutrient Shortfalls on Low-Carb Diets.

More isn't better

Two cautions from the literature.

First, the NIH Office of Dietary Supplements notes that multivitamins can push some intakes above upper limits, especially when you stack single-nutrient supplements on top (NIH ODS). That line was read from search snippets, not the page itself, so check it before quoting.

Second, high-dose antioxidant pills, which aren't the same as multivitamins, have a bad record. A meta-analysis of trials found beta-carotene, vitamin A and vitamin E supplements associated with higher mortality (Bjelakovic 2007), and the Task Force recommends against beta-carotene and vitamin E for prevention (USPSTF 2022).

What the research says

KetoDick's take

He has suggested a multivitamin in more than one video, and always as a backstop, not a cure. In 2017 his order was leafy greens for calcium and magnesium, salt and light salt for sodium and potassium, and a one-a-day multivitamin to help get you to acceptable levels, especially if you won't drink pickle juice (video lbRPlGZ3Rvw). His close: salt your food and maybe consider a multivitamin to cover your bases (from the video lbRPlGZ3Rvw). In 2018 he suggested a common multivitamin with magnesium and calcium, with no brand push (video UygKGoP_BtA).

That lines up with the gap-filling question; in these videos he doesn't pitch it as disease prevention. Where the literature adds something he didn't say: the gap-filling case rests on diet studies, not on trials showing the pill closes the gap. And he pushes back on specialty products marketed as things keto requires (video DWI8qQl5BbY), which fits the prevention data too.

If you take other medications or single-nutrient supplements, run the list past your doctor or pharmacist so nothing stacks over an upper limit.

Key takeaways