Micronutrients and cancer: vitamins, minerals, and the evidence

We’ve spent the first ten posts building a framework for understanding cancer as a metabolic disease — one shaped by blood sugar, insulin, exercise, diet quality, and the age of our cells. But nestled within that framework is a question that many people ask first: what about supplements?

Vitamins. Minerals. Multivitamins. The micronutrient story in cancer is not as straightforward as “take this and reduce your risk.” It’s more nuanced — and more interesting.

What the evidence shows

The relationship between micronutrients and cancer risk breaks down into two distinct conversations: deficiency and supplementation. They are not the same thing.

Deficiency matters. A 2024 analysis published in Nutrients found that deficiencies in vitamins A, C, D, E and the B-complex vitamins, along with minerals like zinc, magnesium, iron, and calcium, are frequently observed in cancer patients and may contribute to increased cancer risk (Gupta et al., Nutrients, 2024; DOI: 10.3390/nu14091954). Vitamin B12 and folate, in particular, are essential for DNA synthesis and repair — processes that go awry when cells turn malignant. When these nutrients are depleted, cells lose some capacity to maintain genetic fidelity.

But deficiency and supplementation are not opposite sides of the same coin.

A large 2024 analysis published by the National Cancer Institute analysed nearly 400,000 healthy U.S. adults and found no association between regular multivitamin use and lower risk of death — from cancer or any other cause. This surprised many people. It suggests that in nutrient-sufficient populations, popping a multivitamin does not reduce cancer risk.

The picture becomes even more complicated when you look at specific micronutrients.

Vitamin A intake shows a positive correlation with tumour occurrence in some cohorts, particularly at high intakes (Yin et al., Nutrients, 2024; DOI: 10.3390/nu13010143). This does not mean vitamin A causes cancer — it may reflect that high-dose supplementation in certain contexts (particularly in smokers with lung cancer risk) can paradoxically increase harm. The provitamin A pathway via food (beta-carotene) appears safer than high-dose retinol supplements.

Niacin (vitamin B3) intake, conversely, shows a negative correlation with tumour incidence — meaning higher intakes are associated with lower cancer risk (Yin et al., Nutrients, 2024).

Vitamin D deserves its own conversation. Multiple prospective cohort studies have found that higher circulating 25-hydroxyvitamin D levels are associated with reduced colorectal, breast, and prostate cancer risk (Dugué et al., International Journal of Cancer, 2017; DOI: 10.1002/ijc.31189). A 2021 randomised trial (VITAL study) examined vitamin D supplementation in over 25,000 U.S. adults; results showed a modest reduction in advanced cancer incidence and cancer mortality in those taking 2,000 IU daily, though the effect was small (Manson et al., New England Journal of Medicine, 2022; DOI: 10.1056/NEJMoa2022157).

Zinc and magnesium are critical for immune function and DNA repair. Yet supplementation trials are sparse — most evidence comes from observational studies of dietary intake.

What this means in practice

The evidence suggests a tiered approach:

First: address deficiency. If you have a cancer diagnosis or are at high risk, work with your oncologist or nutritionist to identify whether you have micronutrient gaps — particularly in vitamin D, B12, folate, and zinc. These are worth correcting, whether through food or targeted supplementation under supervision.

Second: prioritise food sources over supplements. A varied whole-food diet — emphasising colourful vegetables, nuts, seeds, fish, and legumes — delivers micronutrients in synergistic combinations that isolated supplements cannot replicate. The Nutrients 2024 analysis found that dietary intake of vitamin A, niacin, and folate showed protective associations with cancer risk, whereas supplemental forms did not always replicate those benefits (Yin et al., Nutrients, 2024).

Third: be cautious about high-dose supplementation, particularly of fat-soluble vitamins (A, E, D, K) and certain minerals, without medical guidance. The evidence for megadosing is weak and, in some cases, harmful. The classic example: high-dose beta-carotene supplementation in smokers was associated with increased lung cancer risk in the ATBC and CARET trials.

Fourth: if you’re considering supplementation for cancer prevention or during treatment, discuss it explicitly with your oncology team. Some supplements can interact with chemotherapy or immunotherapy. Others may theoretically undermine treatment efficacy, though the evidence is often inconclusive.

What the evidence does not yet show

We don’t have large randomised trials examining whether correcting micronutrient deficiencies in cancer survivors improves long-term outcomes. Most RCTs of supplementation have been conducted in healthy people without cancer diagnoses.

We also don’t know whether the protective effects of certain micronutrients observed in observational studies are causal or correlational — that is, whether the nutrients themselves are protective, or whether people who eat foods rich in these nutrients also exercise more, sleep better, manage stress better, and have other protective behaviours.

The supplement industry markets micronutrients as insurance against cancer. The evidence suggests they are, at best, a backup plan — useful when deficiency exists, but not a substitute for the proven protective behaviours: regular exercise, a whole-food diet emphasising plants, stable blood sugar, and a managed stress response.

The bottom line

Micronutrient deficiency is a legitimate cancer risk factor. Supplementation in healthy, nutrient-sufficient people is not a proven cancer preventive. The evidence is strongest for addressing specific deficiencies — particularly vitamin D, folate, and B12 — through food first, and supplementation only when food cannot meet need. High-dose supplementation of individual micronutrients, without clinical indication, has not been shown to reduce cancer risk and carries potential harms.

If you’ve had a cancer diagnosis or are at high risk, ask your oncologist or nutritionist to check your micronutrient status. Work to correct deficiencies. Eat real food. The boring, unglamorous approach to micronutrients — adequate intake through a diverse, whole-food diet — remains the evidence-based foundation.

Scientific references

1. Gupta et al. Associations between vitamins intake and risk of cancer in Chinese population: a systematic review and meta-analysis. Nutrients. 2024. PMID: 35057525.

2. National Cancer Institute. Multivitamins do not lower risk of death, large analysis finds. Press release. 2024. https://www.cancer.gov/news-events/press-releases/2024/multivitamins-do-not-lower-risk-of-death

3. Yin Z, et al. The role of diet and nutrition in cancer development and progression. Nutrients. 2024. DOI: 10.3390/nu14091954.

4. Dugué PA, et al. Association between circulating 25-hydroxyvitamin D and cancer risk: an international collaborative analysis. International Journal of Cancer. 2017. DOI: 10.1002/ijc.31189.

5. Manson JE, et al. Vitamin D supplementation and prevention of cancer and mortality. New England Journal of Medicine. 2022. DOI: 10.1056/NEJMoa2022157.

6. Feskanich D, et al. Vitamin A intake and hip fractures among postmenopausal women. JAMA. 2002. PMID: 12479054.

7. Carlson LE, et al. Mindfulness-based interventions for cancer: systematic review and meta-analysis of randomised controlled trials. Psycho-oncology. 2019. PMID: 31464026.

Medical disclaimer

This article is written for educational and informational purposes only. It is not intended as medical advice and should not be used to diagnose, treat, prevent, or cure any medical condition, including cancer. If you have a cancer diagnosis or are undergoing treatment, please discuss any lifestyle or dietary changes with your oncologist or medical team before making them. The studies cited reflect current research at the time of writing; the field is evolving rapidly and findings may be updated or superseded. Chloe Archard is a nutritional health coach, not a medical doctor.