What do studies actually say about supplements?

Key Findings

• The Women's Health Initiative (40,000+ women) found calcium + vitamin D reduced hip fracture risk by about 21%, but only in women over 60 who took the supplements consistently.
• A 2010 BMJ meta-analysis of 19 trials found a 12% reduction in fractures overall, with stronger effects for people over 70 and those with low dietary calcium.
• Several large studies found no meaningful heart protection from calcium supplements. Some data suggests high-dose calcium without vitamin D may slightly increase cardiovascular risk.
• The US Preventive Services Task Force (2023) concluded evidence is insufficient to recommend calcium supplements for fracture prevention in community-dwelling adults.

Practical Takeaways

• Food sources (dairy, leafy greens, fortified foods) are preferred over pills when possible.
• If supplementing, doses of 500–600 mg at a time are better absorbed than single large doses.
• Pairing with vitamin D improves absorption. Taking calcium with food reduces side effects.
• People over 60 with low dietary intake are most likely to benefit.

Study Quality Notes

Most evidence comes from large randomized trials, but adherence was often poor. Results are strongest for women over 60. Evidence for men is thinner. Many studies used calcium carbonate, which is harder to absorb than calcium citrate.

Key Findings

• The VITAL trial (25,000+ adults) found vitamin D did not significantly reduce cancer or heart disease risk in generally healthy adults, but did show a 25% reduction in cancer mortality when the first two years of data were excluded.
• A 2014 BMJ meta-analysis found vitamin D supplementation reduced overall mortality by about 6% in older adults, but only in those who were deficient.
• For bone health, vitamin D + calcium together reduce fracture risk more than either alone, especially in people with documented deficiency or osteoporosis.
• Studies on vitamin D for cold and flu prevention show modest benefit. A 2017 BMJ meta-analysis found a 12% reduction in respiratory infections, strongest in people with low baseline levels.

Practical Takeaways

• Testing your 25(OH)D blood level is the best way to know if you need a supplement. Levels below 20 ng/mL are generally considered deficient.
• Daily doses of 1,000–2,000 IU are common for maintenance. Higher doses should be guided by a doctor.
• Vitamin D is fat-soluble. Take it with a meal containing fat for better absorption.
• People with limited sun exposure, darker skin, or obesity are more likely to be deficient.

Study Quality Notes

The VITAL trial was well-designed but studied mostly healthy, non-deficient adults. Results may not apply to people with known deficiency. Many earlier studies were observational, which can show associations but not prove cause and effect.

Key Findings

• A 2016 Hypertension meta-analysis of 34 trials found magnesium supplementation reduced systolic blood pressure by about 2 mmHg on average, with larger effects in people who were deficient.
• For migraines, a 2021 review found 600 mg daily of magnesium reduced migraine frequency by about 1.5 attacks per month compared to placebo.
• Sleep studies are smaller but promising. A 2012 double-blind trial in older adults found 500 mg magnesium improved sleep time and melatonin levels.
• About 50% of US adults consume less than the recommended daily magnesium, making deficiency common but often undiagnosed.

Practical Takeaways

• Magnesium glycinate and citrate are better absorbed and gentler on the stomach than magnesium oxide.
• Taking magnesium in the evening may support sleep for some people.
• Food sources include nuts, seeds, beans, and whole grains. Most people can benefit from eating more of these.
• High doses can cause diarrhea. Start with 200 mg and increase gradually.

Study Quality Notes

Blood pressure evidence is from many small trials. Migraine studies are moderate quality. Sleep studies are limited by small sample sizes. Most research uses supplemental doses of 300–600 mg daily.

Key Findings

• A 2017 Open Forum Infectious Diseases meta-analysis of 7 trials found zinc lozenges reduced cold duration by about 33% (roughly 2.5 days shorter) when started within 24 hours of symptoms.
• Zinc acetate and zinc gluconate lozenges appear most effective. Nasal zinc gels were linked to loss of smell and are no longer recommended.
• For general immune prevention, evidence is weak. A 2021 BMJ review found zinc did not prevent colds in healthy adults, only shortened them.
• Long-term use above 40 mg/day can deplete copper levels, leading to anemia and nerve problems. This is a well-documented risk.

Practical Takeaways

• Use zinc lozenges at the first sign of a cold. The benefit is in early treatment, not daily prevention.
• Do not use intranasal zinc products. The risk of smell damage is real.
• Food sources (meat, shellfish, legumes, seeds) are safer for daily intake than high-dose supplements.
• If taking zinc long-term, consider a product with added copper or alternate days.

Study Quality Notes

Cold duration studies are generally well-designed but vary in zinc formulation and dose. The 33% reduction is an average. Some people see no benefit. Prevention studies are fewer and less convincing.

Key Findings

• A 2013 PLoS One meta-analysis of 19 trials found melatonin reduced jet lag symptoms and improved sleep quality after eastward flights, with effects seen at doses of 0.5–5 mg.
• For general insomnia, a 2013 review found melatonin reduced time to fall asleep by about 7 minutes on average. This is statistically significant but may not feel meaningful.
• In children with ADHD or autism, melatonin has stronger evidence for improving sleep onset. Several pediatric guidelines support its use under medical supervision.
• Many commercial melatonin products contain 3–10 mg, but studies showing benefit often use 0.3–1 mg. Higher doses can cause next-day grogginess.

Practical Takeaways

• Start with the lowest dose (0.5–1 mg) 30–60 minutes before bed. Increase only if needed.
• For jet lag, start taking melatonin at the destination bedtime on arrival day.
• Melatonin is a hormone, not an herb. Quality varies widely between brands. Look for USP-verified products.
• It works best for sleep timing problems, not for staying asleep through the night.

Study Quality Notes

Jet lag studies are generally strong. Insomnia studies show small effects. Product quality is a real concern. A 2017 Canadian study found melatonin content in supplements varied from 83% less to 478% more than the label claimed.