NOW Foods Magnesium Ascorbate

Vitamin C· evidence: Moderate

Vitamin C (ascorbic acid, or mineral ascorbates such as sodium, calcium or magnesium ascorbate) is an essential vitamin found in fruit and vegetables. Adults need about 40 mg a day in the UK, and most people who eat fruit and vegetables get enough. The body cannot store much, and absorption falls off at higher doses, so the extra from a 1,000 mg tablet is largely passed in urine.

The best evidence on colds (a Cochrane review) found that taking vitamin C every day does not stop most people catching colds, but may shorten them slightly. People doing very heavy physical exercise are the exception, where regular vitamin C roughly halved the number of colds in trials.

A supplement makes sense if your diet is low in fruit and vegetables, if you smoke, or around periods of hard training. Most people do not need mega-doses, and fancy forms (liposomal, ester or buffered) have little evidence of meaningful benefit over plain ascorbic acid, though buffered forms can be gentler on the stomach.

Typical dose: The UK reference intake for adults is 40 mg a day (NRV on labels: 80 mg). Supplements commonly give 200 to 1,000 mg.

Safety: The FSA advises that 1,000 mg (1 g) a day or less from supplements is unlikely to cause harm. Higher doses can cause stomach pain, diarrhoea and wind. People with a history of kidney stones, haemochromatosis (iron overload) or kidney disease should speak to their GP before taking high doses. Chewable and powder forms are acidic and can affect tooth enamel.

Research we looked at:
NHS. Vitamin C. nhs.uk/conditions/vitamins-and-minerals/vitamin-c
Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev 2013;(1):CD000980.
EFSA NDA Panel. Scientific opinion on dietary reference values for vitamin C. EFSA Journal 2013;11(11):3418.
Carr AC, Vissers MCM. Synthetic or food-derived vitamin C: are they equally bioavailable? Nutrients 2013;5(11):4284-4304.

Magnesium· evidence: Moderate

Magnesium is an essential mineral found in green vegetables, nuts, seeds, wholegrains and beans. UK surveys show that some groups, particularly teenagers and some adults, eat less than the recommended amount. A supplement reliably raises intake, and people with a low intake or a diagnosed deficiency are the ones most likely to notice a difference. Most people eating a varied diet get enough from food.

Labels often show the weight of the whole compound (for example 1,000 mg of magnesium citrate), but only part of that is magnesium itself. What matters is the elemental magnesium per daily dose, compared with the NRV of 375 mg. Organic forms such as citrate, glycinate (bisglycinate), malate and taurate are generally absorbed a little better than oxide in small studies, but differences between them are modest and no form has been shown to be better for any particular use.

Magnesium is widely sold for sleep, stress and cramp. These are not authorised claims. Reviews have found the evidence for sleep is of low quality, and a Cochrane review found magnesium is unlikely to prevent ordinary muscle cramps in older adults.

Typical dose: Most supplements give 100–400 mg of elemental magnesium a day (NRV 375 mg). The NHS says adults need about 270–300 mg a day in total, mostly from food.

Safety: High doses loosen stools or cause diarrhoea, especially citrate and oxide. EFSA sets an upper level of 250 mg a day of magnesium from supplements, and the FSA advises that up to 400 mg a day from supplements is unlikely to cause harm. Magnesium can reduce absorption of some antibiotics (tetracyclines, quinolones) and osteoporosis drugs (bisphosphonates), so take them a few hours apart. Speak to your GP first if you have kidney disease.

Research we looked at:
NHS. Vitamins and minerals: others – magnesium. nhs.uk.
EFSA NDA Panel (2015). Scientific opinion on dietary reference values for magnesium. EFSA Journal 13(7):4186.
Expert Group on Vitamins and Minerals (2003). Safe upper levels for vitamins and minerals. Food Standards Agency.
Garrison SR et al. (2020). Magnesium for skeletal muscle cramps. Cochrane Database Syst Rev 9:CD009402.

Cost per daily dose lets you compare packs of different sizes fairly.Worked out from our current prices.