Table 1.

Summary of selected recent meta-analyses of micronutrients and respiratory infections

Micronutrient	Authors	Sample size	Main findings	Stated conclusion in abstract
Vitamin A	Imdad et al 57	47 RCTs (1 223 856 children)	Vitamin A did not affect incidence of, or mortality from, respiratory disease; Note: vitamin A decreased all cause mortality and mortality from diarrhoea and decreased incidence of diarrhoea and measles	Vitamin A supplementation is associated with a clinically meaningful reduction in morbidity and mortality in children.
Vitamin A	Hu et al 58	15 RCTs (3021 children)	Vitamin A did not affect mortality of children with pneumonia. Vitamin A decreased pneumonia morbidity, increased the clinical response rate, shortened clearance time of signs and shortened length of hospital stay.	Vitamin A supplementation helps to relieve clinical symptoms and signs (of pneumonia) and shorten the length of hospital stay.
Vitamin C	Hemila and Louhiala65	3 prophylactic trials (2335 participants) two therapeutic trials (197 patients)	All three trials found vitamin C decreased the incidence of pneumonia. One trial found vitamin C decreased severity and mortality from pneumonia; the other trial found vitamin C shortened duration of pneumonia.
Vitamin C	Hemila and Chalker66	29 prophylactic RCTs investigating incidence (11 306 participants) 31 prophylactic RCTs investigating duration (9745 episodes)	Vitamin C did not affect incidence of the common cold in the general population (24 RCTs) but decreased incidence in people under heavy short-term physical stress (5 RCTs). Vitamin C shortened duration of common cold in all studies (31 RCTs), in adults (13 RCTs) and in children (10 RCTs) and decreased severity of colds.
Vitamin D	Bergman et al 86	11 RCTs (5660 participants)	Vitamin D decreased the risk of respiratory tract infections.	Vitamin D has a positive effect against respiratory tract infections and dosing once daily seems most effective.
Vitamin D	Martineau et al 87	25 RCTs (11 321 participants)	Vitamin D decreased the risk of acute respiratory tract infection, effects greater in those with low starting status	Vitamin D supplementation was safe and it protected against respiratory tract infection.
Vitamin D	Pham et al 88	24 studies; 14 included in meta-analysis of risk of acute respiratory tract infections and 5 in the meta-analysis of severity	Serum vitamin D was inversely associated with risk and severity of acute respiratory tract infections.	There is an inverse non-linear association between 25-hydroxyvitamin D concentration and acute respiratory tract infection.
Vitamin D	Zhou et al 89	8 observational studies (20 966 participants)	Participants with vitamin D deficiency had increased risk of community-acquired pneumonia.	(There is] an association between vitamin D deficiency and increased risk of community-acquired pneumonia.
Zinc, copper and iron	Mao et al 122	13 studies in Chinese children	Children with recurrent respiratory tract infection had lower hair levels of zinc, copper and iron.	The deficiency of zinc, copper and iron may be a contributing factor for the susceptibility of recurrent respiratory tract infection in Chinese children.
Zinc	Hemila123	7 RCTs (575 participants)	Zinc shortened duration of common cold.
Zinc	Science et al 124	17 RCTs (2121 adults and children)	Zinc decreased duration of common cold symptoms overall and in adults but not in children.	Oral zinc formulations may shorten the duration of symptoms of the common cold.
Zinc	Lassi et al 125	6 RCTs (5193 children)	Zinc decreased incidence of pneumonia. Zinc decreased prevalence of pneumonia.	Zinc supplementation in children is associated with a reduction in the incidence and prevalence of pneumonia.
Zinc	Wang and Song126	6 RCTs (2216 adults with severe pneumonia)	Zinc given as an adjunct therapy decreased mortality. No effect of zinc on treatment failure or antibiotic treatment.	Zinc given as an adjunct to the treatment of severe pneumonia is effective in reducing mortality.

RCT, randomised controlled trial.