Table 1.
Characteristics of included studies
| Author, Year [Ref.] | Reported Study Design | Country of study; follow-up duration | Population size and description (% Men); Mean Age (years); | Micronutrient in question | Cut-offs for different micronutrient categories, as defined by study (nmol/L) | Outcome reported | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| Deficient | Inadequate | Sufficient | Optimal | |||||||
| Met inclusion criteria clearly | ||||||||||
| Laaksi, 2007 [19] | Prospective cohort | Finland; 6 months | 756 Young Finnish military conscripts (100%); 18–29* | Vitamin D | <40 | – | ≥40 | – | – |
• Clinical RTI incidence • RTI duration |
| He, 2013 [17] | Prospective cohort | UK; 4 months | 225 healthy endurance exercise athletes with ≥3 h of total moderate/high-intensity training per week (69.8%); 21 | Vitamin D | 12 to 30 | 30 to 50 | 50 to 120 | ≥120 | – |
• Self-reported upper RTI incidence • Self-reported upper RTI duration • Subjective symptom severity per episode |
| Jovanonich, 2014 [18] | Retrospective case-control | US; 3–15 months | 132 patients hospitalised for pneumonia (cases) or other reasons (controls) (29%); 60 | Vitamin D | < 37 | 37 to 50 | 51 to 75 | > 75 | – | • Laboratory-confirmed pneumonia incidence |
| Rafiq, 2018 [20] | Cross sectional | Netherlands; 1 month prior to interview | 6138 men and women living in the greater area of Leiden with self-reported BMI ≥27 kg/m2 (43.9%); 55 (deficient), 55.9 (inadequate), 55.7 (sufficient) | Vitamin D | < 50 | 50 to 75 | ≥75 | – | – | • Self-reported common cold incidence |
| Highly likely to meet inclusion criteria | ||||||||||
| Sabetta, 2010 [24] | Prospective cohort | US; 4 months | 198 healthy adults (42.9%); 20–88* | Vitamin D | < 95 | – | ≥95 | – | – |
• Clinical acute viral RTI incidence • Acute viral RTI duration |
| Berry, 2011 [21] | Cross sectional | UK; 3 weeks prior to interview | 6789 non-pregnant free-living Caucasians born in England, Scotland and Wales during a week in March 1958 (birth cohort) (49.9%); 45 | Vitamin D | < 25 | 25 to 49.9 | 50 to 74.9 | 75 to 99.9 | ≥100 | • Self-reported influenza, pneumonia, bronchitis or severe cold incidence |
| Nanri, 2017 [23] | Nested Case-control | Japan; 6 months | 532 employees from 4 companies in Kanto and Tokai regions (83.1%); 37.6 | Vitamin D | < 50 | 50 to < 75 | ≥75 | – | – | • Clinical influenza incidence |
| Lee, 2018 [22] | Retrospective cross sectional | US; 1 year post-vaccination | 437 Young, healthy US military members vaccinated with 2009 monovalent influenza A (H1N1) vaccine (50.3%); Not reported by 90.6% of the population were 19–39 years old | Vitamin D | < 50 | 50 to 75 | > 75 | – | – | • Clinical influenza-like illness incidence |
RTI Respiratory tract infections, UK United Kingdom, US United States of America, *age range