Travel Immunity Kit

Vitamin D deficiency is one of the most consistently identified modifiable risk factors for respiratory infection severity. A large meta-analysis of 25 randomized trials found supplementation reduced the risk of acute respiratory tract infection, with the largest benefit in those who were deficient at baseline. Fat-soluble — bioavailability is much higher with a fat-containing meal.^{[1, 2, 3]}

Zinc is essential for immune cell function. A meta-analysis of zinc lozenge trials found that supplementation during the early phase of a cold reduces duration by approximately one-third. Picolinate and bisglycinate forms are well-tolerated and absorbed; gluconate is the form used in most lozenge trials. Take with food — zinc on an empty stomach causes nausea reliably. Do not exceed 40 mg/day for more than 10-14 days at a time (chronic high zinc depletes copper).^{[4, 5, 6]}

The Cochrane review of vitamin C and the common cold found that regular supplementation does not reduce cold incidence in the general population but does modestly reduce duration (about 8% in adults) and severity. Effect size is small but the safety margin is wide. Higher doses (over 1 g) can cause loose stools — split into morning and afternoon doses if you want to take more.^{[7, 8]}

Quercetin is a flavonoid found in onions, apples, and capers. In vitro studies show antiviral and anti-inflammatory activity; small human trials suggest modest reduction in upper respiratory infection severity in physically active adults. Often co-formulated with bromelain or vitamin C to improve bioavailability. Treat this as the most speculative item in the stack — the supporting human data is thinner than for the other three.^{[9, 10, 11]}