An experimental beverage containing heat-killed Levilactobacillus brevis KB290 and beta-carotene, a pigment which the body converts into vitamin A, was shown in a recent large-scale clinical study to substantially reduce rates of influenza infection during the winter season in people under the age of 40. The results, which appeared in the journal Nutrients, concluded that younger participants who took the supplement saw a 52% lower incidence rate of flu infection compared to an equivalent placebo group.
 
Vitamin A has been shown to benefit the immune system and immune function, particularly in regard to protective effects against flu infection. Likewise, lactic acid bacteria have been thoroughly investigated and reported to have beneficial immunomodulatory effects on the respiratory tract. However, the combination of lactic acid bacteria and beta-carotene have very rarely been investigated in human clinical studies.
 
In the present study, the authors recruited 2,200 healthy Japanese subjects; half were assigned to a placebo group, and half were assigned to a group which was administered a daily supplement beverage containing heat-killed KB290 and 7.4-12.4 mg of beta-carotene. The participants of the study were asked to maintain a number of lifestyle factors throughout the duration of the study, and, over 12 weeks, reported to the researchers if they developed a fever and had to seek medical attention (or received a flu diagnosis), refrained from taking pharmaceuticals, supplements, and a number of functional foods. Temperatures were taken daily, and each subject recorded all degrees of symptoms, including sore throat, cough, headache, and abdominal pain.
 
While in the subgroup of participants older than 40 saw no differences regardless of whether they took the supplement or the placebo, a substantial difference was achieved in participants under the age of 40. In the experimental <40 subgroup, subjects had an influenza incidence rate of 1.9%, compared to the placebo <40 subgroup which saw an incidence rate of 3.9%. While it was anticipated that placebo groups would achieve a flu rate of 5% over the study duration, social distancing and other measures taken against the spread of COVID-19 likely resulted in a significantly lower flu rate among the general population. The flu incidence rates were calculated after researchers determined whether fever and other flu-like symptoms were actually caused by flu or by other ailments, such as common colds, gastroenteritis, joint pains, mumps, or for reasons that are unexplained.
 
“The strength of the present trial lies in using a large-scale randomized controlled design in healthy adults,” the authors of the study noted. “Previous studies focusing on the preventive effects of food components on influenza have methodological limitations, including inadequate study design, limited diversity of the subjects, and small-scale size. Despite subdivisions of the study population into two groups according to age, each group still had more than 500 subjects, eliminating the risk of random effects unrelated to the intervention.”