Modulation of natural killer cell activity by supplementation of fermented milk containing Lactobacillus casei in habitual smokers

Background Habitual smoking significantly reduces natural killer (NK) cell activity. To clarify whether the intake of fermented milk containing lactic acid bacteria restores NK cell activity in habitual smokers, we conducted a placebo-controlled double-blind test. Methods Ninety-nine subjects with smoking habits were randomly divided into two groups and daily for 3 weeks were given fermented milk containing Lactobacillus casei or placebo. NK cell activity in the peripheral blood mononuclear cells (PBMNC) was determined before and after the intake. Average number of cigarettes smoked and number of cigarettes that the subjects smoked before giving blood after getting up on the inspection day were asked, and these data were used to adjust the influence of smoking on NK cell activity. Results NK cell activity in individuals was inversely correlated to numbers of cigarettes smoked. Averaged NK cell activity adjusted by the numbers of cigarettes was significantly higher in individuals drinking fermented milk containing L. casei than those drinking a placebo. However, the proportion of NK cells was not different between individuals drinking either fermented milk containing L. casei or the placebo. Conclusions Intake of fermented milk containing lactic acid bacteria was considered effective for restoring the NK cell activity of habitual smokers.