Lactoferrin's antibacterial action

That lactoferrin inhibits bacterial growth in culture is not in dispute. What that means for someone swallowing a capsule is a separate question, and the two are routinely merged.

Two mechanisms

The first follows directly from what the protein is for. Bacteria need iron, and most cannot obtain it unless it is available in a usable form. Lactoferrin binds ferric iron with high affinity and retains it across a wide range of acidity, including conditions where other iron carriers release their load. In a culture medium, adding lactoferrin lowers the free iron available and bacterial growth slows. The effect is bacteriostatic: growth is inhibited rather than the organism killed.

The second mechanism does not involve iron at all. The protein carries a strongly positively charged region near one end, which interacts with negatively charged components of bacterial surfaces. A fragment released when lactoferrin is cleaved by pepsin, usually called lactoferricin, is more potent in this respect than the intact protein, and can be bactericidal rather than merely inhibitory.

Both are well described. A reader looking for a mechanism has one.

What the laboratory work does not establish

Culture conditions are chosen to make effects visible. Free iron is limited, competing proteins are absent, and the concentration of lactoferrin is one the experimenter selected. None of those conditions describes a human gut.

Swallowed lactoferrin also has to survive being digested. Some fraction does, particularly in infants, whose stomach acidity and protease activity are lower. How much survives in an adult, in what form, and whether it reaches concentrations comparable to the ones used in culture, is not well characterized.

Where the human evidence went

The largest clinical test of a lactoferrin infection outcome was conducted in very preterm infants, the population where the argument for benefit was strongest and where supplementation replaces something they would otherwise receive in milk. That trial found no reduction in late-onset infection.

That result does not overturn the culture work. It illustrates the gap between the two, which is the point of grading them separately. The preterm infant page covers that trial in full.

One of the reviews cited below is titled "The Lactoferrin Phenomenon — A Miracle Molecule". It is a competent summary of the mechanistic literature. The title is also a fair illustration of the register this subject is usually written in, and of why a grade sitting above the prose is useful.