Synbiotic

What is a synbiotic?

The word was coined in 1995 for products combining a probiotic and a prebiotic. In 2020 an ISAPP panel (Swanson et al., Nat Rev Gastroenterol Hepatol) redefined it as a mixture of live microorganisms and substrates selectively utilised by host microorganisms that confers a health benefit on the host.

The revised definition contains an important nuance: it does not require each component to meet probiotic and prebiotic criteria on its own. What must be demonstrated is a benefit from the mixture. The substrate may also feed either the administered microbes or those already resident in the person’s microbiota.

Types of synbiotic

The consensus describes two categories:

• Complementary. A probiotic and a prebiotic acting independently. Each must meet its own evidence and dose requirements, and the combination must show a benefit in the host.
• Synergistic. The substrate is designed to be selectively used by the co-administered microorganisms. Here the components need not prove benefit separately; the mixture is tested in a controlled trial.

Mixtures containing inactivated microbes fall outside the term; they sit closer to the concept of a postbiotic.

What the evidence says

Trials of specific combinations exist in areas such as digestive health, neonatal infection prevention in some settings and the vaginal microbiota, with variable results. Comparisons are difficult because each product combines different strains, substrates and doses. The panel recommends study designs that test the mixture against its components whenever synergy is claimed. Outside the combinations actually studied, benefit cannot be assumed.

Example

A gynaecologist reads a randomised placebo-controlled trial of a vaginal synbiotic built on Lactobacillus crispatus strains and a supporting substrate. Before considering it, she checks what the trial measured (microbiota composition or symptoms), how long follow-up lasted and whether the participants resemble her patients. She concludes that the findings apply to that formulation, not to any product containing Lactobacillus.

Synbiotics in clinical practice

Xenogene does not sell synbiotics or recommend products. Microbiota tests such as the gut microbiota analysis or the vaginal microbiota analysis describe the composition of the ecosystem and can give the clinician context, but they do not indicate which synbiotic to use or predict a response. The clinical decision rests with the healthcare professional. The blog discusses the L. crispatus trial in its article on the vaginal microbiota.