Some honeys are 100 times more powerfully antimicrobial than others. The possible antimicrobials include:

• Hydrogen peroxide, produced if added water activates the enzyme glucose oxidase. It accounts for most of the antibacterial activity of most honeys.

• Flavonoids in particular as well as pinocembrin.

• Phenolic acids such as caffeic and ferulic acids.

• Methylglyoxal, present in certain manuka and jellybush honeys in levels up to 1,000 times higher than in other honeys, and doubled in antibacterial activity by an unknown synergist in these honeys.

• Sugars.

• Furanones.

• Defensin-1, apidaecins and abaecin, which are proteins that bees add to honey.

Methylglyoxal – a byproduct of sugar metabolism that is present in many foods and drinks and is also produced by gut bacteria. Methylglyoxal in manuka honey is dubbed 'unique manuka factor' (UMF). In jellybush honey it's called 'unique leptospermum factor' (ULF) as 'UMF' is trademarked. Some jellybush honeys contain more methylglyoxal than the richest manuka honeys.

Medical-grade honey – is heated only minimally, then passed through a fine filter and gamma-irradiated to kill microorganisms. It's checked for pesticide residues and heavy metals and sold in dark-glass containers so that light cannot destroy its antimicrobials. Some honeys, including certain manuka honeys, are produced as medical-grade. Medical-grade honey is also used to make certain medical honey-containing products such as honey-impregnated wound dressings.

Health benefits

Honey's antimicrobials inactivate or kill more than 250 strains of bacteria and certain fungi, including Candida albicans and viruses. Honey's anti-infective power depends on its nectar and honeydew sources and its processing.

Because honey contains hundreds of antibacterial compounds, bacteria are highly unlikely to become resistant. The compounds are quickly absorbed into the blood, so don't destroy 'good' bowel bacteria.

Hydrogen peroxide – is effective even though present in a 1,000-fold smaller amount than in a typical hydrogen-peroxide wound disinfectant. It has a valuable slow-release action. It also stimulates white cells that boost immunity.

Furanones – act against biofilms, slimy sheets of bacteria that are much more resistant to antibiotics and antiseptics than free bacteria.

Methylglyoxal – Certain honeys have a bioactivity rating according to the antibacterial strength of their methylglyoxal. Manuka honey can have a UMF (unique manuka factor) rating, and jellybush honey a ULF (unique leptospermum factor) rating. A rating of 5, for example, means it's as strong as a 5 per cent solution of phenol, an antiseptic. A rating of 0–4 indicates undetectable methylglyoxal; 5–9, low levels; 10–15, therapeutically useful levels; and 16–30, high potency.

Manuka honey with a UMF or ULF of 10 or more can act against biofilms and skin infections with antibiotic-resistant bacteria such as MRSA (methicillin-resistant Staphylococcus aureus). It doesn't have this action in blood.

The term 'Active Manuka Honey' was coined for manuka honeys with antibacterial activity from methylglyoxal rather than hydrogen peroxide. But some honeys are now labelled 'Active Manuka Honey' even though their antibacterial activity results mainly, as in most honeys, from their hydrogen peroxide.

In the test tube, methylglyoxal can damage genes and cells. Large amounts have been linked with premature ageing, cancer and reduced efficacy of insulin, the hormone that lets blood glucose enter cells.

Methylglyoxal is safe for healthy people as their cells break down much of it. But this may not happen in people with diabetes, so researchers are concerned it may then do damage by attaching to certain body proteins. Damage to insulin could worsen diabetes, while damage to certain other proteins could encourage Alzheimer's and Parkinson's. Until we know more, it's probably better for people with diabetes and, perhaps, pre-diabetes too to avoid consuming manuka or jellybush honey. Interestingly, the diabetes drug metformin was designed to reduce methylglyoxal's insulin-damaging effects.

Sugars – attract water, thus dehydrating and deactivating local bacteria and fungi.

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