Rare sugars

Rare sugars are monosaccharides that occur in nature only in trace amounts. Two matter commercially: D-allulose (also called D-psicose), the C-3 epimer of D-fructose, and D-tagatose, the C-4 epimer of D-galactose. Their interest is that they are genuine sugars — with sugar’s chemistry — that the human body largely does not use for energy.

Made by moving one hydroxyl

Both are produced enzymatically from an abundant sugar by inverting the configuration at a single carbon. Allulose is made from fructose by a ketose 3-epimerase; tagatose from galactose by an isomerase, or from other starting points by multi-enzyme cascades.

Because an epimerisation is a reversible isomerisation rather than a synthesis, the reaction reaches an equilibrium that favours the starting sugar. The practical consequences are structural: conversion per pass is modest, the product must be separated chromatographically from a large excess of unreacted substrate, and the substrate is recycled. Yield economics are set by that separation, not by the enzyme.

Absorbed, then discarded

Allulose is taken up from the small intestine by the same transporters that carry fructose, so it enters the bloodstream. What it does not do is get metabolised: humans lack an efficient route to phosphorylate and catabolise it, so most of what is absorbed is filtered by the kidney and excreted unchanged in urine. Energy yield is therefore very low — conventionally treated as a small fraction of a kilocalorie per gram rather than four.

This mechanism explains a second property. Because most of it is absorbed rather than reaching the colon, allulose causes less gastrointestinal disturbance at a given dose than sugar alcohols, which are poorly absorbed and osmotically active in the gut. The tolerance limit is higher, but it is not absent: above a threshold, unabsorbed sugar draws water into the lumen and causes laxation, and that threshold varies between people.

Tagatose behaves differently — it is only partly absorbed and the remainder is fermented in the colon, giving it a lower tolerance ceiling and a modest prebiotic character.

Why they behave like sugar in a recipe

This is the property that distinguishes rare sugars from high-intensity sweeteners. They are reducing sugars with free carbonyl groups, so they participate in the Maillard reaction with amino acids on heating — in fact allulose browns more readily than sucrose. They contribute bulk and mass, depress freezing point, and lower water activity.

A baked or browned product reformulated with allulose therefore still browns, still has body, and still has the mouthfeel that comes from dissolved solids. That is precisely what a high-intensity sweetener cannot provide, and it is why these ingredients occupy a different formulation role rather than competing directly.

The trade-offs are real: they are roughly seventy and ninety percent as sweet as sucrose, so they are not more potent, and faster browning can mean over-colouring in a recipe designed for sucrose.

The honest caveats

Metabolic-health claims — effects on postprandial glucose and on liver fat — rest largely on small human studies and animal work, and should not be presented as established. Regulatory status also diverges internationally: authorisation and the way these sugars are counted in nutrition labelling differ between jurisdictions, so a claim that is lawful in one market may not be in another.