Introduction
What are common sugars – and why are they a concern?
What are rare sugars?
How do rare sugars influence glucose levels?
Rare sugars vs. regular sugars: Key metabolic differences
Could rare sugars improve metabolic health?
Are rare sugars safe?
Practical challenges of using rare sugars daily
The future of rare sugar research
References
Further reading
Rare sugars may offer a smarter way to sweeten foods, but their real promise depends on dose, sugar type, and stronger long-term human evidence.
Image Credit: Erhan Inga / Shutterstock.com
Introduction
The rising prevalence of obesity and type 2 diabetes throughout the world is largely attributed to the excessive intake of refined carbohydrates and added sugars. Common sugars such as sucrose, fructose, and high-fructose corn syrup have been specifically implicated in adverse cardiometabolic outcomes. In response to this growing public health burden, researchers and food manufacturers are increasingly exploring alternative sweeteners, such as rare sugars, that may offer similar sweetness with a lower metabolic burden.3
What are common sugars – and why are they a concern?
‘Normal’ sugars like glucose, fructose, and sucrose are easily digested and absorbed in the gastrointestinal tract, which, following ingestion, can cause a sudden rise in blood sugar and insulin levels. Dietary patterns that are high in sugar are associated with a greater risk of obesity and type 2 diabetes. These sugars are rapidly metabolized and contribute directly to energy intake and glycemic excursions. Due to their functional properties, such as sweetness, texture, and preservation, natural sugars are widely used in food manufacturing. However, growing evidence linking excessive intake of these sugars to non-communicable diseases has raised concerns about their widespread use and prompted interest in healthier alternatives.1,3
What are rare sugars?
Rare sugars are monosaccharides, with their derivatives found at extremely low concentrations in fruits, grains, and honey. They are more formally defined as monosaccharides and their derivatives that occur only in limited quantities in nature.3 The most abundant rare sugars are allulose, which is also known as D-psicose, as well as tagatose, while other studied examples include L-arabinose, trehalose, and isomaltulose,3 with chemical structures similar to common sugars like glucose and fructose but slightly different in their configuration.
These structural properties cause rare sugars to be absorbed, excreted, or metabolized in sugar-specific ways in the body. For example, much of allulose is absorbed but excreted unchanged in urine, whereas tagatose, L-arabinose, trehalose, and isomaltulose have distinct digestive and metabolic profiles.1,3 Consequently, rare sugars are generally lower in caloric content and cause a lesser glycemic response than traditional sugars. Some rare sugars can be excreted unchanged, which contributes to their minimal contribution to energy intake. 1,2,3
How do rare sugars influence glucose levels?
Rare sugars have different effects on blood glucose regulation as compared to conventional sugars due to their unique metabolic handling. Human and experimental studies indicate that rare sugars, such as allulose and tagatose, can reduce postprandial glucose and insulin responses when consumed with carbohydrates. In human trials, these effects are generally modest and appear more consistent in people with impaired glucose regulation than in healthy participants.1,3 These effects are partly explained by their incomplete absorption in the small intestine and limited metabolism, with a proportion of these sugars excreted unchanged in urine or passing into the large intestine.1,2
Some rare sugars may slow glucose absorption or influence the activity of enzymes involved in carbohydrate digestion, thereby improving glycemic control. For example, L-arabinose may reduce sucrose digestion by inhibiting sucrase, while isomaltulose and trehalose produce slower or lower glycemic responses than rapidly absorbed sugars in some trials.3 However, these benefits are not consistent across all rare sugars and vary based on the specific type of sugar, dosage, and individual metabolic status, with some studies showing modest or inconsistent outcomes.1,2
Does the Sweetener Allulose Have Side Effects?
Conventional sugars like glucose, fructose, and sucrose are rapidly digested and absorbed, leading to a significant rise in blood glucose and insulin levels after consumption. Comparatively, rare sugars like allulose and tagatose may lead to reduced or blunted postprandial glycemic and insulin responses.
Rare sugars differ substantially from one another: some are only partially absorbed in the small intestine, some are absorbed but largely excreted unchanged, and others are digested more slowly, thereby contributing fewer calories and a lower glycemic impact than conventional sugars. In addition, rare sugars often retain functional properties similar to sucrose, including bulk and browning, unlike high-intensity sweeteners.2 This contrast highlights their potential as alternative sweeteners, particularly in dietary strategies aimed at improving glycemic control and reducing overall caloric intake.1,3
Rare sugars have been associated with several potential metabolic benefits, particularly for glycemic control. Human and experimental studies suggest that allulose and tagatose may reduce postprandial blood glucose levels and improve short-term glycemic responses when consumed with carbohydrates, particularly among individuals with metabolic dysfunction. Some longer-term studies also report reductions in body weight or fat mass, although findings are inconsistent across trials and not all studies show improvements in glucose, insulin, or HbA1c.1,3 Importantly, most of these data have been obtained from short-term or small-scale studies, with the long-term clinical significance of these metabolic effects unclear.1,3
Image Credit: A9 Studio / Shutterstock.com
Are rare sugars safe?
Rare sugars are generally regarded as safe and tolerated when consumed in moderate amounts, with few adverse effects reported in human trials. Certain rare sugars have been assessed in controlled environments and established acceptable safety profiles. However, safety designations should not be generalized to all rare sugars; GRAS status has been reported for specific sugars, such as allulose and tagatose.2
Like other carbohydrates that are low in digestibility, rare sugars can cause gastrointestinal symptoms like bloating, flatulence, or diarrhea when taken in larger amounts. These side effects likely result from incomplete absorption of sugars in the small intestine, with residual sugars undergoing fermentation or exerting osmotic effects in the large intestine. Sugar tolerability is often dose-dependent, with lower concentrations generally well tolerated and higher intake associated with a greater risk of discomfort.1
Practical challenges of using rare sugars daily
The integration of rare sugars into daily dietary patterns depends on their availability, cost, and their role in overall nutrition. As rare sugars have functional properties and taste similar to those of sucrose, they are being explored for use in beverages, baked goods, and commercial sweetener blends.
Rare sugars occur naturally in only small amounts and require either enzymatic or fermentation processes to be produced in large amounts. Modern production relies on enzymatic conversion processes, such as epimerization, as well as emerging microbial fermentation strategies that may improve scalability.2 These specific requirements lead to production constraints that reduce their affordability and restrict their potential for mass-market use.
The future of rare sugar research
To date, most studies examining the health effects of rare sugars have been short-term or limited in scale, thus emphasizing the need for well-designed long-term clinical trials in diverse populations. Additional research is also needed to elucidate the mechanisms by which rare sugars affect glucose metabolism, insulin response, and energy balance. More evidence is also needed on long-term safety, effective doses, and whether rare sugars retain benefits when incorporated into real-world reformulated foods rather than tested as isolated ingredients.1 Although preliminary data on the metabolic effects of rare sugars are encouraging, future studies are needed to provide clear clinical recommendations.1,2,3
References
- Smith, A., Avery, A., Ford, R., et al. (2022). Rare sugars: metabolic impacts and mechanisms of action: a scoping review. British Journal of Nutrition 128(3); 389-406. DOI: 10.1017/S0007114521003524. https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/rare-sugars-metabolic-impacts-and-mechanisms-of-action-a-scoping-review/9F897F8CFEB966C31D123483CC12338F
- Dai, D., & Jin, Y. S. (2024). Rare sugar bioproduction: Advantages as sweeteners, enzymatic innovation, and fermentative frontiers. Current Opinion in Food Science 56. DOI: 10.1016/j.cofs.2024.101137. https://www.sciencedirect.com/science/article/pii/S2214799324000158
- Ahmed, A., Khan, T. A., Dan Ramdath, D., et al. (2022). Rare sugars and their health effects in humans: a systematic review and narrative synthesis of the evidence from human trials. Nutrition reviews. 80(2); 255-270. DOI: 10.1093/nutrit/nuab012. https://academic.oup.com/nutritionreviews/article/80/2/255/6335867
Further Reading
Last Updated: Apr 28, 2026
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