Allulose in human nutrition: the known and the unknown

Fate of ingested allulose

In mammals, allulose uses essentially the same pathways for uptake, distribution, and excretion as dietary fructose.

Absorption of allulose from the intestine is mediated by the GLUT5 transporter in the apical membranes of epithelial cells, while export across the basolateral membrane is via GLUT2 ^{( Reference Kishida, Martinez, and Iida 7 )} .

Since both transporters are uniporters that mediate equilibrium across the membrane, the direction of transport is determined solely by the concentration gradient.

Intestinal fructose uptake appears to be dependent on its metabolism in epithelial cells, resulting in the release of glucose and several organic acids into the portal circulation. This has recently been elegantly demonstrated in mice using ^13C -labeled fructose ^{( Jang, Hui, and Lu, refs . 8 )} .

Similar to humans, mice have limited fructose absorption in the small intestine, likely due to the low density of GLUT5 in apical membranes compared to SGLT1, the electrogenic sodium-coupled GLUT responsible for the high-throughput transport of glucose and galactose.

Limited fructose absorption causes intestinal discomfort, which is associated with osmotic diarrhea and fermentation by microbial communities in the distal parts of the intestine, leading to gas formation and bloating ^{( Reference: Jones, Butler, and Brooks 9 )} .

Gas formation (mainly hydrogen) is easily monitored in exhaled air following fructose ingestion, and many consumers experience limited fructose absorption and increased gas formation with acute fructose doses exceeding 25-50 g.

Malabsorption has become a common problem in recent decades due to the increased fructose intake from foods and beverages.

Importantly, fructose malabsorption can be overcome by co-administration of glucose. This effect is dose-dependent, and fructose malabsorption is no longer detectable at a 1:1 fructose-glucose stoichiometry ^{( Reference Truswell, Seach, and Thorburn 10 )} .

This finding is consistent with the observation that the majority of patients with fructose malabsorption do not experience unpleasant intestinal effects when consuming sucrose.

The molecular mechanisms underlying the enhancement of fructose absorption from the intestine by glucose are not yet fully understood, but it has been hypothesized that glucose enables GLUT2 to integrate into the apical membrane, allowing for the absorption of more fructose and glucose ^{( Ref. Kellett and Helliwell 11 , Ref. Röder, Geillinger and Zietek 12 )} .

It is also currently unknown whether glucose can enhance allulose absorption through the same mechanism.

Allulose, like fructose, has a limited absorption capacity, with doses greater than 0.4 g/kg body weight/day being tolerated without intestinal complaints ^{( Reference: Han, Choi, and Kim 13 )} .

The extent of absorption of allulose from the entire intestine is estimated to be approximately 70% of the oral dose ^{( Reference Tsukamoto, Hossain, and Yamaguchi 14 , Reference Moura 15 )} .

It remains to be determined whether additional negative side effects of allulose intake occur when allulose is consumed alone and not in combination with other dietary items (foods or beverages) that also contain glucose or sucrose.

Like fructose, allulose has been shown to mediate glucagon-like peptide 1 (GLP-1) secretion from rat enteroendocrine L cells.

This epithelial cell type is found throughout the intestine, but shows the highest density in the ileum and colon.

GLP-1 reduces the rate of gastric emptying and, in addition to its acute effect on glucose-dependent insulin production, is responsible for maintaining vital ß -cell mass in the pancreas.

A study in C57BL/6 mice, in which allulose was administered intragastrically at doses of 1 and 3 g/kg, described increased GLP-1 secretion and a transient decrease in food intake, which was no longer observed when animals lacking the GLP-1 receptor were used ^{. Reference Hayakawa, Hira, and Nakamura 16 )} .

In another study in healthy and obese-diabetic animal models, allulose also induced GLP-1 release, reduced food intake, and showed positive effects on glucose tolerance ^{( Reference Iwasaki, Sendo, and Dezaki 17 )} .

It is unknown whether this metabolic activity of allulose is relevant in humans, given the expected total daily intake of 10–30 g.

Furthermore, hormone secretion and sensing in the gut show notable differences between rodents and humans. Therefore, human studies that specifically examine the gastrointestinal response to allulose are needed.

Since human data on the fate (i.e. organ distribution) of absorbed allulose are not yet available, the results of animal studies and the analogy with fructose help to determine the current status of the “known” substances and allow estimates to be made based on the missing information.

Early animal studies in rats investigated the fate of allulose using ^{a 14C} radioactive tracer administered intravenously (iv) or orally ^{( reference 3 by Whistler, Singh, and Lake )} .

Radioactivity recovery after intravenous administration showed that >95% was excreted and was detectable in urine as early as 6 hours.

In contrast, 72 hours after oral administration, approximately 40% of the tracer remained in the carcass, 37% in the urine, and 15% was detectable as carbon dioxide.

The latter strongly suggests that the gut microbiota is able to metabolize allulose reaching the colon, thereby releasing CO2, while 12% of the tracer was excreted in the feces.

In a later study using radiolabeled allulose in mice, autoradiography was used to identify organs that retained the tracer ^{( Reference Tsukamoto, Hossain, and Yamaguchi 14 )} .

Only the liver and urinary bladder showed significant accumulation 30 minutes after intravenous injection. Whole blood analysis showed rapid elimination with a half-life of <15 minutes. However, when analyzed 7 days after oral administration of 100 mg allulose per kilogram of body weight, there was still some tracer in both the liver and intestines (including the contents).

Currently, only one human study reports the postprandial plasma concentration of D-allulose after oral administration at a single dose of 0.5 g/kg body weight ^{( Reference Kuzawa, Ikeda, and Kagaya 18 )} .

Compared to glucose and fructose administration, the maximum plasma level of allulose reached nearly 3 mM, thus increasing similarly to that of glucose, but fructose levels only increased by approximately 300 µM.

While glucose levels peaked at 30 minutes, allulose levels reached their maximum at 1 hour but remained above fasting levels for more than 6 hours.

These results are consistent with significant metabolism of fructose in the gut, which prevents significant increases in peripheral blood, while in the absence of metabolism, allulose levels mimic glucose levels – although due to insulin, glucose can be cleared from the blood more rapidly than allulose.

Rapid clearance of the molecules from the blood occurs in the kidneys by glomerular filtration and subsequent urinary excretion. However, the apical membrane of the proximal tubular cells of the kidney also contains GLUT5, which means that there will always be some level of allulose remaining in the kidney tissue. Assuming that 50-70% of the allulose dose absorbed in the intestine ultimately appears in the urine, the urogenital tract is exposed to quite high concentrations of allulose.

When calculating the average urinary concentration of allulose, for a daily dose of 10-40 g of allulose, which comes from fortified foods and beverages, the urinary allulose level can easily exceed 10 mM.

Experts and authorities have raised the potential impact on urogenital tract infections as a safety concern, as it provides extra substrate for bacteria.

However, treatment of diabetic patients with SGLT2 inhibitors, which aim to increase urinary glucose excretion by blocking tubular reabsorption of filtered glucose via SGLT2, also raises glucose levels above 10 mM ^{( Reference List and Whaley 19 )} .

This was also identified as a potential risk of promoting urinary tract infections during the early stages of SGLT2 inhibitor development.

However, a recent meta-analysis based on long-term follow-up of thousands of patients found no evidence of such an increase in the incidence of urinary tract infections ^{( Reference: Dave, Schneeweiss, and Kim 20 )} .

Allulose and commensal gut microbes

Since GLUT5 has a limited ability to transport allulose, amounts that are not fully absorbed in the upper small intestine reach the small intestine and colon and can be metabolized by commensal bacteria living there.

A certain level of utilization was already _{indicated by the finding} of CO2 production from the labeled allulose in the previously mentioned early rodent experiments. ^{( Whistler, Singh and Lake reference number 3 )} .

More systematic studies on allulose utilization/fermentation in rats have examined the dose-response relationship of 10, 20, or 30% (w/w) allulose in the diet, followed by analysis of short-chain fatty acids (SCFA) in the cecal contents. A dose-dependent increase in the concentrations of acetate, propionate, and butyrate was observed ^{( Reference Matsuo, Tanaka, and Hashiguchi 21 )} , and this profile was similar to other fermentable carbohydrates, including fructose or fructans. Although SCFAs are considered beneficial, ^{studies in mice with 13 C-labeled fructose ( Ref. Zhao, Jang, and Liu (22 )} have shown that acetate, as the dominant short-chain fatty acid species in colonic fermentation, can be utilized as a starting substrate by rodent liver fatty acid synthase, which may be one mechanism by which high-dose fructose induces nonalcoholic fatty liver disease in rodents and humans.)

However, this would suggest that allulose could also essentially cause hepatic fat synthesis via acetate produced by commensal bacteria. In addition, these studies also address the energetic value of allulose.

Although sugar is not metabolized in mammalian cells, the production of SCFA would provide energy for the host – similar to other fermentable substrates.

However, for allulose, a lower “calorie yield” of approximately 0.4 kcal/g has been suggested ^{( Reference Moura 15 )} , as only a fraction of the ingested dose reaches the colon, and in addition, the fermentation rate of allulose may be much lower in humans than in rodents.

This is suggested by a study that used analysis of hydrogen gas (H2) in exhaled air as an indicator of fermentation, similar to the standard fructose malabsorption test.

When administered in three doses of up to 0.33 g/kg body weight and compared with the same dose of fructooligosaccharides, allulose did not increase exhaled H2, _but A dose-dependent increase was observed for oligofructoses ₍ ^{Iida , Hayashi and Yamada reference 23 )} .

The authors therefore concluded that allulose at a reasonable dose (23 g for a 70 kg person) does not show significant fermentation and thus does not produce energy through short-chain fatty acids (SCFA).

Furthermore, in vitro studies using 35 bacterial strains commonly present in the commensal microbiome did not show significant fermentation of allulose when incubated with 0.5% D-glucose or D-allulose solutions for 96 hours, followed by pH measurements to measure the production of organic acids from the substrates ^{( Iida, Hayashi, and Yamada ref. 23 )} .

No significant increase in fecal short-chain fatty acids (SCFA) levels was observed in mice fed a high-fat diet (40% energy from fat) for 16 weeks and ^{supplemented with 5% of their body weight of D-allulose (supplemented or not). Reference: Han, Park, and Choi 24 )} .

Microbiome analysis of fecal samples from mice using 16S rRNA gene sequencing revealed some changes in microbiota composition, but these changes were all correlated with changes in body weight of the animals, leaving it unclear whether allulose had a direct or indirect, weight-mediated effect on the microbiome. The same authors, using the same diets and the same study design (presumably from the same study), reported similar microbiome changes, but the effects on SCFA levels were different ^{( Reference: Han, Yoon, and Choi 25 )} .

These partially identical data were presented in three studies and have already been identified (PubPeer: https://pubpeer.com/publications/1C5ADDF6BF9F4E555B598F39AE9B5A ).

In summary, the current knowledge base on the fate of unabsorbed allulose in rodents and humans is insufficient to draw any conclusions about whether regular consumption of higher amounts of allulose from fortified products would have a significant impact on the microbiota composition or the colonic metabolite spectrum.

Allulose and pathogenic bacteria

Safety concerns have been raised regarding the consumption of allulose, as reports have suggested that allulose may provide a substrate for potentially harmful bacteria ^{( Blin, Passet, and Touchon reference 26 )} .

Consumption of allulose may therefore provide a growth advantage to these bacteria and increase the incidence and/or severity of infection ^{( Reference: Martin, Cao, and Wu 27 )} .

A similar scenario has previously been proposed for another carbohydrate, trehalose. Medically relevant strains of Clostridioides difficile , an opportunistic pathogen that causes colitis after antibiotic therapy, mainly in elderly hospitalized patients, metabolize trehalose ^{( Reference Collins, Robinson, and Danhof 28 )} .

Based on these findings, trehalose has been proposed for use in humans to increase susceptibility to C. difficile infection, and to promote colonization and symptomatic infection. However, subsequent studies have failed to confirm trehalose-induced C. difficile toxin production or a more severe course of infection in a mouse model of infection, and the presence of the trehalose gene cluster in C. difficile isolates has not been associated with adverse outcomes in a large human cohort ^{( Eyre, Didelot, and Buckley 29 , Saund, Rao, and Young 30 )} .

Thus, the ability of bacteria to break down a nutrient does not necessarily increase the risk of infection in individuals exposed to the nutrient.

Similarly, the ability to metabolize allulose has been demonstrated in environmental and clinical isolates belonging to the Klebsiella pneumoniae species complex and associated with a utilization gene cluster ^{( Blin, Passet, and Touchon reference 26 , Reference: Wyres, Lam, and Holt 31 )} .

It is noteworthy that K. pneumoniae can utilize a wide spectrum of over 100 different carbon sources, and the detailed function and specificity for allulose of the eight genes encoded by the operon that enables utilization have not yet been determined.

Allulose utilization has also not been associated with increased expression of virulence factors. Rather, it is thought to provide a competitive metabolic advantage, allowing for faster growth in its ecological niche or site of infection. K. pneumoniae is a common member of the human enteric microbiota, and colonization per se does not result in adverse outcomes.

Although K. pneumoniae can cause infections, such as urinary tract infections, in healthy individuals, The majority of K. pneumoniae infections occur in individuals with underlying medical conditions and in intensive care units ^{( Reference Martin and Bachman 32 )} .

However, under such circumstances , properties of K. pneumoniae such as rapid growth, the ability to develop resistance to almost all antibiotics, and its resistance to environmental factors play a critical role ^{( Reference Chen, Mathema, and Chavda 33 )} .

However, it cannot be ruled out that renal excretion and elevated urinary concentrations of allulose promote local growth and infection of the urinary tract ^{( Reference Tsukamoto, Hossain, and Yamaguchi 14 ). More recently, especially in the Asia/Pacific region, there have been reports of} Community-acquired liver abscesses caused by K. pneumoniae in the absence of biliary tract disease in apparently immunocompetent individuals ^{( Ref. Russo and Marr 34 )} .

The underlying mechanisms have not yet been identified. However, the consumption of allulose, which has been available in some countries in the region for years, has not been linked to these infections.

By comparing K. pneumoniae isolates from infected and colonized hospitalized patients, Martin RM et al. ^{( Reference Martin and Bachman 32 )} found an overrepresentation of allulose-utilization cluster-positive and putative allulose-utilization competent strains among isolates from infected individuals compared to colonized individuals. They also observed that bacterial growth occurred in the presence of allulose, and that this growth was dependent on a functional permease that serves to import allulose. Finally, using a co-infection of a permease-competent and a permease-deficient K. pneumoniae strain in a competitive lung infection model, they observed a slightly increased organ growth of permease-competent bacteria ^{( Reference Martin, Cao, and Wu 27 )} .

Based on these results, and similar to the trehalose/ C. difficile example mentioned above, allulose consumption has been suggested to promote colonization and increase susceptibility to K. pneumoniae infection.

However, important questions remain. First, the specificity of allulose utilization genes within the operon has not been formally demonstrated, and the concentration of allulose in different parts of the body is essentially unknown.

Furthermore, the abundance of K. pneumoniae within the gut microbiota after oral allulose consumption, but in the absence of other selection factors such as antibiotic therapy, requires further investigation as this is a potential reservoir for subsequent infections. With regard to infection, urinary tract infections may be of particular interest due to the renal excretion of allulose ^{( Reference Tsukamoto, Hossain, and Yamaguchi 14 , Reference Iida, Hayashi, and Yamada 23 )} .

However, increased urinary carbohydrate concentrations do not necessarily increase the risk of infection. Increased renal glucose excretion following treatment with SGLT-2 inhibitors has not been shown to lead to an increased incidence of urinary tract infections ^{( Reference: Dave, Schneeweiss, and Kim 20 )} .

Finally, the proposed advantage of allulose metabolism for K. pneumoniae in the presence of glucose and other carbohydrates during infection of systemic body sites needs to be demonstrated .

It is noteworthy that K. pneumoniae exhibits a broad metabolism and its growth in the presence of glucose was much faster than in the presence of similar allulose concentrations ^{( Reference: Martin, Cao, and Wu 27 )} .

Allulose and its general metabolic health effects

Most published studies on allulose have focused on its supposed health benefits as a sugar substitute with negligible energy value. Both rodent and human studies have examined the effects of allulose on short-term metabolic responses, such as insulin secretion and glycemic control, and on weight control and disease biomarkers in medium-term studies. We will focus on human studies, as almost all rodent studies have used allulose at doses that are not suitable for humans (based on allometric dosing) due to gastrointestinal discomfort and adverse effects at doses of allulose greater than 30 g.

Several studies have been conducted in healthy individuals and individuals with prediabetes or manifest type 2 diabetes to assess the effect of allulose on glycemic parameters in response to an oral glucose tolerance test with 75 g D-glucose or specific test items.

In a recent study conducted in Canada, 0.5 or 10 g of allulose was added to 75 g of glucose with fructose as a comparator and given to healthy subjects. The highest dose of allulose had no effect on the AUC of glucose profiles ^{( Ref. Noronha, Braunstein and Glenn 35 )} .

The same researchers conducted a similar study in individuals with type 2 diabetes, but here they found a modest effect of allulose on glycemic control, while insulin levels remained unchanged ^{( Reference Braunstein, Jarvis, and Noronha 36 )} .

However, in a study where healthy subjects were given 75 g of maltodextrin instead of glucose in the presence or absence of 2.5, 5, or 7.5 g of allulose, significant decreases in blood glucose and insulin AUC were observed at the two higher allulose doses ^{( Reference Iida, Kishimoto, and Yoshikawa 37 ).}

^{This effect is due to the allulose mucous membrane} It was explained by possible inhibition of α- glucosidase, but this was not experimentally confirmed.

A recent study in healthy European volunteers examined the effects of allulose at different doses against a background of a 50 g sucrose test solution and observed a significant reduction in plasma glucose peaks after 30 minutes when the highest doses of allulose, 7.5 and 10 g, were taken with sucrose ^{( Ref. Franchi, Yaranov, and Rollini 38 )} .

However, a meta-analysis that summarized data from three human studies conducted in Asia on the effects of allulose and glycemic control concluded that allulose did not show any beneficial effects ^{( Reference Noronha, Braunstein, and Blanco Mejia 39 )} .

Allulose has been shown to reduce body weight and body fat stores in several animal models of diet-induced or genetically induced obesity, in animals prone to obesity and diabetes.

In contrast to these rodent studies, there are only very few studies available that have evaluated the effects on humans.

Significant weight loss, with a corresponding reduction in fat mass measured by CT scans, was observed in moderately to severely overweight healthy volunteers who received 4 g or 7 g of allulose in the form of test drinks twice daily for 12 weeks ^{( Reference Han, Kwon, and Yu 40 )} .

A sucralose-sweetened beverage was served as a placebo. However, the effect of D-allulose on body weight was modest, with BMI decreasing by 0.38 kg/m2 in ^the high-dose group compared to the placebo group.

Blood lipid markers (lipoproteins), markers of glycemic control (HbA1c, fasting glucose and insulin, HOMA-IR) and other biomarkers did not show significant changes.

This again confirms that the effects of allulose in humans are less likely than those shown in rodents, which are usually given unrealistically high doses of allulose.

In a small clinical trial in people with impaired glucose metabolism, increases in total and LDL cholesterol were observed after 12 weeks of daily intake of 15 g of D-allulose ^{. Reference Tanaka, Kanasak, and Hayashi 41 )} .

A subsequent randomized, controlled trial involving 90 individuals with moderately elevated LDL cholesterol levels examined the effects of long-term consumption of D-allulose for 48 weeks.

Participants were divided into a high-dose D-allulose (15 g/day), a low-dose D-allulose (5 g/day) and a placebo subgroup. No significant changes were observed in total and LDL cholesterol levels, and long-term D-allulose consumption did not alter other established cardiovascular risk factors.

A modest improvement in liver enzyme activity and fatty liver score was reported. Although anthropometric data were not presented, the authors mention that the changes were not clinically significant ^{( Reference Tanaka, Kanasak, and Hayashi 41 )} .

Due to the structure and absorption of D-allulose, there has been interest in the tolerability of D-allulose since the beginning of human studies. In a recent systematic review, Han et al . examined the dose-response relationship of gastrointestinal tolerability of D-allulose ^{( Reference Han, Kwon, and Yu 40 )} .

When the dose of D-allulose was gradually increased, severe diarrhea and other gastrointestinal symptoms were observed at a dose of 0.5 g/kg body weight. Therefore, the authors recommended a maximum single dose of D-allulose of 0.4 g/kg body weight and a maximum total daily dose of 0.9 g/kg body weight.

This aspect was also taken into account in the long-term study by Tanaka et al ., where the authors reported no significant difference in the incidence of side effects between the placebo group and the D-allulose groups ^{( Reference Tanaka, Kanasak, and Hayashi 41 )} .

Summary