Psicose
D-Psicose (C6H12O6), also known as D-allulose, or simply allulose, is a low-calorie epimer of the monosaccharide sugar fructose, used by some major commercial food and beverage manufacturers as a low-calorie sweetener. First identified in wheat in the 1940s, allulose is naturally present in small quantities in certain foods. The U.S. Food and Drug Administration (FDA) has accepted a petition for Generally Recognized As Safe (GRAS) for allulose as a sugar substitute in various specified food categories. Because it is absorbed and metabolized differently from other sugars, the FDA has exempted allulose from the listing of total and added sugars on the Nutrition and Supplement Facts labels, but requires its weight listing as a carbohydrate, with 0.4kcal/g (about 1/10 the calories of ordinary carbohydrates). Studies have shown the commercial product is not absorbed in the human body the way common sugars are and does not raise insulin levels, but more testing may be needed to evaluate any other potential side effects. In 2020, the U.S. FDA accepted the conclusion by Samyang that the maximum tolerable consumption for a 60 kg adult was 33 to 36 grams per day. Biochemistry
The sweetness of allulose is estimated to be 70% of the sweetness of sucrose. It has some cooling sensation and no bitterness. Its taste is said to be sugar-like, in contrast to certain other sweeteners, like the high-intensity artificial sweeteners aspartame and saccharin. The caloric value of allulose in humans is about 0.2 to 0.4 kcal/g, relative to about 4 kcal/g for typical carbohydrates. In rats, the relative energy value of allulose was found to be 0.007 kcal/g, or approximately 0.3% of that of sucrose. Similar to the sugar alcohol erythritol, allulose is minimally metabolized and is excreted largely unchanged. The glycemic index of allulose is very low or negligible. Allulose is a weak inhibitor of the enzymes α-glucosidase, α-amylase, maltase, and sucrase. Because of this, it can inhibit the metabolism of starch and disaccharides into monosaccharides in the gastrointestinal tract. Additionally, allulose inhibits the absorption of glucose via transporters in the intestines. For these reasons, allulose has potential antihyperglycemic effects, and has been found to reduce postprandial hyperglycemia in humans. Through modulation of lipogenic enzymes in the liver, allulose may also have antihyperlipidemic effects. Due to its effect of causing incomplete absorption of carbohydrates from the gastrointestinal tract, and subsequent fermentation of these carbohydrates by intestinal bacteria, allulose can result in unpleasant symptoms such as flatulence, abdominal discomfort, and diarrhea. The maximum non-effect dose of allulose in causing diarrhea in humans has been found to be 0.55 g/kg of body weight. This is higher than that of most sugar alcohols (0.17–0.42 g/kg), but is less than that of erythritol (0.66–1.0+ g/kg). D-allulose was found to be more reactive than fructose and glucose in glycation reactions. Effect on carbohydrate absorption 
A meta-analysis was conducted of the effect on postprandial glucose and insulin responses of adding a median of 5 grams of allulose (range, 2.5-10 g) to a fixed carbohydrate-containing drink or meal, versus the same meal alone. Overall, compared to the carbohydrate-containing meal alone, the same meal with a small dose of added allulose resulted in a 10% lower integrated area under the curve (iAUC) of postprandial glucose. The quality of the evidence was rated as moderate. Chemistry

Allulose, also known by its systematic name D-ribo-2-hexulose as well as by the name D-psicose, is a monosaccharide and a ketohexose. It is a C3 epimer of fructose. Fructose can be converted to allulose by the enzymes D-tagatose 3-epimerase () and/or D-psicose 3-epimerase (), which has allowed for mass production of allulose. The compound is found naturally in trace amounts in wheat, figs, raisins, maple syrup, and molasses. Allulose has similar physical properties to those of regular sugar, such as bulk, mouthfeel, browning capability, and freeze point depression. This makes it favorable for use as a sugar replacement in food products, including ice cream. In a paper produced for the European Food Safety Authority, the enzyme d-psicose 3-epimerase, manufactured by Matsutani Chemical Industry Co., Ltd, was investigated for safety and allergenicity. No DNA of E. coli (used for production of the enzyme) was found in the enzyme preparation, and no match was found in the enzyme amino acid sequence with those of known allergens. History
Allulose was first discovered in the 1940s. The first mass-production method for allulose was established when Ken Izumori at Kagawa University in Japan discovered the key enzyme, D-tagatose 3-epimerase, to convert fructose to allulose in 1994. This method of production has a high yield, but suffers from a very high production cost. Regulatory history 
In June 2012, the U.S. Food and Drug Administration (FDA) accepted the assertion of CJ CheilJedang, Inc. of South Korea that allulose is Generally Recognized As Safe (GRAS) as a sugar substitute in various specified food categories. In June 2014, a similar GRAS letter was issued to Matsutani Chemical Industry Company, Ltd. of Japan. Non-GMO allulose manufactured by Samyang Corp. of South Korea was approved as GRAS in March 2020. In the European Union, although allulose is a naturally-occurring saccharide, under their regulations, monosaccharides and other saccharides are not considered food additives, and thus cannot be approved as such, but must be approved as ingredients. CJ-Tereos Sweeteners of France filed for such an approval in April 2018. The Allulose Novel Food Consortium (ANFC) was formed in 2021 by four Japanese, Korean, U.S. and European food ingredient companies to speed its approval as an ingredient in Europe, including exemption from sugar labelling. The U.S. FDA in October 2019 announced the exemption of allulose from total and added sugars on nutritional labels, but manufacturers must continue to include allulose in the total carbohydrates declaration, with a value of 0.4kcal/g, "0.4 calories per gram of allulose". Manufacturing
Allulose is produced by an enzymatic reaction that converts fructose into allulose. As of 2018, most commercially available allulose uses corn (maize) as the source of fructose. Allulose is also produced from sugar beet. Commercial application 
Commercial manufacturers and food laboratories are looking into properties of allulose that may differentiate it from sucrose and fructose sweeteners, including an ability to induce the high foaming property of egg white protein and the production of antioxidant substances produced through the Maillard reaction. Commercial uses of allulose include low-calorie sweeteners in beverages, yogurt, ice cream, baked goods, and other typically high-calorie items. London-based Tate & Lyle released its proprietary variant of allulose, known as Dolcia Prima allulose, and U.S.-based Anderson Global Group released its own proprietary variant into the North American market in 2015. The first major food company to adopt allulose as a sweetener was Quest Nutrition in some of their protein bar products. On April 16, 2019, US Food and Drug Administration (FDA) issued a draft guidance, allowing food manufacturers to exclude allulose from total and added sugar counts on Nutrition and Supplement Facts labels. Like sugar alcohols and dietary fiber, allulose will still count towards total carbohydrates on nutrition labels. This, combined with the GRAS designation, has increased interest in including allulose in food products instead of sucrose. References

Ketohexoses
Pyranoses
Furanoses