According to the US Food and Drug Administration (FDA):
Allulose, also known as psicose, is a simple sugar (monosaccharide) with the chemical formula same as glucose (C6H12O6) but with different arrangement of atoms. Unlike glucose or other common sugars, it is only partially absorbed and almost completely excreted unchanged with the urine:
...about 98% of intravenously administered allulose is excreted in the urine within 6 h...When orally ingested, urinary excretion of unchanged D-allulose ranged from 11 to 25%... The data indicate that D-allulose absorbed in the small intestine may pass into the bloodstream and be excreted in the urine without being significantly metabolized. Unabsorbed D-allulose is fermented to short chain fatty acids (SCFA) by intestinal microflora in the colon...or is excreted in the feces.
Some short-chain fatty acids produced from allulose in the colon can be absorbed and provide some calories:
the energy value of D-allulose was predicted to be less than 0.2 kcal/g...
To compare, table sugar (sucrose) has ~4 Cal/gram.
Allulose is manufactured artificially from fructose or extracted naturally from sugar beet. So, depending on the production, it can be considered artificial or natural sweetener.
Allulose is keto-approved.
Ketosis is a physiological state in which your body, due to low-carb diet does not have enough glucose, so it starts to produce ketones and use them as an alternative fuel. A keto-approved carb is a carb that does not yield enough glucose to make your body switch back to usual metabolism. Some people believe that ketogenic diet helps in weight loss or in treating epilepsy in children.
Allulose is a carbohydrate but is not metabolized in your body, which means it is not converted to glucose or anything else, so its consumption does not make you come out from ketosis when you are on ketogenic diet (Food Insight).
FDA allows allulose to be excluded from total and added sugar counts on nutrition facts labels, but not from total carbohydrates (so, it is considered a non-digestible carbohydrate).
Despite being a low-calorie sweetener, the taste of allulose is similar to table sugar (sucrose):
d-Allulose is a rare sugar with almost zero calories, sweetness that is 70% of sucrose, with some cooling sensation and no bitterness. (Pharmacology and Therapeutics, 2015)
The taste is clean-sweet (KF Tiefenbacher, The Technology of Wafers and Waffles, 2017).
Allulose has been generally recognized as safe (GRAS) by FDA.
Like non-digestible oligosaccharides and fiber ingredients, the only known side effect of D-allulose is gastrointestinal discomfort when ingested in large quantities.
According to one study in Nutrients, 2015, the safe dose is 0.4 g allulose/kg body weight (28 g/70 kg) as a single dose or 0.9 g allulose/kg body weight (63 g/70 kg) as a total daily dose. Exceeding that dose can cause diarrhea, abdominal pain, nausea and headache.
Allulose is not that special, though; another sweetener with similar characteristics and metabolism is erythritol.