Tagatose is a kind of sugar that occurs naturally in dairy products. The companies that make tagatose filter it out of the lactose sugar extracted from dairy products. It is 92% as sweet as sugar, but it contains only 36% of the calories. You are most likely to encounter fruit and vegetable juices flavored with tagatose in northern Europe and in South Africa.
There are not a lot of downsides to using tagatose. You can freeze, refrigerate, or even boil fruit and vegetable juices sweetened with tagatose without losing its sweet flavor. Although tagatose provides very few calories for humans, it's a useful food source for the friendly symbiotic bacteria that live in the human colon, such as Lactobacillus acidophilous. All in all, tagatose is genuinely natural and healthy food and beverage additive. Consume in moderation.