Yacon is a name used to refer to the plant Smallanthus sonchifolius, which is a tuber vegetable (similar to a potato) and a common food product in South America. It looks like a sweet potato and tastes like a pear. While the tuber itself is used in cooking, a syrup derived from the tuber (called yacon syrup) is used as an alternative sweetener.
This vegetable is also sometimes called the 'diet potato,' which is thought to be related to its fructooligosaccharide (FOS) content. The FOS benefits gut health and has a prebiotic effect, and they may also have a minor appetite suppressing effect in the obese, which reduces food intake. It is thought to be a good dietary supplement, since its production costs are low and yield is high, so it may be one of the most financially feasible sources of dietary FOS while possibly deter sugar consumption, as yacon syrup itself is sweet.
Human studies on yacon are currently limited. One study supports a weight loss effect, which may be due to a reduction in consumed calories, while the other study noted that yacon possesses an anti-motility property (slowing intestinal transit time down) which is known to reduce appetite somewhat. These properties are thought to be related to the FOS content, and are similar to the benefits of FOS from other sources.
Rodent studies suggest proliferation of the 'good' bacteria in the colon in accordance with yacon's prebiotic effects, as well as an increase in mineral uptake from the colon, the latter of which is beneficial for bone health. These properties are not yet proven in humans with, but thought to occur since they have been noted to occur in humans with any other source of FOS.
There is a tea product made from the leaves of yacon, which is said to be anti-diabetic. There is no human evidence for this claim and due to at least one study noting kidney damage associated with the tea (kidney damage tends to reduce glucose levels in the blood), it is thought that this underlies the supposed anti-diabetic effects and consumption of the tea is not recommended.