D-mannose is a simple sugar found in many fruits. It is related to glucose. It also occurs naturally in some cells in the human body.
Other names for D-mannose are:
It's estimated that we get about 2 grams of glycine a day from food sources. As a supplement, it's taken in much higher amounts.
Why do people take glycine?
Glycine has numerous proposed uses. Few of those suggested uses have enough evidence to fully back glycine's effectiveness.
Glycine has shown the most promise as a part of a treatment plan for schizophrenia. In several studies, glycine boosted the effectiveness of other schizophrenia drugs when taken at doses that range from 15 grams to 60 grams per day. However, glycine may have the opposite effect when paired with the antipsychotic drug clozapine.
A small study suggests that glycine may help people with type 2 diabetes control their blood sugar. But more research is needed to back up that result.
In a much larger study, small doses of glycine (1 to 2 grams dissolved under the tongue) showed some potential for limiting brain damage caused by ischemic stroke if treatment begins within several hours of a stroke. There is some concern, though, that high doses of glycine could make the damage caused by a stroke worse.
Studies done on animals hint at glycine's potential as an anticancer agent. But there's no evidence yet that it could help prevent or treat cancer in people. The same can be said for its ability to protect the liver and kidneys from damage caused by chemicals such as alcohol.
Leg ulcers, which can be caused by poor circulation, diabetes, kidney failure, and other health problems, have shown some improvement after treatment with a cream containing glycine and other amino acids.
One study showed some improvement in memory among young and middle-aged men. But the results need to be confirmed by more research.
Glycine is also marketed for a host of other uses, despite the lack of scientific evidence that it is effective or safe for any of them. For example, glycine is marketed as a way to:
Promote the healing of overworked or damaged muscles.
Again, there is no reliable evidence that it works for such uses.
Optimal therapeutic doses for glycine have not been set for any condition. Also, as with supplements generally, the quality of the active ingredients in products that contain glycine varies from maker to maker.
Can you get glycine from food?
High-protein foods provide small amounts of glycine. But supplements are required to get glycine in high doses.
What are the risks of taking glycine?
Glycine appears to be safe, even at doses as high as 60 grams a day. But glycine's safety has not been fully tested or studied. Particular caution should be taken when considering glycine for young children, pregnant or breastfeeding women, and people with liver or kidney disease.
People being treated with clozapine should avoid taking glycine. Also people who have had a stroke should not take glycine without the supervision of a doctor.
A few people have reported nausea, vomiting, and upset stomach after taking glycine. Such reports have been rare, and the symptoms have gone away after glycine was discontinued.