Guava leaf tea is approved in Japan as a ”Food for Specified Health Uses” for prediabetics, based on its antidiabetic properties.82
For example, human studies have found that drinking guava tea before eating carbohydrate reduces post-meal blood sugar.82
Longer-term studies found that drinking guava tea with every meal for 12 weeks reduced insulin resistance in type 2 diabetics and prediabetics.82
Based on these findings, drinking guava leaf tea with meals, especially carbohydrate-rich meals, could be helpful in controlling blood sugar levels.
Whether the effect is achieved with other guava products, such as fruit or fruit juice, is uncertain. I haven’t seen any studies that have evaluted it. Fortunately, guava tea is easy to obtain in most countries.