Calamansi is a small citrus fruit that is more sour than lemon and lime. This small fruit is common in tropical countries and used for food condiment. Calamansi is also used as a drink. The juice of calamansi is mixed with water and sugar which is a very effective treatment for the common cold. The calamansi juice is a good source of vitamin C. If you are on OMAD, I guess calamansi juice is a good way of enhancing the weight loss.