With respect to anabolism, Deca is only slightly stronger than testosterone. It has a strength rating of 125, while the strength rating of testosterone is 100. But testosterone has a very low androgenic rating of just 37, and this makes it more suitable for people who experience side effects of androgen.
Nandrolone has very low activity of estrogen and it binds very badly with the enzyme, aromatase.
Another point is that, Aromatase is responsible for converting androgens into estrogen. Only approximately 20% of nandrolone is converted into estrogen which is lower than testosterone’s conversion percentage.
It is because Deca is a progestin like all other 19-nor compounds, that its estrogenic activity is so low. If you delve deeper, you will find out that nandrolone is aromatized into estrogen in the liver. However, in other areas of the body where normally there is high rate of estrogenic conversion, like in fat tissue, Deca is not conducive to estrogen conversion.
Deca is categorized as a progestin and classified as a 19-nor compound because it does not have the 19th carbonate that testosterone has. Being a progestin, it has an attraction for the progesterone receptor. This can result in some possible side effects and problems that only 19-nor compounds exhibit, and are not found in other anabolic steroids.