What are the advantages in the different strengths?

It is not about an advantage, it is more of a process. You need to experiment with this to see how it works for you. Some say they don't notice a difference but I swear by it. I am 84 years old and I work with numbers. I need to be sharp. If you are starting out with taking Prevagen, start with the lowest dose, once a day, for a month or so to see how it may work for you. If you like the results, bump it up to two a day of the low dose, or go to one a day of the next highest dose.

Why does this have vitamin D, an oil absorbed only vitamin, in it?

Vitamin D is used to help memory improve. Hope this helps you.

differance between professonial nd extre strength?

Professional strength was strongly recommended to me. I was told the ingredients included in this were stronger and more able to improve memory. I've only been taking for about a week or so consistently and I do feel sharper with improved memory and attention. I would recommend trying it for one month and I think you'll know too within a couple weeks.

What is the research on taking 40 mg. instead of the "extra strength 20 mg.?"

Either one is beneficial but we tried the 40mg to see if it would work better. It said that the 40mg was for medical practitioners and I was one before I had to retire. My husband and I took it for a month and we didn’t see any benefit or improvement taking it. On the commercials the people said that they have taken this for years and that they feel like it works. So perhaps it needs to be taken seriously for a year or so to see its effectiveness.