I started taking these at the recommendation of my doctor due to being close to 50. I had fatigue due to a low D level which in a million years I never would have thought. I thought my B was going to be the one that was low, but my doctor advised me that many people have low D because of age (Menopause - even pre-Menopause), and the fact that we use so much sunscreen that our levels get low because D is considered the "sunshine vitamin". In addition, pre-Menopause and Menopause immediately starts your calcium levels to start decreasing. This product has always gotten good reviews due to how well the calcium absorbs in the body and the D is a plus. (D also helps you process calcium). I wish I would have started taking these a few years ago. I have noticed a difference in my energy level. I'm not at 100% and not sure I ever will be, but if you are questioning your energy levels suddenly dropping this can help