I used to be a beauty therapist and nail technician so I can actually answer this one for you. These ridges and dents can be caused by literally anything, a false/acrylic nail pinging off, accidentally hitting your hand on something, even just a slight tap that you don't even notice can cause a dent in your nail. They can also be caused by things like psoriasis, eczema, arthritis and alopecia. There's nothing really that can be done to stop the pitting but it can be disguised by buffing your nails and applying a ridge filler which are generally colourless, I'd also recommend using a good cuticle oil, this doesn't only soften the cuticle, it can also help nourish the nail, nail bed and its root.
Hope that helps, if there's anything else I can help with please feel free to ask xx

I too have dents/ridges on my nails. Nails are very brittle. Was told to take more calcium in. Drink hot chocolate et take calcium tabs every day. My nails split up et down which really hurts et sux cuz you can only cut or file so far:( I have looked into it not sure if fibro related but have found could be related to psoriatic arthritis??? I'm just slowing watching them et these dry shining patches I get on my skin since blood work always comes back normal. My mother had psoriatic arthritis, rheumatoid arthritis et fibro secondary.

My nails are brittle, especially in the winter my right thumb nail has a week fault line down the middle and I think it could actually split down into the quick. It also is like thetop layer is flaking away. I've always kept good care of my nails but can't seem to help my poor thumb nail. I keep lotion on them.

I have ripples in some of my nails and they never go away. They are not brittle though. Sometimes its from injuries below the nail bed that can cause it to grow differently. Also many illnesses can cause rippling such as diabetes. As far as being brittle you might also want to have your thyroid checked because hypo thyroid can cause not only brittle nails but also brittle hair and dry skin along with fatigue.

Thank you all for the replies. I will look into the Vitamin D deficiency as well. Hope you are having a good day! :)