system
Given the following post, respond as if you're commenting on Reddit:
human
Is there anything that a human  can take to rebuild their teeth?
gpt
No, and nothing will ever be as good as your natural teeth. enamel is ~98% mineral content, made up of hydroxyapatite. it is harder than bone. caries (tooth decay) involves bacteria forming a biofilm (plaque) on your teeth, and metabolising the sugar in your diet into acids. These acids draw out the mineral content from your teeth, weakening them. They eventually use the microscopic voids created in the enamel matrix to travel into the second layer called dentine, which is not as mineralised as enamel, ~70%. Also, dentine has hollow tubules running through it, fastracking access to your pulp (nerve).

Non-Carious Tooth Loss includes several categories; Erosion involves acids of non-microbiological origin, endogoneously (reflux, bulimia) or exogenously (coke, juices); Attrition - grinding of teeth; Abrasion - brushing too hard or in same cases chewing an abrasive substance like sand; and others like trauma/abfraction/congenital issues.

Your pulp will detect this attack when it reached the dentine and try to sclerose these tubules, laying down tertiary dentine using odontoblasts. It cannot lay down extra enamel, only dentine. this is limited to the existing matrix as it can only "close-off" tubules, or shrink the pulp by walling itself in bit by bit. We lose our enamel producing cells (ameloblasts) during tooth formation, so enamel can NEVER be regrown, except in the case of pathology (odontoma, dentigerous cysts, ameloblastomas aren't mineralised but have those cells). It can be "repaired" however.

Mineral loss is in a dynamic equilibrium. Calcium and Phosphate ions in your saliva will constantly replace minerals lost, with caries and erosion with constantly removing minerals. When the balance swings towards minerals lost you get decay. You can swing it in your favour by adding fluoride to your oral environment, which replaces minerals in your teeth as well, removing the biofilm causing caries (brushing/flossing), and reducing acidity in your diet.

given that your enamel is dynamically replaced, it is better than any filling material, which will degrade over time.

TL;DR: The outer layer, enamel, can be repaired. The second layer, dentine, can be bulked up by the nerve. Both are limited to existing structure and cannot form outside what already exists.