Almost a century ago, two independent researchers, Dr. Edward Mellanby and Weston A. Price both observed the importance of diet and especially fat-soluble vitamins in the development of healthy teeth. Their pioneer work has since been confirmed by modern science.

Dr. Edward Mellanby’s observations

One of the most popular researchers of his time and the discoverer of vitamin D, Dr. Edward Mellanby and his wife, Dr. May Mellanby, have conducted many interesting experiments on the relationship between different diets, nutrients and dental health. 
In their research they found that the key to healthy teeth is proper development during growth, as teeth that are not structurally sound are much more prone to decay later in life. 
The abnormal development of tooth enamel has since been termed hypoplasia, and it is well known that inadequate micronutrient intake plays a role in its development.
Dr. Edward Mellanby identified the following factors as the most important influences on tooth development:

• Mineral and fat-soluble vitamin content of the diet
• The amount of substances that inhibit mineral absorption, such as phytic acid

To keep our teeth strong, it is important to have enough minerals, especially calcium and phosphorus, which we also need vitamin D to absorb. Phytic acid and other "anti-nutrients" reduce the absorption of minerals, so when their intake is high, the minerals we need are all less available. 

Can diet help reverse dental caries? 

Teeth, like our other tissues, are capable of some degree of regeneration if given the necessary nutrients and conditions. Weston A. Price observed that the teeth of people in traditional societies were very well developed and healthy, and almost completely free of decay - despite the fact that they did not brush and had no access to the dental treatments of the time. 
As soon as modern, processed foods such as white flour and granulated sugar appeared, their teeth began to deteriorate rapidly, much like those of Westerners. When, for some reason, the trade was curtailed for a while and they returned to their traditional diet, the health of their teeth began to improve again.  
Edward Mellanby writes in his book, "It has long been known that when enamel and dentin are damaged by wear or decay, the teeth react to the damage, resulting in the deposition of a secondary dentin over time."
His wife, May Mellanby, began to study this phenomenon in dogs and found that the amount of secondary dentin deposited in the animal's teeth in response to wear could be regulated, and was independent of the original tooth structure in both quality and quantity. 
When the dogs were fed a diet rich in vitamin D, calcium and phosphorus during wear, the new secondary dentin formed well, regardless of the original structure of the teeth. In contrast, a diet rich in cereals and low in vitamin D resulted in poorly calcified secondary dentin, and this occurred even when the primary dentin was properly developed.

The Mellanby's experiments on children

Upon seeing positive changes in dogs, they tested the effect of different diets and vitamin D on dental caries in young children. (3)
In their experiments, they divided the children into 3 groups:

• The first group was given extra oatmeal (because of its phytic acid content) in addition to the traditional diet.
• The second group received vitamin D in addition to the traditional diet
• And the third group was on a low phytate diet free of cereals, supplemented with vitamin D

The results were quite astonishing: children who followed a vitamin D-free diet with oatmeal developed nearly 6 new caries over the 6-month trial, compared to just 1 when vitamin D was added to their regular diet. 
However, the best result by far was achieved with the grain-free diet supplemented with vitamin D: on average, only 0.3 caries teeth were formed, or 1 in every 3 children. 
That's not all, as the children in the trial had a lot of decayed teeth at the start of the trial, so they also looked at how diet affected the condition of existing bad teeth. All teeth deteriorated on the oatmeal diet, while the dentin layer thickened on ~3.9 teeth on the vitamin D supplemented diet. Again, the grain-free diet supplemented with vitamin D proved to be the best, improving the condition of ~4.7 teeth. 
Interestingly, all of these diets contained some sugar, jam and fruit, and still managed to improve the condition of the teeth. 

Weston A. Price’s similar experiment

Inspired by the excellent dental health of the traditional tribes, Weston Price tested on poor children whether their caries teeth would improve if they were given a diet rich in minerals and vitamins every day. 
The meal consisted of lots of cooked vegetables, fruit and meat, and a roll made from whole meal flour. As a supplement, the children were given butter extract, which is very rich in fat-soluble vitamins, and cod liver oil.
Although this diet was not grain-free, the flour was freshly ground, and the much longer leavening time at of that day and age probably degraded much of the phytic acid in the bread. It is also possible, of course, that simply replacing the fat-soluble vitamins was the more important factor. 
Price has also confirmed with X-rays that the children's decayed teeth were strengthened by the new diet. 

Modern research on the relationship between vitamin D and teeth

Since the discovery of vitamin D's role in calcium absorption, it has been used in an increasing number of studies to prevent and treat tooth decay.
A 2013 meta-analysis  based on pooled results from 24 clinical trials found that vitamin D supplementation reduced the risk of tooth decay by 47%. (4) In practice, this meant that children who took vitamin D had half as many cavities as those who did not. 
In a 2015 observational study, every 10 nmol/L increase in vitamin D levels reduced the risk of tooth decay by 11%. (5)
A meta-analysis published in 2023 found that vitamin D deficiency increased the risk of dental caries by 22%, compared with 62% in observational studies alone. (6) It would have been interesting to have also examined vitamin D levels above the 50 nmol/l threshold.  
And a follow-up of a randomized trial  demonstrated the benefits of vitamin D supplementation during pregnancy, showing that the health of a child's teeth is influenced by the vitamin intake of the pregnant mother; (7) half as many children had poorly developed teeth when the mothers supplemented with high doses of vitamin D.

Summary

Modern research confirms the pioneering work of Mr. and Mrs. Mellanby and Weston Price, who have shown that vitamin D plays a crucial role in the development of healthy teeth and the prevention of tooth decay. It's also worth paying attention to your intake of fat-soluble vitamins A and K1, as they also play a role in bone and tooth health in combination with vitamin D. 
In addition to vitamin D, of course, a mineral-rich, natural diet is essential, avoiding grains, legumes and other phytate-rich foods, or including them only in traditional ways such as soaking, sprouting, fermenting and thorough heating. 
It may also be important to eat harder, more chewy foods from time to time, as physical stress strengthens teeth just as it does bones and is also necessary for a strong and healthy jaw.
Although it is probably much harder to reverse tooth decay in adults, it is safe to say that these strategies, along with thorough dental care and regular dental check-ups, may help to keep our teeth healthy. 
The information in this article is for early-stage caries and is unlikely to work for advanced caries. If you try them, do so in consultation with your dentist