The two main results of our study were obtaining similar values in BMD determined by two radiomorphometric indices in mandibles between two populations, one medieval and the other current. In addition to similar values, there were only three cases in the medieval samples versus two in the current population with a diagnosis of a higher risk of osteoporosis according to MCW.

The second result was that δ15N levels showed a statistically significant inverse correlation with the degree of bone mass.

To the best of our knowledge, there is no scientific literature on this type of methodology for the determination of BMD applied to osteological populations. However, bone mass loss has been studied in ancient populations using other techniques, mainly densitometry, the gold standard for the diagnosis of osteoporosis [22, 23], in 185 femurs of a medieval Norwegian population, obtained a higher BMD in the rural population compared to a population of monks from a monastery in the area. The authors related it to greater physical activity of the former.

In an ancient population of Southern Patagonia, also showed a reduction in BMD by over 25% in female over 30 years old by densitometry and related it to food deficits, pregnancies, and lactation [24].

On the remains of six elite Moche characters in Peru, showed by densitometry a decreased bone mass density (in the II and VII centuries AD) [25]. The authors' conclusions were that the sedentary lifestyle of these noble members would have more influence on the loss of BMD than their diet.

All these investigations can provide information on the role of extrinsic factors in the appearance of osteopenia and osteoporosis in these past societies. With our study, we demonstrate, with different methodology, how BMD is similar in two populations in southern Spain, separated in time by almost 900 years. Even in the case of a medieval rural population with totally different eating habits and social life, this did not influence a different BMD.

One aspect to consider in archeological bone remains, when comparing their characteristics, would be the problems of correctly assigning the estimation of the subjects’ age. In the case of BMD, the positive correlation between increasing age and bone mass loss is well known [26]. To avoid this bias, we dated the mandibles according to the changes observed in three different types of bones of the skeleton, according to [16], and estimated an age range.

We then matched the age to compare them with current mandibles. Another aspect in our favor is that the measurement of the two radiomorphometric indices, unlike the authors who described them [11, 13] who did it manually with rules and calipers, was carried out using a digital image program (Image J®) that ensures more accurate and reliable results. Our medieval population, sharing the opinion of previous researchers, although from a poor social class, with a deficient diet, could present greater physical activity, which, in terms of bone mass, would compensate for possible nutritional deficiencies compared to the current population, where a more complete diet would balance the excess of sedentary lifestyle characteristic of modern civilization.

Plants use atmospheric CO2 (12C) in three radically different photosynthesis routes, this implies the appearance of different fractionations in carbon isotopes (12C and δ13C), which once known, show us the types of plants and the parameters organic ingredients used in each human diet [27]. Therefore, depending on the results for δ13C, we will distinguish a diet rich in C3 plants from consumers of C4 plants that are even more enriched in 13C [28]. The C3 plant group is represented by plants typical of temperate and cold climates (wheat, barley, rice…), while the C4 plants are typical of tropical, savannah and dry climates (millet, corn, sorghum, sugar cane…) [29].

The results obtained in the Nasrid population with respect to this 13C isotope show that the diet followed by this community, surely based on carbohydrates, did not influence their BMD, as demonstrated by the total absence of relationship between both variables.

Regarding the isotopic ratio of δ15N, higher figures are indicative of the consumption of foods rich in animal proteins, lower values, in principle, would indicate the consumption of foods of vegetable origin [30].

An excessively rich diet in terms of its protein content can alter bone health. Protein intake is related to increased bone mass in childhood and adolescence [31, 32]. However, in older adults, an inverse relationship has been shown between the amount of protein in the diet and bone mass [33]. A diet very rich in protein of animal origin has been related to a decrease in bone mass, especially in individuals with poor calcium intake [34]. Hyperproteinemia would have a metabolic acidosis as a mechanism of action, which in turn would cause greater bone resorption and, as a consequence, hypercalciuria and/or an increase in glomerular filtration of calcium at the renal level [26].

In our study, we have found a negative correlation between the values of δ15N and the radiological indexes of bone mass. These data corroborate the first finding of this study, dealing with a rural population with a normal BMD far removed from a hyperproteic diet. The consumption of cheese, cottage cheese, eggs, and meat (protein diet), in older adults, would cause metabolic acidosis, bone resorption, hypercalcemia with hypercalciuria, and an effect on their bone mass [26]. In our study, one woman and three adult men, who presented the highest values of this isotope, corresponded to a possible diagnosis of osteoporosis according to the radiomorphometric indices. A prominent aspect in the Muslim world, which could explain these cases of possible osteoporosis, is the ingestion of vegetables rich in oxalates (spinach, artichokes, cabbage) [35], which would induce by chelation, a decrease in the intestinal absorption of calcium and therefore, hypocalcemia, secondarily the parathormone (PTH) would be activated, with bone resorption and corresponding decrease in bone mass. Added to a possible deficit in physical activity, we would have the entire pathophysiological mechanism that could justify the changes in bone mass density observed in some individuals in this study.

The limitations of this study are the lack of quantification of both isotopes in the current population and the use of densitometry as a BMD quantification technique, the most reliable method. However, our study has put into practice a method, already used for screening for osteoporosis and osteopenia, which constitutes an original and inexpensive methodological advance for use in physical anthropology.

In conclusion, the bone mass density in mandibles belonging to two diachronic populations compared and determined by two quantitative radiomorphometric indexes is similar. Within the medieval population, there is an inverse correlation between the δ15N value and bone mass density, this would indicate a hypoproteic diet without repercussions on BMD.