In a quest to better understand calcium supplementation I found and read this article Influence of the calcium content of the diet offered to leopard tortoises (Geochelone pardalis) where the tortoises were fed a calcium poor diet.  Unfortunately, the authors did not actually test their “as fed” diet or even offer a calculated amount of calcium as their starting point.  They did however suggest their ‘as fed’ quantities and evaluated the effect on the tortoises with a special bone density measuring device and a few post-mortem examinations.

The high, low, and average calcium of the base diet in grams per kilo is carrots at 33gm/kilo, iceberg lettuce at 18gm/kilo, sweet peppers and tomatoes tie for 10gm/kilo, and “hay” guessed at 2gm/kilo for an average that is 14.8gm/kilo – assuming the diet constituents were all fed in equal proportions.  Using that same assumption, the calcium phosphorus ratio is 1:0.9 of the foods, but gets tilted to phosphorus with the use of the supplement to an end ratio of 1:2.4  .  This ratio is considered poor and would lead to metabolic bone disease even if all other nutrition was exemplary.

There are four groups, one group fed the diet as described with a no-calcium supplement, and then three more groups with higher amounts of calcium offered along with the no-calcium supplement.  These are group 2 – 2.9 grams more calcium/kilo, group 3 – 8 grams more calcium/kilo, and group 4 – 23.3 grams more calcium/kilo making the respective C:P rations group 2 – 3.9:2.4, group 3 – 9:2.4, and group 4 – 24.3:2.4.   Normalized for calcium group 1 – 1:2.4, group 2 – 1:0.61, group 3 – 1:0.26, and group 4 – 1:0.01.

The authors found sign of MBD in groups 1 and 2, and found calcium hardening soft tissue in group 3 and 4.  The best growth in terms of bone mineral density was group 3.  I infer that the highest level of calcium diet may have caused nutrient competition, but that is speculative on my part.

Some of the take-aways for me in reading this paper…

  • Most offered diets start with a better C:P ratio and overall nutrient balance.
  • Calcium supplementation should not be every day, but staggered with days of no additional calcium. I think this allows for some mechanism/pathway for the removal or mitigation of ‘too much’ if indeed that happens, and nutrient competition would then, be reduced.
  • That every other day or three times a week supplementation would land at about 4gm calcium/kilo of ‘As Fed’ diet. To put this ratio in terms of common kitchen use, that’s near 1 heaping teaspoon of calcium carbonate powder for each two heads of chopped romaine (about 6 grams of calcium for 1000 grams of chopped greens, or 6 gms/kilo.)  or a heaping 1/8 teaspoon per head of greens.  Assuming most people feed higher calcium items than just Romaine with a C:P of 28:26 this amount should be in the range of just enough.

If offered every other day or three times a week nutrient competition should not be a problem (again speculative) and in the presence of a small pile of powder near the feeding area the tortoises can supplement themselves at-will.