This is a simplified, but updated, article including advice incorporating our latest research, experience and observations. Obtaining perfect, natural-looking growth in tortoises and turtles is a multi-factorial challenge. The keeper must balance many different aspects of diet with extraneous (environmental) factors if this is to be achieved. There is no ‘quick fix’ or ‘ABC’ method guaranteed to produce reliable results under all circumstances. Instead, it is vital that keepers understand the basic mechanisms involved. These include growth rates and activity periods, the influence of dietary fibre, and of course provision of a suitably formulated base diet for the species in question.
The consequences of getting this wrong are severe. See our in-depth article describing 'shell deformities from the inside' for a full explanation and examples.
In this updated article we look at calcium, and calcium supplementation in general. Is this really needed, and if it is, what is the 'best' form to use?
In nature, tortoises obtain their calcium requirement in several ways. The bulk of their requirement is typically supplied by consuming calcium-rich vegetation. This vegetation is itself typically growing on calcium-rich soils, a situation that leads to plants that are themselves rich in this mineral. Tortoises also obtain some additional calcium by incidental consumption of calciferous sand/soil particles when feeding, and by deliberately seeking out calcium-rich items in their environment, such as snail shells or sun-bleached bones. Surprisingly, even desert environments often have extremely large snail populations, and it is not unusual to find literally hundreds of estivating snails per square meter, and thousands of particles of broken snail shell in the same area. Tortoises have been observed to seek these out, and to consume them enthusiastically. Note that live snails are rarely consumed by Testudo graeca, more so by Testudo hermanni - the reason for this is that the live snails mostly attach to higher vegetation and only move or 'activate' during wet periods, which are very rare in extremely arid habitats. Testudo graeca therefore usually only has access to dead snail shells, or particles of these snail shells that have fallen upon the ground. They provide a concentrated, readily absorbed source of calcium carbonate. The same observation applies to Testudo kleinmanni and Testudo nabeulensis.
Examples of small snails in Testudo habitats
In most captive situations, the gross calcium content of the diet rarely, if ever, approaches that of wild diets, and the calcium-to-phosphorus ratio of captive diets is also typically far lower overall than is seen in the wild. The typical diets adopted by many keepers, which are frequently based upon commercial salads, 'vegetables' and fruit, often contain very little calcium and a massive excess of phosphorus.
Simply by modifying the diet so that it as closely replicates the wild diet as possible, many of these dietary and growth problems can be avoided entirely, at source.
Do we then still need to give a calcium supplement? The answer to that is somewhat complex as there are countless variables.
It is normally accepted that a minimum Ca:P (calcium to phosphorus) ratio is 2:1, but our own observations suggest that it is typically much higher in most wild diets. For example, a recent study done on plants eaten by Egyptian tortoises (Testudo kleinmanni) in the wild revealed that the mean Ca: P ratio was about 14:1, but as noted, there are many variables and each species, in each habitat will be different, hence why it is so hard to generalise. All we can say is that there should certainly be considerably more calcium in the diet than there is phosphorus, and by a considerable margin. A diet with a mean Ca:P ratio between 3:1 to 5:1 would appear to be a reasonable objective to aim for.
If indeed you can categorically ensure that excess phosphorus levels are avoided and that adequate calcium is present, it is perfectly possible to rely entirely upon a suitable plant-based diet without recourse to extra supplements. This does require careful planning, however, and is not easy to measure or assess either.
We can only go by our own experience here. What we have found over very many years (several decades) is that where tortoises are maintained outdoors in well-designed, well-sited enclosures with 'wild flower' type vegetation, development and bone density has been excellent without additional supplements of any kind. This may vary depending upon the soil type involved, however (we did say there are numerous variables). There are ways to try to increase the soil calcium content by 'seeding' with limestone, but that in turn is also affected by various things (permeability, rainfall, soil pH, etc.), so it is far from easy to be sure exactly what is happening over time. There are soil testing services that can shed light on this, but again, being certain is difficult. We ourselves, however, have never seen any evidence whatever of calcium deficiencies in tortoises maintained as described on suitable diets. Certainly if you live in a limestone-rich locality, there should be no issue with this. In a locality with acidic soils and low calcium levels, however, this could prove probematic and might still require some additional supplementation with the feed.
As a 'security measure', though, you might well decide to offer an additional source of calcium maybe once or twice a week. This should easily ensure an adequate level. We do not feel that daily supplementation should be necessary on a well-balanced and varied diet.
We used to suggest daily supplementation with a modest amount of calcium carbonate, but in more recent years we began to feel that more effort in correcting the base diet to the point where this is not necessary is a preferable approach. That is not to say you will cause harm by daily supplementation, you won't.
It is quite difficult to actually overdose on calcium carbonate (some would argue practically impossible), so there really is a very wide safety margin here. There is a theoretical risk of creating other imbalances, but again, we have not seen any convincing evidence of this in practice. Very large calcium intakes at a time can also affect gut pH (this is why humans take calcium-based products as an 'anti-acid' medication), but there is very little reliable data on how this might impact herbivorous chelonian digestive tracts. It could certainly be more of an issue for highly carnivorous reptiles, however, that rely far more upon digestive acids to break down proteins, e.g., crocodilians or snakes. In any event, reptiles that consume 'whole prey' normally receive a balanced diet from that alone and do not require separate supplementation. Insectivorous reptiles, however, are a different category again and cases of calcium deficiency in these are commonplace. As always, different classes of reptile, and even different species, can have very different dietary requirements. It is requisite upon keepers to understand these needs, and to meet them adequately.
Reviewing literally many hundreds of rescue and rehab cases over 40 years, it may be reassuring to know that:
All of the cases (without exception) involving moderate to severe MBD that we have encountered were associated with truly very poor diets indeed. Diets so poor or inappropriate that we would hope that most educated keepers would know to avoid them in the first place. Many of these cases also involved total deprivation of UV-B, or the use of seriously expired lamps for long periods and almost all of them involved tortoises kept indoors, either wandering around a house (certainly not recommended) or in various 'vivarium' habitats.
We have seen good results where daily calcium was provided, or where weekly calcium was provided, and also when no additional calcium was provided. To our knowledge we have yet to encounter a case where overdosing on calcium could be confirmed. That is not to say it is impossible, but it is certainly highly unlikely under normal cirumstances. Certainly too much calcium given at one time could theoretically impact the digestion, but it would take rather more than most people would normally provide, even if being 'generous'. As always, moderate and sensible use should be absolutely fine. We basically suggest "a fine sprinkling". See the main photo for this article. That is the kind of amount that we're talking about.
As a general rule, calcium carbonate is the preferred source for use as a routine supplement where required. It is both safe and effective. Limestone powder (also known as 'limestone flour') is also available at very low cost in bulk from agricultural feed merchants. There are other forms of calcium supplement available including calcium lactate (17.5% Ca), calcium gluconate (9% Ca) or calcium citrate (24% Ca) these do contain a lower percentage of calcium than calcium carbonate, but their absorption potential, or bioavailability, is considerably higher. In general, we feel that they are not necessary for routine use, but that they can be very helpful indeed when treating existing cases of calcium deficiency under veterinary direction.
Perfect carapace growth with high density bone from a wild Testudo graeca graeca (road casualty).
Some texts recommend the use of ground poultry eggshells as a calcium supplement. While it is true that these do contain useful amounts of calcium (up to 39% available Ca), and have been shown in human studies to help prevent osteoporosis in mammals, it should be noted that for chelonian use, there are a number of potential drawbacks. These include the fact that studies show that such eggshell material also contains hormone traces (with unpredictable effects upon tortoises) and that any remaining egg membrane material can also contain antibiotic traces as well as representing a potential reservoir for contamination by salmonella organisms. Eggshell derived calcium supplements intended for human use are extracted and purified under strict laboratory conditions. The use of home prepared eggshells from store-bought eggs is not recommended due to the lack of quality control over feed inputs and the potential for contamination with residues due to the difficulty of removing all membrane traces.
Cuttlefish bone is another long-standing method of providing extra calcium to tortoises, and especially to turtles, as it floats readily in water. The main chemical constituents of cuttlebone are calcium carbonate, sodium chloride, calcium phosphate, magnesium salts and an abundance of micro-trace elements. Despite its high gross calcium content, cuttlefish bone also has quite a high phosphorus content, and as such should not be relied upon as the sole source of calcium supplementation. That said, there are many trials (mostly with mammals and birds) that demonstrate very useful increases in serum calcium levels when cuttlefish bone is used vs. where no supplement is used, so there is no doubt that it can prove effective. It is also readily accepted by most tortoises. Contamination with heavy metals (especially lead) is a concern. It does however, have a long history of use and many keepers employ it regularly. It is also worth knowing that cuttlefish (in life) are quite intelligent, responsive animals. In fact, one of the most intelligent invertebrates, so there is an ethical dimension to supporting trade in their body parts.
One form of calcium supplement often sold in pet stores are known as 'turtle blocks'. These are mostly made of Plaster of Paris (calcium sulfate hemi-hydrate) combined with calcium carbonate, often in a 50-50 combination. Plaster of Paris itself contains almost no useable calcium, and is merely used to adhere the block into shape. Weight-for-weight, therefore, 'turtle blocks' contain 50% or less calcium carbonate, of which, in turn, only 40% may be bioavailable. There are again concerns over heavy metal contamination in gypsum with these producs, from which Plaster of Paris is derived. This is not a form of calcium supplementation that we therefore recommend.
Acceptance
Tortoises can be quite resistant to accepting 'new' things. This is especially so in the case of things that have a very strong taste or smell. It should be noted that commercial supplements, particularly those with multiple ingredients, are a particular problem. Plain calcium carbonate, on the other hand, has virtually no taste or smell and is much more readily accepted. It merely has a slightly 'gritty' texture, but fine-ground or 'refined' versions can avoid even this. If applied to the food as shown above, or mixed in well, and allowed to 'soak' up some smell from the leaves or flowers, most tortoises soon habituate to it.
Vitamin D3
Vitamin D3 plays a pivotal role in bone formation, allowing the body to effectively absorb and use calcium, and for maintaining the proper balance of calcium and phosphorus. A tortoise could eat calcium all day long, but if it was not getting an adequate amount of vitamin D3, it would not be able to use it properly.
In nature, herbivorous tortoises acquire all of their vitamin D3 requirements as a result of a chemical reaction in the skin, following exposure to the UV-B spectrum of sunlight. A compound is formed that is known as 7dehydroxycholesterol (this is sometimes also shortened to 7DCH or provitamin D). This in turn is converted, by means of temperature, to vitamin D proper. It is vital that both sufficient UV-B plus adequate basking temperatures are available if this process is to function properly. A UV-B fluorescent tube alone will not do this. If such a tube is used, a separate source of basking heat is mandatory. Without an adequate source of heat, the conversion will not take place efficiently. In nature this is achieved by basking under natural sunlight which provides both deep-heat infra-red, UV-A, visible light and UV-B all in one complete and balanced 'package'.
A combination of a natural high-fibre, high-calcium diet and exposure to natural sunlight which offers optimal UV-B, UV-A, visible light and Infra-Red is all that is needed to produce the absolutely perfect growth and development seen in this wild Testudo graeca graeca here in Spain.
As a general rule, if your tortoises are able to spend at least three or four hours outdoors in unfiltered sunlight daily, this should be adequate. Even in colder, cloudier countries the outdoor levels of UV-B are higher than you might expect. Alternatively the 'Climate Frame' design offers many advantages.
Vitamin D3 can also be provided in oral form. The difficulty here is knowing how effective, and especially, how safe it is. There is considerable debate on this topic. There are conflicting studies. One study found oral supplementation of vitamin D3 was ineffective in raising plasma concentrations of 25(OH)D3 and 1,25(OH)2D3 to the same concentrations observed in UV-B exposed animals, but again, some other studies suggest that adequate levels can be achieved.
Again, from long-term experience, we found that using a combined phosphorus-free powered supplement that also contained a small amount of vitamin D3 did appear to be safe and effective when used as directed (no more than a few times per week), and there was no evidence of any overdosing. Liquid D3 supplements are a different matter entirely, however, and overdosing on these is certainly possible. We therefore do not recommend their use. 'Pure' vitamin D3 may be required, under qualified veterinary direction only, when treating existing, severe cases of metabolic bone disease, but it should not be used routinely. The risks are just too great.
Ideally, the best results are almost invariably achieved under natural unfiltered sunlight combined with an as near-natural calcium-rich diet as possible, but for indoor animals with sub-optimal UV-B lighting a reputable combined calcium and D3 supplement might be helpful and should certainly be considered.
Calcium and Vitamin D3 are certainly not the only components required to facilitate healthy bone development in tortoises and turtles:
Magnesium is essential for proper calcium absorption and is an important mineral in the bone matrix. It has specific effects on the parathyroid hormone, which helps regulate proper calcium metabolism.
Phosphorus is the second most prevalent mineral in bones and makes up more than half the mass of bone mineral. Thus, the diet needs to have sufficient phosphorus in order to build healthy bones. For most tortoises and turtles, this is not a problem, as most vegetation is rich in this element. When phosphorus levels in the blood are too high, however, the body takes calcium out of the bones to bind with the phosphorus to facilitate its removal from the bloodstream. Bones can become soft and deformed as a result. This is commonly seen in tortoises that are fed on phosphorus-ruch fruits and store-bought 'vegetables'.
Many other micro trace elements are also important, including Manganese, Zinc, Boron and Strontium. An appropriate and varied diet will certainly provide these easily, but for tortoises kept indoors (which we do not recommend exclusively anyway) a wide-ranging mineral supplement can also be used occasionally to ensure that they are present. We would recommend such a supplement be used no more than once weekly or imbalances can occur.
To summarise, in order to provide the calcium that tortoises and turtles need, especially during growth phases or for egg-laying females:
Try to provide a diet that provides an overall positive calcium-to-phosphorus ratio of at least 2:1 and preferably, higher.
Carefully consider vitamin D3 requirements. Provide access to adequate levels of natural sunlight, or use an adequate and correctly installed and maintained artificial UV-B lighting source together with suitable basking facilities. If these are unavailable then a combined phosphorus-free calcium supplement with a low level of added oral D3 should be considered.
There are many variables and it is almost impossible to give 'absolute' guidelines that match every possible set of circumstances. We can safely say, however, that moderate use of a calcium carbonate supplement is not going to cause harm, and may be very helpful, and is indeed, essential in some circumstances.
Commercially available products
It would be impossible to cover every supplement marketed to reptile keepers. There are many hundreds of them. Many of the comments made in our article on commercially available packaged foods also apply here. Unfortuately the use of unproven or even damaging additives and ingredients is widespread. Buyer beware.
One area of concern is that many of these products also include a wide range of other vitamins (or even amino acids) for which no real need for routine supplementation has ever been demonstrated. It often appears to be a case of "throw everything possible in" - it looks great on the packet! Whether it is needed, or even safe, however over the long-term is highly questionable. Genuine cases of vitamin deficiency in tortoises are really very, very rare indeed (with the exception of D3 deficiency in tortoises kept indoors). A balanced, varied diet is more than adequate to provide near total protection against such deficiencies without added supplements. To put that in perspective, we ourselves have only ever encountered a very few such cases in over 40 years of handling many, many hundreds of rescues and rehabs, and in each of these cases the diet was incredibly poor and restricted to the point of neglect and abuse. We have not encountered one single case, ever, where a suitable diet has been provided.
Vionate
This is perhaps the 'grandaddy' of multi vitamin/mineral supplements. It seems to have been around forever. We used to use it ourselves many years ago. At the time it was one of the few products widely available. Ingredients: Degermed Corn Meal, Dicalcium Phosphate, Calcium Carbonate, Salt, Choline Chloride, Sodium Ascorbate, Ferrous Carbonate, Magnesium Oxide, Niacin, Calcium Pantothenate, Riboflavin, D-Alpha Tocopherol Acetate, Vitamin A Acetate, Thiamine Mononitrate, Mandanous Oxide, Cupric Sulfate, Calcium Iodate, Pyridoxine Hydrochloride, Cobalt Carbonate, Folic Acid, D-Activated Animal Sterol (Source of Vitamin D3), Vitamin B12 Supplement.
The inclusion of cornmeal is not ideal for herbivorous reptiles, but the amount consumed would be very small. Whether some of the other ingredients are actually required as a regular supplement is debatable, but as a 'general purpose' multi-vitamin and mineral supplement for ocassional use it remains a viable option. It should not be used as a promary source of calcium, however.
SF-50 Stress Formula Supplement (SA-37 replacement)
This is another very well-established product, especially in its original form, SA-37, where it was the main competitor to Vionate. Ingredients per kg: Vit A 360,000 iu, Vit D 3,3600 iu, Vit.E 2,700 iu, Vit. K3 45mg, Vit.B1 45mg, Vit. B2 108mg, Vit. B6 54mg, Vit.B12 360mcg, , Nicotinamide 720mcg, Calc.Pantothenate 216mcg, Biotin 3.6mg, Folic Acid 90mg, Copper 360mg, Iron 540mg, Manganese 1440mg, Cobalt 18mg, Iodine 18mg, Zinc 1440mg, Selenium 3.6mg, Choline Chloride 1800mg, Monocalcium Phosphate 78.6mg, Magnesium 720mg, Sodium (as chloride) 191mg, Dried Brewers Yeast 100g, Natural Limestone Flour to 1kg, Ash 55%, Fibre 0%, Oil 0%, Protein 4.6%
It offers a decent range of trace-elements and vitamins but the inclusion of brewer's yeast (a probiotic) essentially makes this completely unsuitable for herbivorous reptiles. Probiotics drastically affect the slow, fermentation-based digestion of these species. We advise against using it.
Nutrobal
One of the first 'specialist' products aimed at reptile keepers. Good primary source of calcium with very high Ca:P ratio. Ingredients: Calcium carbonate, Dextrose, Dicalcium phosphate, Sodium chloride, Aminoacids: L-lysine-monohydrochloride 3.2.3 mg 79,000. Vitamins: Vitamin A IU 600,000 – Vitamin D3 IU 180,000 – Vitamin E 3a700 IU 23,500 – Choline chloride 3a890 mg 6,000 - Vitamin C 3a300 mg 3,000 – Niacinamide 3a315 mg 3,000 - Vitamin B2 mg 720 – Calcium D-Pantothenate 3a841 mg 651.9 - Vitamin B1 3a821 mg 600 – Vitamin B6 3a831 mg 360 – Folic acid 3a316 mg 180 – Vitamin K3 3a711 mg 50 – Vitamin B12 mg 0.145 – Biotin 3a880 mg 1.2. Trace elements: Coated granulated cobalt (II) carbonate 3b304 (Cobalt) mg 50 – Calcium iodate anhydrous 3b202 (Iodium) mg 50 – Sodium selenito 3b801 (Selenium) mg 5 - Copper(II) sulphate pentahydrate 3b405 (Copper) mg 751 - Iron(II) sulphate monohydrate 3b103 (Iron) mg 3,000 - Manganous sulphate monohydrate 3b503 (Manganese) mg 1,750 - Zinc sulphate monohydrate 3b605 (Zinc) mg 3,000. Anti cacking agents: Silicic acid, precipitated and dried E551a mg 1,000.
The need for all of these vitamins as a routine, regular supplement is questionable, and even more so in the case of amini-acid additives. Dextrose is a simple sugar and would have some influence upon the very sensitive digestive processes of hind-gut fermenters such as herbivorous lizards and similar animals such as tortoises. As a calcium balancer and oral D3 supplement it has proven effectiveness, but the other ingredients now give cause for some concern.
Rep-Cal Calcium with Vitamin D3 (Phosphorous-Free Original powder)
This is another product that has been around a very long time and that has established a solid reputation with keepers. It is mainly available in the US. It has a very simple formulation: Phosphorus-free calcium carbonate from ground oster shells and a D3 additive. Nothing else.
A simple and straightforward product with nothing 'extra' or unnecessary. The only concern here is using ground oyster shells as the calcium source as there are valid concerns about heavy metal contamination (in one survey of the lead content of 70 brands of calcium supplements, more than half were found to have lead ingestion rates exceeding the safe benchmark for children). Oyster shells are very high risk in this regard.
NEKTON-Rep-Calcium+D3
Primarily a calcium carbonate and magnesium-carbonate supplement with added D3 and 'two amino acids'. Ingredients: Calcium, 50,000 I.U. vitamin D3, 500 mg iron (ferrous(II)-sulfate, monohydrate), 491 mg zinc (zinc sulfate, monohydrate), 250 mg manganese (manganese(II)-sulfate, monohydrate), 144 mg copper (cupric(II)-sulfate, pentahydrate), 20 mg iodine (calcium iodate, free of water), 16 mg selenium (sodium selenite).
Our own view is that adding amino-acids is almost certainly unnecessary and that it is hard to predict the long-term consequences. We therefore do not recommend any products that include amino-acid additives.
Arcadia Earthpro A
Described as "full spectum mineral with bee pollen" and "safe and potent". It does not include any oral D3 so is intended to be used with sunlight or UV-B lighting systems. Ingredients: , Protein Powder (Vegetable), Carrot Powder, Algal Carotenoid, Vitamin B Premix, Bee Pollen, B vitamin complex; Vitamin B1 8mg/kg, Vitamin B2 12.8mg/kg, Vitamin B6 16mg/kg, Vitamin B12 40mcg/kg, Vitamin K 4.8mg/kg, Nicotinic 64mg/kg, Pantothenic 32mg/kg, Folic Acid 4mg/kg. Biotin 800mcg/kg. Mineral Clay; SiO2 57.74%, Al2O3 18.23%, Fe2O3 3.09%, CaO 1.93%, MgO 4.19%, K2O 3.03%,Na2O 1.72%, P2O3 0.13%, TiO2 0.39%, Mn3O4 0.08%, V2O5 <0.05%, Cr2O3 <0.05%. Amino Acid complex (vegetable base) g/kg finished product; Lysine 6.36, Methionine 1.99, Cysteine 1.27, Threonine 4.93, Trytophan 1.19, Valine 5.57, Leucine 8.59, Isoleucine 4.69, Phenylalanine 5.25, Tryosine 4.77, Histidine 1.75, Arginine 4.37, Glycine 4.05, Alanine 4.05, Aspartic Acid 10.26, Glutamic Acid 9.06, Proline 4.05, Serine 4.37
We'd love to see the scientific justification and any credible 'proof of need' for these things in a regular supplement. Carrot powder? Bee pollen? Protein powder? Amino acids? We'll pass. We suggest you do too.
Arcadia Earthpro Ca (Essential Calcium)
A very simple calcium supplement with nothing else added. Ingredients: Limestone Flour 100% (Ca 39.4%).
Perfectly safe and effective. Our only comment is that you are paying rather a lot extra for the fancy packaging and marketing. You can purchase a full kilo of limestone flour for the price that this is for 100g.
Exo Terra Calcium + D3
Another fairly basic calcium + D3 supplement (though this is all that is ever really needed). Ingredients: Calcium (min): 35.0%; Calcium (max): 37.0%; Vitamin D3: 14,740 IU/lb, Dextrose.
We prefer to avoid products with dextrose, but other than this, a straightforward combination calcium+D3 supplement.
Zoo Med Repti-Calcium + D3
A long-established product. Simple and direct. Ingredients: Precipitated Calcium Carbonate, Cholecalciferol (Source of Vitamin D3).
If you need a combined calcium + D3 supplement, this is a safe bet.
IN CONCLUSION
Try to get the baseline diet as varied and as close to natural for that species as possible. This alone will avoid 95%+ of the common problems and issues seen in so many captive tortoises.
Do take into account natural behavioural and activity/innactivity cycles. These are really important in avoiding growth problems.
Provide as much exposure to natural unfiltered (not through glass) natural sunlight as possible. Even the very best artificial light/heat sources fall far short of the sun. Allow as much outdoor time as seasons and conditions permit.
A simple, plain calcium carbonate supplement once or twice a week should be more than adequate if the above advice is followed. You can provide more if you wish (it is very safe), but don't overdo it.
If tortoises are kept without access to natural sunlight, or indoors under artificial lamps of questionable effectiveness, then a combined calcium+d3 supplement may be justified. Again, this should not be necessary every single day, but normally no more than two or three times per week is required. Ideally, get the tortoises outdoors as much as possible. If tortoises are kept mostly outdoors, even in colder, cloudier climates our own experience suggests that no oral D3 supplementation is required.
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(c) 2024 A. C. Highfield/Tortoise Trust
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