dovolím si ocitovat kousek z http://www.tortoisetrust.org/articles/dietaryfibre .html
(už dříve tu zadaný P60 , který jste zřejmě ještě nečetl)

The challenge of captive diets
Keeping and breeding tortoises in captivity is very popular, and a number of conservation projects also employ the technique of “head starting” captive-bred tortoises for eventual release. Provision of adequate herbivorous tortoise diets can be challenging, requiring the sourcing of a wide range of plant materials (which may suffer seasonal lack of availability) and additional supplements. Many keepers and program managers therefore seek a more convenient way of providing a satisfactory diet for their animals. A number of manufacturers have tried to address this market with pre-packaged pelleted foods. Many of these are described as “complete diets”. Stated analysis of such foods indicates that with few exceptions the crude fibre content is typically substantially less than 20% and in some cases is below 12%. Most manufacturers also label their products to indicate the maximum fibre content, and fail to divulge either the average level or the minimum level, which can prove very misleading. Studies by Hatt, Clauss, et. al. (2005) suggest that if high rates of digestibility in tortoises are to be avoided, the crude fibre content on a DM basis needs to be in the order of 30 to 40%. As we noted in the example of grinding dry vegetable matter, the crude fibre content remains the same, but digestibility varies enormously according to the size of the resulting particles and also according to how much damage has been done to the protective cuticle surrounding the plant cells. For ease of extrusion, commercial manufacturers typically utilise fine particles and softer grade fibres. Donoghue and Langenberg (op. cit.) also point out that processes used in extrusion and pelleting involve high temperatures that partially destroy labile vitamins. Such processes typically also degrade the protective integrity of the plant cells by damaging the waxy cuticle. It has already been noted (above) that the more a whole plant part is chopped, ground, degraded or processed the greater the number of cells that will be immediately exposed to immediate microbial attack. There is, therefore, a major difference in derived energy between a tortoise consuming nearly-whole plant parts and a tortoise consuming highly processed plant parts with very much more surface area available to the gut microflora.

If highly fermentable sugars and starches are added to a mix already inadequate in particle size, the digestion will accelerate rapidly. Unfortunately, many pelleted foods are extremely poor performers in this regard as they typically include a high proportion of starch-rich food industry by-products such as corn (maize) meal, soya derivatives, oats and other grain-based ingredients. Typically comprised of finely chopped short fibres, they are rapidly assimilated and are associated with promoting extremely high growth rates, unless intake is very strictly rationed, either by quantity or feeding frequency. When combined with inadequate calcium or vitamin D3 availability this is a particularly dangerous situation, and consistently, animals reared on such regimes have a very high incidence of bone developmental disorders. A secondary effect of providing excessively digestible diets is an increase in generated blood urea, that it turn places considerable demands upon the renal system. One consequence is high rates of kidney disease and gout in tortoises maintained on such diets (Highfield, 1987, MacArthur, et. al., 2004).