This information is intended only as a rough guide, for a true understanding of the dietary requirements and effects we would need to follow several more scientific paths, which is beyond the scope of this document, Here we will only touch the surface in a more generalised approach.

Importance of a correct diet

As with all animals a correct diet is essential to its health and well being, An incorrect diet maybe an excess or lack of vitamins, minerals, trace elements, etc... maintained long term this could lead to illness, disease, major organ failure, paralysis, and eventually death.

One of the many reasons why an incorrect diet is discussed so often in conjunction with tortoises is the fact that we are still hearing cases of family pets being feed a diet of tinned cat food, banana, chocolate, bread, and so on, with the vast amounts of published research  that now exists there is no excuse, we can only presume it to be a lack of research by the owner.

Calcium to Phosphorus Ratio

In all food items a Ca:P ratio can be calculated, and the suggested balance for tortoises is around 2:1 possibly as high as 4:1 in their native foods. 
In order for our tortoise to continue to grow its shell and bones properly we need to maintain a positive Ca:P ratio, not everybody has a laboratory at their disposal to check the ratio on all the food items they offer so we have to rely upon research by others for tested food items this will give us a general idea to select the positive ratio items and additionally add Calcium.

The most popular method of adding additional calcium to food items is in the form of cuttlefish bone, many pet shops stock this for cage birds, some keepers prefer to soak this cuttlefish in water and dry it before hand to reduce the odour.
Prior to feeding the tortoise food can be dusted with cuttlefish bone scraping the back of a spoon across it, this is a good way of supplying additional calcium and one that can be used every feed, additionally you can offer chunks of the cuttlefish to allow the tortoise to consume extra at will (we have had tortoises that like it so much they have eventually ware their beaks down).

" Cuttlefish bone used for additional calcium "

Although having high calcium balanced food items alone is not the complete answer we also need an adequate supply of vitamin D3, without this our tortoise will be unable to utilise the Calcium intake, there are two recognised ways to provide this and a combination of both is generally practiced, Firstly via UVB rays either from unfiltered sunlight (not likely here in the UK!) or specially designed lights which omit the UVB rays, the other method is via oral supplementation (see Supplements)

Supplements

" Multivitamin used as a supplement "

Dried Foods

In recent years there has been an array of 'formulated tortoise food', Again these can have their own benifits especially when wild foods are unavailiable or out of season, these are readily availiable and purchased off the shelf all year round.

Many of these products contain a nutritial information section on the pacakaging which can be very useful in determining the correct supplementry diet combination and as always make sure you read the packaging carefully before using, its also advisable to always have plenty of fresh drinking water which can help rehydrate the food if it starts to dry.

" Dry Foods "

Recommended - Wild Plants as Food Items

Conclusion

Remember: An adult tortoise can survive many years on an incorrect diet, but a younger tortoise will be affected far more rapidly without the correct balance of the required elements.