1. A process for providing egg yolk from an egg-producing bird, the egg yolk being enriched with regard to natural anti-secretory proteins (NASP), characterised by feeding the bird with a NASP-inducing feed. 2. The process of claim 1, characterised in that the bird is a gallinaceous bird. 3. The process of claim 1 or 2, characterised in that the NASP-inducing feed contains amino acids, sugars and amides in an amount such that the formation of NASP is stimulated. 4. The process of claim 1 or 2, characterised in that the feed contains products having enzymatic activity stimulating the formation of NASP. 5. The process of claim 4, characterised in that the products having enzymatic activity are malted cereals. 6. An egg yolk having a high level of natural anti-secretory protein (NASP) containing at least 1,000 FIL units/ml obtained by the method of any one of claims 1-5. 7. The egg yoke according to claim 6, characterized in that NASP contain is between 1,000 to 10,000 FIL units/ml. 8. The egg yolk according to claim 6 or 7 originating from eggs of gallinaceous birds. 9. Use of the egg yolk according to any of claims 6-8 for the preparation of NASP-enriched functional food products. 10. Use of the egg yolk according to any of claims 6-8 for the preparation of pharmaceutical products for the treatment and prophylaxis of disease-like conditions caused by extreme body fluid discharge. 11. The use according to claim 9 or 10, characterised in that the egg yolk is used as such or in a form obtained by processing such as grinding, leaching, extraction, evaporation, membrane filtration, drying and similar standard operations. 12. The use of NASP concentrate from egg yoke according to any one of claims 6-8 for the preparation of pharmaceutical products for the treatment and prophylaxis of disease-like conditions caused by extreme body fluid discharge. 13. A food product having a high level of natural anti-secretory proteins (NASP), characterised by containing egg yolk according to any of claims 6-8 or NASP from egg yolk in a more or less purified form. 14. The food product of claim 13, characterised in that the high level of NASP has been obtained by mixing into the food product, when preparing the same, NASP-enriched egg yolk according to any of claims 6-8 or processed forms thereof as source of NASP. 15. The food product according to claim 13, characterised in that the high level of NASP has been obtained by adding to the food product, when preparing the same, NASP concentrate from NASP-enriched egg yolk according to any of claims 6-8. 16. The food product according to claim 13, characterised by consisting of egg yolk as such. 17. The food product according to any of claims 13-15, characterised by consisting of pancake, omelette, ice cream, hard- or soft-boiled eggs and the like. 18. Pharmaceutical product for the treatment and prophylaxis of disease-like conditions caused by extreme body fluid discharge, characterised by containing, in addition to customary excipients and adjuvants, natural anti-secretory proteins (NASP) originating from NASP-enriched egg yolk according to any of claims 6-8. 19. A process for preparing a food product having a high level of natural anti-secretory proteins (NASP) according to any of claims 13-17, characterised by feeding, into a food or feed, NASP-enriched egg yolk according to any of claims 6-8 or processed forms thereof or more or less purified NASP obtained from egg yolk. 20. The process of claim 19, characterised in that the processed forms of the egg yolk have been obtained by grinding, leaching, extraction, evaporation, membrane filtration, drying and similar standard operations.