1. A composition on basis of soybean ingredients comprising (a) isolated soy protein  (b) soybean fibres, wherein  i) the amount of isolated soy protein is such that the protein content provides at least 15 percent of the total energy content of the composition, ii) the weight ratio between (a) and (b) is at least 2, iii) the isolated soy protein is present in an amount of at least 75 weight percent of the total protein content of the composition. 2. A composition on basis of soybean ingredients comprising (a) isolated soy protein (b) soybean fibres, wherein i) the amount of isolated soy protein is such that the protein content provides at least 15 percent of the total energy content of the composition, ii) the weight ratio between (a) and (b) is at least 3. 3. The composition according to claim 2 wherein isolated soy protein is present in an amount of at least 50 weight percent of the total protein content of the composition. 4. The composition according to claim 2 wherein isolated soy protein is present in an amount of at least 75 weight percent of the total protein content of the composition. 5. The composition according to claim 1 or 2 wherein isolated soy protein is present in an amount of at least 90 weight percent of the total protein content of the composition. 6. The composition according to claim 1 or 2 wherein substantially all of the protein is isolated soy protein. 7. The composition according to claim 1 or 2, said composition further comprising an additional protein source. 8. The composition according to claim 1 or 2, said composition further comprising a carbohydrate source. 9. The composition according to claim 8 wherein the carbohydrate source is present in an amount of less than 50 weight percent of the composition. 10. The composition according to claim 9 wherein the carbohydrate source is glucose, fructose and/or maltodextrine. 11. The composition according to any of claims 1, 2 and 8, said composition further comprising a fat source. 12. The composition according to claim 11 wherein the fat source is present in an amount of from 3 to 50 weight percent of the composition. 13. The composition according to claim 11 wherein the fat source is present in an amount of from 4 to 40 weight percent of the composition. 14. The composition according to claim 11 wherein the fat source is present in an amount of from 4 to 12 weight percent of the composition. 15. The composition according to claim 11 wherein the fat source is present in an amount of from 5 to 10 weight percent of the composition. 16. The composition according to claim 11 wherein the fat source comprises essential polyunsaturated fatty acids, monounsaturated fatty acids and saturated fatty acids. 17. The composition according to claim 16 wherein polyunsaturated fatty acids and monounsaturated fatty acids are present in an amount of from 35 to 50 weight percent of the total fat content. 18. The composition according to claim 16 wherein polyunsaturated fatty acids and monounsaturated fatty acids are present in an amount of from 38 to 44 weight percent of the total fat content. 19. The composition according to claim 16 wherein saturated fatty acids are present in an amount of from 20 to 30 weight percent of the total fat content. 20. The composition according to claim 16 wherein saturated fatty acids are present in an amount of from 22 to 26 weight percent of the total fat content. 21. The composition according to any of claims 1,2, 8 and 11, said composition comprising at least one further ingredient selected from the group consisting of a flavoring agent, a vitamin source, a mineral source, an electrolyte, a trace element and a conventional additive. 22. A use of a composition according to claim 21 in the form of a very low calorie diet (VLCD) comprising an energy content of from 400 to 800 kcal per day. 23. The use of a composition according to claim 21 in the form of a very low calorie diet (VLCD) comprising an energy content of from 450 to 800 kcal per day. 24. The use of a composition according to claim 21 in the form of a low calorie diet (LCD) comprising an energy content of from 800 to 1200 kcal per day. 25. The composition according to claim 1 wherein the weight ratio between (a) and (b) is at least 2.5. 26. The composition according to claim 1 wherein the weight ratio between (a) and (b) is at least 3. 27. The composition according to claim 1 or 2 wherein the weight ratio between (a) and (b) is between 3 and 4. 28. The composition according to claim 1 or 2 wherein the protein content provides at least 20 percent of the total energy content of the composition. 29. The composition according to claim 28 wherein the protein content provides at least 25 percent of the total energy content of the composition. 30. The composition according to claim 29 wherein the protein content provides at least 30 percent of the total energy content of the composition. 31. The composition according to claim 1 or 2 wherein the soybean fibres are isolated from the cotyledon of soybeans. 32. The composition according to claim 1 or 2 wherein the soybean fibres are present in an amount of at least 5 weight percent on a dry basis. 33. The composition according to claim 1 or 2, said composition being a powdery mixture, said powdery mixture being suspendable, dispersible or emulsifiable in a water-containing liquid. 34. The composition according to claim 33 wherein the water-containing liquid is water, coffee, tea or fruit juice. 35. A liquid nutritional preparation comprising a composition according to any of claims 1 to 34, said composition being suspended, dispersed or emulsified in a water-containing liquid. 36. The liquid nutritional preparation according to claim 35 wherein the composition is present in an amount of from 10 to 25 weight percent of the preparation. 37. A drinkable nutritional preparation comprising the liquid nutritional preparation according to claim 35 or 36, said drinkable nutritional preparation further comprising a flavoring agent. 38. The drinkable nutritional preparation according to claim 37 wherein the flavoring agent is selected from the group consisting of cocoa, vanilla, lime, strawberry. 39. The drinkable nutritional preparation according to claim 37 wherein the flavoring agent is a suitable soup flavor selected from the group consisting of mushroom, tomato and bouillon. 40. The use of a composition according to claim 35 or 36 for intravenous administration or for probe administration. 41. The composition according to any of claims 1, 2, 8, 11 and 21 wherein the composition is solid. 42. The composition of claim 41, said composition being in the form of a nutritional bar, a fruit bar, a cookie, a cake, a bread or a muffin. 43. The use of a composition according to any of claims 1 to 42 in combination with an anti-smoking nicotine preparation. 44. The use of a composition according to any of claims 1 to 42 as a partial or total diet for overweight or obese subjects. 45. The use of claim 44 wherein the composition is a very low calorie diet (VLCD) and comprises an energy content of from 400 to 800 kcal per day. 46. The use of claim 44 wherein the composition is a very low calorie diet (VLCD) and comprises an energy content of from 450 to 800 kcal per day. 47. The use of claim 44 wherein the composition is a low calorie diet (LCD) and comprises an energy content of from 800 to 1200 kcal per day. 48. The use of claim 44 wherein the overweight or obese subjects have a total serum cholesterol level of 5.7 mmol/l or below. 49. The use of claim 44 wherein the overweight or obese subjects have an increased total serum cholesterol level and/or an increased total serum triglyceride level. 50. The use of the composition according to any of claims 1 to 42 as a total or partial meal replacement in combination with a lipid-lowering medicament. 51. The use of claim 50 wherein the lipid-lowering medicament is a statin. 52. The use of the composition according to any of claims 1 to 42 as a total meal replacement. 53. The use of the composition according to any of claims 1 to 42 as a medicament for normal weight subjects having an increased serum cholesterol level and/or an increased serum triglyceride level. 54. The use of a composition according to any of claims 1 to 42 for the preparation of a medicament for lowering the cholesterol level and the triglyceride level and for increasing the HDL/LDL-cholesterol ratio in serum. 55. The use of a composition according to any of claims 1 to 42 for the preparation of a nutritional preparation, such as soy milk or soy margarine, for lowering the cholesterol level and the triglyceride level and for increasing the HDL/LDL-cholesterol ratio in serum.