There are many views on this subject and I invite comments from the members.
The latest views are that Athletes should eat 40-50% carbohydrates, 20-30% proteins and 20-30% fat. Recommendations are also that meat or eggs are taken with every meal. Lots of fruit and vegetables are recommended, while avoiding sugars and starches.
Personally, I believe in a high protein diet. I find lots of fruit taken approximately two and a half hours after a main meal, keeps the digestive system working well. On early morning rides, I never ride on an empty stomach. I divide breakfast into two parts, one before the ride and the second after the ride. Sometimes, but very carefully, I would supplement with Whey Protein, as it takes the appetite away and helps keep weight down. One has to be careful though, as too much protein can also increase your weight.
50% view says if you eat healthy and a wide range of foods you don't need any. The other 50% states supplements are good. They can't do any harm, provided the dosage is not excessive and it may be good insurance. A lot of endurance athletes use vitamin C, E and magnesium. A good protein and carbo drink, after a race or hard long ride, speeds up recovery.
An excess of protien or minerals will cause problems with kidneys and the liver, but the excess will have to be pretty severe. At most it may lead to kidney stones with excess minerals. Urinary tract problems can come about also.