The blood brain barrier has a transporter that transports amino acids into the brain. When you eat protein your blood becomes full of amino acids. These amino acids out compete access to those brain transporters over tryptophan. Tryptophan is the only amino acid that isn't taken up by muscle cells. When you eat carbohydrates your body releases insulin. This insulin pushes both glucose and amino acids into muscle cells. If you've eaten too much protein, insulin will not be able to push all the amino acids from your blood into your muscles. Tryptophan will enter the brain after you've exercised the blood proteins away or you've eaten enough carbs to put out enough insulin. Too much carbs not enough exercise too much protein = no brain trytophan, no niacin production, too much insulin, insulin desensitization, diabetes