We can absorb L-tryptophan though our digestive system. Since nearly everything we eat comes from forms of life, nearly everything we eat contains at least a trace of L-tryptophan. Some things we consume do contain larger amounts of L-tryptophan than others. The best sources of L-tryptophan from food are meats (especially turkey and chicken), dairy products (especially cottage cheese and milk), brown rice, peanuts, almonds, soy products (soy contains 5 times as much as oats) and green leafy vegetables. According to some information I've seen from several sources, humans can't produce L-tryptophan - we have to consume it. That is why it is sometimes called an "essential" amino acid. Apparently, since L-tryptophan has been widely available to us, our human bodies have evolved to just rely on getting it from our environment rather than forming it inside our bodies. This is not unsual. It is the same situation for humans with vitamin C. It is the same way for cats with taurine.