Monosodium glutamate is pure free glutamate. When you eat it or drink it (it's often added to packaged soups) it is quickly and readily absorbed. In your rice protein shake the glutamate is mostly bound (still linked to other amino acids). This slows down the absorption of the glutamate. But, you are still ultimately ingesting that much glutamate, and depending on your personal brain chemistry may cause problems for you. Some people can be crazy-sensitive to glutamates and glutamate analogues (like theanine). When I was on lipitor (which blocks CoQ10 synthesis) and still had my amalgams, any amount of free glutamate (MSG or high-glutamate flavorizers) would tip me over. I'd get woozy, an instant headache, slur my speech, get cold feet, suffer insomnia, or have wild hallucinations if I did get to sleep. Not a good scene. There is also evidence that when glutamate and mercury are simultaneously an issue they act synergistically on certain receptors in the brain.

One final note, drinking protein shakes will also increase the amount of sulfur-bearing amino acids in your system, which can cause issues for some folks with mercury poisoning.