Here's the abstract:
In this study, betaine [TMG] content in cereal grains, cereal-based products, gluten-free grains and products of mainly local origin was surveyed. Estimates of betaine are currently a topic of considerable interest. The principal physiologic role of betaine is as an osmolyte and methyl donor. Inadequate dietary intake of methyl groups causes hypomethylation in many metabolic pathways which leads to alterations in liver metabolism and consequently, may contribute to numerous diseases such as coronary, cerebral, hepatic and vascular. Cereals are the main sources of betaine in human diet. Results showed that betaine content in grains is variable. Spelt grain was found to be a richer source of betaine (1848 mg/g DM) than that of common wheat (532 mg/g DM). Gluten-free ingredients and products were mainly low in betaine (less than 150 mg/g DM). Amaranth grain is a remarkable gluten-free source of betaine (5215 mg/g DM). Beet molasses is an ingredient which may increase betaine content in both cereal-based and gluten-free products.
I highly recommend reading the entire paper if you're at all interested in this topic. I'll pull out what I consider the highlights:
Rice contains no TMG. Corn, millet, and buckwheat have a little TMG, and oats and barley have a little more. Good sources of TMG include wheat (spelt wheat is best, and bread wheat has more than durum) and rye. Amaranth and quinoa have very high levels of TMG, as does beet molasses.
I pestered a Belarussian relative recently to find out whether she eats a lot of borscht and beet greens. She said, no, that's more her mom and aunt who do that, but she drinks kvass often. Kvass is a rye bread-based beverage popular in eastern Europe. Rye has about four times the amount of TMG as wheat. Sadly for me, kvass is mildly alcoholic, and I don't drink alcohol for religious reasons. (I'd love to play with a reverse osmosis filter that could allow me to make my own de-alcoholized drinks, but from what I read online, such an apparatus is hard to come by.)
Amaranth used to be a very popular grain in Guatemala, Mexico, and Peru. Amaranth has been becoming a regularly-consumed pseudocereal again in the past couple decades, but it's happening very, very slowly. I started putting a tablespoon full of amaranth in with my batch of rice in the rice cooker in order to get more TMG in our family diet; my family doesn't notice the addition.
I've liked quinoa since I was sixteen. I think my dad was introduced to quinoa while on a trip to Machu Picchu, and he ate it like it was couscous, complete with canned spaghetti sauce over it. It tastes quite good that way, especially with some grated cheese on top. In the Andes, quinoa is mostly eaten in soups and porridges. I think it's unfortunate that we tend to drain our quinoa here in the USA, for that disposes of the TMG that leaches out into the cooking water.
Beet molasses is marketed as a bread topping in Germany. A German friend brought me some a couple of weeks ago, and it is quite tasty. I'll have to look for it in our local German deli/market after I run out. Which will be in about 2-3 more weeks, based on how quickly we're eating it. It's rather like pancake syrup, but one feels healthier eating it due to the knowledge that it is high in TMG.