"The first time I ate vegetables again I soon developed some kind of lethargy, but it was familiar, and had once been common. I realized that since I had stopped eating plants, I had not felt this at all. I thought it was just normal to eat dinner and not want to do anything for 1-2 hours? But when I only eat meat, I don’t get this. Eating plants, even just a bit, makes my stomach feel bloated— But I can eat 2lbs of steak in one sitting and feel great."

I consider myself vegetarian although not 100% strict and in the past 4/5 years I ate red meat when I wanted to try specific foods, or dishes and every single time I felt in the exact same way that the author felt when eating vegetables. When I eat red meat, I am tired, I don't sleep well, I can't exercise well and I can't concentrate. And all my (mostly) vegetarian friends feel the same way when eating red meat.

This to say that his single data point doesn't mean anything: most likely my body is not used to red meat anymore, in the same way that his is not used to vegetables anymore.