It's pretty much an observation. Not under control not clinical supervision. Following a sick elderly woman for 6 months est. and observe the effects of the change in diet on her health.https://pmc.ncbi.nlm.nih.gov/articles/PMC4921549/
This is a pretty interesting one subject intervention study.
We do not know her definition of healthy American diet. Just like some saying our hawker food is healthy, it does raises some questions too.
It's definitely a great improvement moving from the standard American diet or our hawker type food to a whole food plant based food.
However, it will come to a point when deficiencies manifest. Takes many years. That's how resilient our bodies are. Probably That's why many vegans and vegetarians gave up..