Keen observation can be better than modern misguided measurements. Gary Taubes writes about William Banting, an undertaker and coffin maker in Britain in the 19th Century who struggled with weight for most of his life.
William Banting tried a variety of methods to control his weight, all recounted in his Letter on Corpulence, Addressed to the Public. Nothing worked. One doctor recommended a rowing machine. He rowed like a demon, ate like a horse, and continued to expand. Finally in his 60s, he encountered Dr William Harvey, who had attended a lecture by Claude Bernard, a French physiologist who is credited with establishing the use of modern science in experimental medicine. Harvey was a keen observer who noticed that eating sugar and other carbohydrates made people gain fat.
Banting was so pleased with his new-found control of his body that he published a pamphlet at his own expense. It was wildly successful. The third and later printings were published commercially. The Letter was used as a nutrition tool for decades until 20th Century scientists used their preconceptions instead of the scientific method. Except perhaps for the amount of alcohol consumed, you could use his plan today, I think. Here’s the Third Edition–