With this oxygen our life is in some sort a continual battle; we must either supply it with especial food, or it will prey upon ourselves;--a body wasted by starvation is simply eaten up by oxygen. It likes fat best, so the fat goes first; then the lean, then the brain...
A human is like a fire, you have to keep throwing fuel on it or it goes out.
The reason for this hungriness of frosty air is simply that our lungs hold more of it than they do of hot air, and so we get more oxygen a fact that any one can prove, by holding a little balloon half filled with air near the fire, it will soon swell up, showing that hot air needs more room than cold.
How fast the human fire burns is entirely dependent on the amount of oxygen that comes in contact with the lungs. Since there is more oxygen in a volume of cold air than warm air your body needs more fuel when it is cold. Put a balloon near "the fire" if you don't believe me.
The Englishman in India provokes a make-believe appetite for meat; he has no notion of changing his home-habits because he has left home a few thousand miles away; he goes to war with sun and air, eats meat abundantly; in short, stops up the grate with throwing on fuel where there is but little of the fiery oxygen to consume it, grows sickly yellow, and so pays in suffering the common penalty of ignorance.
If you move from a cold (high oxygen) environment to a warm (low oxygen) environment you need to cut back on cold weather foods, like meat. Otherwise 'fuel' will build up in your body, you'll turn yellow, and die.
--From The Illustrated London Cookery Book--
So far we have these "modern" ideas:
-The food you're eating can make you sick.
-Toxins can build up in your body.
Later on in the book they tell you the ideal food (human milk) and provide a nutritional chart to help you match it. They recognized two main nutrients, the flesh producing and the warmth giving, but you had to navigate an elaborate ratio system (each food has its own ratio of nutrients) if you wanted to eat in a healthy manner.