"[N]o less mischief happens to a man from unseasonable depletion than from repletion... to the bulk of mankind it is of little consequence which of these rules they observe... whether they make it a practice to take one or two meals [per day]. But there are certain persons who cannot readily change their diet with impunity; and if they make any alteration in it for one day, or even for a part of a day, are greatly injured thereby."