Individuals require different diets and lifestyles to achieve maximum health and wellness. This idea can be found in the wisdom of many holistic medical systems throughout the ages including Ayurveda and Traditional Chinese Medicine. It is also manifest today in various systems based on bio-chemical and genetic markers: Blood typing, Genotyping, Metabolic Typing, etc.
As the Roman philosopher Lucretius famously noted, one person's food is another person's poison. In Middle Age England, it was said Jack sprat could eat no fat, his wife could eat no lean. Somehow in our society's seemingly endless quest for the one correct diet, this notion has been lost.
But as bio-chemist Roger Williams illustrated in his landmark text "Biochemical Individuality," healthy individuals have tremendously varying nutritional needs. As far back as the 1950's Williams decried medicine's trend towards increasing homogenization, asserting there is no such thing as an "average person". In the data he presented, blood levels of nutrients (for example, Vitamin C) varied between healthy individuals by as much as 20-30 fold.
The human body is incredibly creative and often able to adapt to a variety of situations. Yet adapting isn't the same as thriving. The goal of any system of therapeutics should be to help people reach their greatest health potential.