The same diet should be adequate for all individuals of the same species. An adequate diet allows individuals to develop fully. On the other hand, development can be stunted through (generational) epigenetics, with chronic deficiency for example.
Take a look at the various photos: http://journeytoforever.org/farm_li...e/pricetoc.html
In every example of traditional populations where they eat their traditional diet, they all have excellent features, nothing out of place, no particular deformation. And in every example of the same traditional populations that eat a modern diet, we can instanly recognize certain traits we see regardless of body type or culture or whatever. Overbite for example does not exist in traditional populations, unless they eat a modern diet. We eat a modern diet, overbite is common. Dental arrangement is due to skeletal differences, and this is due to (generational) epigenetics - the effects of a modern diet on a developing human.
I put (generational) in parenthesis because epigenetics also occurs in the same lifetime, but with different effects. For example, an adult who adopts a modern diet will suffer, but won't get the skeletal deformations seen in children. These deformations are due to the effects of a modern diet on growth, and the most growth occurs during a child's growth over several years.
Now imagine certain traits and differences that don't stand out much, but when affected by epigenetics, get amplified. Compare overbite and crowded teeth. Normally, neither exist, but when amplified through epigenetics, subtle differences in teeth shapes produce significantly different overall dental arrangements. Now imagine the same thing happens with other tissues and organs, we get what we call body types. Normally, body types don't exist, we're all pretty much designed genetically to grow into the same overall shape - human. Think of any other species, all individuals of the same species look pretty much the same, it's hard to find any special difference in body shape. The exception is animals we use for various purposes, where we breed them for or against certain traits.
The point is, every example of bad health and deformations in that book is due to the same thing - a modern diet. How is it possible that the same cause produces so many different outcomes in so many different individuals? It's the small and subtle individual differences that get amplified through epigenetics, especialy chronic deficiency or poisoning, especially in children.
Conversely, every case of bad health due to a modern diet, the same positive effects are produced by the same diet - a low-carb diet. Why? Because a low-carb diet removes the thing that causes deformations and bad health for all humans - sugar and wheat primarily, carbs generally.
As a side note, generational epigenetics make us more sick with every new generation, ending with sterility in the last generation. Accordingly, if we compare two individuals of different generations (one of first generation, the other of third generation, for example), we will see differences with insulin sensitivity, or rather with the current poisoning level. The subtle differences will be amplified further with each new generation. Also, the degree or level of chronic deficiency or poisoning will affect each new generation accordingly, so that a lower level will correlate with better health and vice versa. Then, health correlates with fitness, better health better fitness.
With insulin and obesity, insulin makes us fat. The more insulin, the fatter we are. With each new generation, the more insulin, the fatter our children. With body types, we consider fuel partioning, where the level of insulin determines where energy goes - to lean tissue or to fat tissue. If more goes to fat tissue, less goes to lean tissue. Overall growth, especially bones, will also be affected this way, so that obesity also correlates with shorter stature. Other hormones, especially gender hormones testosterone and estrogen, also determine specific tissue distribution.
Everything that determines our shape, is affected by (generational) epigenetics.
That's just how I see it, not necessarily how it actually works.