The types of food consumed during the Paleolithic varied with time and geography. In that sense there might have been infinite number of diets and nutrient compositions. But I am starting to conceptualise two distinct dietary modalities: one when food was aplenty, the other during times of food shortage.
It all started when I looked at mTOR signalling, which is nutrient dependent. In a nutshell, more protein (particularly rich in leucin) and carbohydrates, as well as more calories (where abundant fat would also enter the picture), activate mTOR. mTOR is responsible for growth, reproduction and learning. But it is also responsible for increased oxidative stress, ageing and cancer. When mTOR is upregulated, you grow, reproduce, learn, you conquer the world. When mTOR is depressed, you slow down, regenerate, repair your DNA, and prepare for better times.
This corresponds to seasonality with abundance occurring in late summer/early autumn, and lean times in winter/early spring. Probably higher metabolism during the summer would benefit from more sunshine and vitamin D, while dark cave would not be problem in the winter (you would have stored a few-months supply of vitamin D from the summer). Most children would be conceived in late summer, to be born just before the next summer begins. When nutritious food was easy to find your brain was working at high speed, with maximum synaptic plasticity for future survival advantage.
Now we have a choice, but we can't have a cake and eat it too. If you want to grow, reproduce and learn, you go higher protein, more occasional fruit/honey, more calories, more exercise. If you want to survive hoping to extend your life, you go caloric restriction, lower protein, higher fat (during the winter months your own stored fat would have been burnt). The optimal strategy: cycle with the seasons?
Possibly, you can beat the system a little bit towards the end of long winter. When food is scarce and little fat storage remains, one option is to intensify hunting or gathering, meaning: more exercise. This would stimulate mTOR and can possibly sharpen your brains again.
By the way, there can be other important reasons for reproducing during the summer or winter months, one having to do with epigenetic inheritance discussed in my previous post.
All this makes be think again about vegetarians. Those who stick to low starch and mild caloric restriction, being in most cases on a relatively low protein diet, can possibly live longer than the summer season Paleo eaters. Vegetarians would not be optimally fit, might suffer from gluten related conditions, but if they don't gorge on soy, their mTOR would be downregulated. Possible nutrient deficiencies can possibly augment the effect. They could probably do better on a diet based on meat and leafy vegetables with mild caloric restriction, but are still better off than well fed summer hunters. At least in terms of longevity. When it comes to growth and reproduction, vegetarianism is probably the worst option.