I'm still trying to figure out optimal.  I was very low carb (<15 g/day) for 4-5 years and felt good, got to a healthy weight, etc.  After a while I got to the point that I would gain weight if I ate more than 1600 calories a day, my gut got really messed up, I started having food sensitivities, and I had trouble with high cortisol in the morning waking me up early feeling like a furnace, so I was unable to get enough sleep.   Raising my carbs (vegetables & fruits only.  I still don't do grains or processed food other than 88%+ and usually 92% chocolate sometimes) and upping my fiber, helped with all of those problems.  My gut isn't fully healed yet, but the other issues are  gone.  

I have verified the cortisol issues are due to my blood sugar being low in the early morning, so my body produces a bunch of cortisol to convert protein into glucose to fuel the parts of my body that need glucose.  Overall, I feel a little better with more carbs (but still much less than the standard American diet).  I agree 100% on avoiding grains, seed oils, and processed food, although I'll admit to breaking down and eating sushi (including rice) up to 4x/year.