For the followers of the Paleo diet, the one that allows you to eat mainly meat, with a few carefully chosen vegetables and fruits thrown in for garnish, it might be instructive to read about the exploits of a famous, infamous devourer of only flesh: Attila the Hun.
Attila is considered one of the most ferocious warriors in history.  Born in 406 A.D. in what is now Hungary, he and his brother (until Attila killed him) ruled over the tribes of the Hun kingdom. But soon his conquests went far beyond being the ruler of the neighborhood. Known as the Scourge of God, Attila attacked, devastated and conquered lands from the Black Sea to the Mediterranean with a ruthlessness that made his name synonymous with terror and savagery.
Could his Paleo-type diet have anything to do with his successful, murderous objectives? According to eyewitness accounts of those who dined with him, and as reported in Gibbons' The Decline and Fall of the Roman Empire, Attila followed the Paleo diet:
"The royal table was served in wooden cups and platters, flesh was his only food, and the conqueror of the North never tasted the luxury of bread." 
Many in the 21st century will applaud Attila's diet, although not many might want him as a neighbor. "Down with bread!" these nouveau-Paleos will say. "Out with any carbohydrate! Dump the dairy, bump the beans, discard most fruit and vegetables and eat animal protein or perish!" Adherents of this diet, cleansed of the evil nutrient carbohydrate, claim unfettered amounts of energy, a lean strong body and an alert, mentally-active brain.
Attila died at 47, but the circumstances of his death are unclear. He either died from an unstoppable nose bleed or just as likely was poisoned or stabbed to death. But it is also possible that his "flesh" diet may have contributed to his early demise.
A highly-respected clinical researcher, Dean Ornish, in a recent article in the New York Times, elegantly described the health hazards of a predominantly animal protein diet. His research confirmed the dramatic improvement in cardiovascular health from eating high-fiber carbohydrates along with vegetables and fruits and eating only small amounts of animal protein.  Might Attila have died from an early heart attack?
Perhaps his cognitive abilities were impaired by his diet, so he did not take precautions to prevent himself from being murdered. A study published this March by the journal The Lancet  looked at changes in cognition, including executive function and mental alertness, among 2,654 Finnish subjects ages 60-77. Half were told to stay on their normal diet that was relatively high in animal protein, and the test group was given a diet in which over half the calories came from complex carbohydrates, fruits and vegetables, and minimal consumption of animal protein and fat. Those eating a diet somewhat similar to that proposed by Dean Ornish in this NYT op-ed did significantly better on tests of cognition such as executive function and mental alertness.
So did a diet only of animal meat leave Attila a little loopy?
Or maybe he was in such a bad mood all the time that he became impossible to live with. It is said that he died on his marital bed. (Lucky bride?) A zero carbohydrate diet all his life could have done a job on his serotonin levels. We know from decades of research on how eating carbohydrates indirectly causes serotonin to be made.  Without carbs in his diet, his brain may have had so little active serotonin that he was depressed, angry, or just apathetic about conquering more lands. Those familiar with two pesky mood problems, PMS and/or winter depression, know the feeling of fatigue, irritability, depression and anger associated with too little serotonin activity.
Of course, Paleo man and Attila would not have known that even though protein contains tryptophan, the amino acid out of which serotonin is made, very little tryptophan gets into the brain when protein is eaten. It took several more centuries for this to be discovered by some carbohydrate-eating scientists. 
But maybe we are judging Attila too harshly. Gibbons says that bread was a luxury. Could it be that Attila had tired of his Paleo diet and his conquests were driven by the need to find a good loaf?
2.) Gibbon Edward, The Decline and Fall of the Roman Empire, 1826 Vol 3. Chapter 28, p.210