The Paleo diet mantra is that you should eat unprocessed foods in a bid to live as virtuously as our hunter-gatherer cavemen ancestors did - so that means absolutely no sugar, processed meat and simple carbs.
But there's no getting around human nature, and the fact is that our predecessors were sugar-whores long before the arrival of fizzy drinks and chocolate bars.
New findings have revealed that eating acorns and pine nuts - the caveman equivalent of junk food - caused tooth decay.
Teeth from 52 skeletons dating back between 13,700 and 15,000 years showed evidence of widespread decay, with only three individuals showing no sign of cavities.
The hunter gatherers pre-dated the rise of farming, which has previously been blamed for a big increase in dental problems linked to carbohydrate-rich foods.
Scientists believe the cause of their molar misery was a diet largely dependent on wild acorns and pine nuts.
Both contain high levels of fermentable carbohydrates that are especially destructive when they lodge in teeth because of the oral bacteria they attract.
Relying on harvesting abundant wild nuts may also have led the hunter gatherers to live a more sedentary lifestyle than was previously thought.
"These people's mouths were often affected by both cavities in the teeth and abscesses, and they would have suffered from frequent toothache," said researcher Isabelle De Groote, from London's Natural History Museum, who studied the teeth.
The skeletons were recovered from Grotte des Pigeons, a cave system at Taforalt, Morocco - a site containing a plethora of preserved Stone Age remains.
Together with humans bones, scientists have found charred remains of foods that would have been cooked and eaten by the cave residents.
"We use the charred fragments to identify plants that were carried back to the cave including foods items, such as acorns and pine nuts, and grasses that were used to make baskets," said palaeobotanist Dr Jacob Morales, another member of the team from Cambridge University.
Evidence of decay was found in more than half the dental specimens examined, with only three of the skeletons having cavity-free teeth.
The results, published in the journal Proceedings of the National Academy of Sciences, were unexpected because tooth decay is strongly associated with agriculture.
Generally, tooth decay affects up to 48% of farming populations, but between zero and 14% of hunter gatherers.
Dr Louise Humphrey, a human origins scientist at the Natural History Museum, said: "Reliance on edible acorns as a staple food could account for the high caries (tooth decay) prevalence at Taforalt, since frequent consumption of fermentable carbohydrates is a key factor in the initiation and progression of this disease.
"The acorns may have been boiled or ground to make flour. Cooking the acorns would have added to their stickiness, and abrasive particles from grindstones contributed to rapid tooth wear so that caries started to form on the roots of the teeth."