Experts have voiced a longstanding concern that ketosis might fog up people’s thinking, but it wasn’t formally tested until 1995. As reported in the International Journal of Obesity article “Cognitive Effects of Ketogenic Weight-Reducing Diets,” researchers randomized people to either a ketogenic or a nonketogenic weight loss diet. Although both groups lost the same amount of weight, those on the ketogenic diet suffered a significant drop in cognitive performance.[195]
After one week in ketosis, higher order mental processing and mental flexibility significantly worsened into what the researcher called a “modest neuropsychological impairment.”[196]