LEP and dementia: Leptin-deficient rodents display several brain structural abnormalities, which can be improved with external leptin administration.51 In humans, increased leptin levels in middle age have been associated with a reduced risk of dementia in non-obese people.52 On the other hand, in groups of subjects with a wider range of BMIs, elevated levels of leptin have also been linked to brain volume deficits,53 perhaps due to leptin resistance.