Objective: To help determine whether mid-life obesity is a cause of dementia, and whether low BMI, low caloric intake and physical inactivity are causes or merely consequences of the gradual onset of dementia, we recorded these factors early in a large 20-year prospective study and related them to dementia detection rates separately during follow-up periods 0-4, 5-9, 10-14 and 15+ years. Methods: 1,136,846 UK women, mean age 56 (SD=5) years, were recruited in 1996-2001 and asked about height, weight, caloric intake and inactivity. They were followed until 2017 by electronic linkage to National Health Service records, detecting hospital admissions with mention of dementia. Cox regression yielded adjusted rate ratios (RRs) for first dementia detection during particular follow-up periods. Results: 15 years after the baseline survey only 1% were lost to follow-up and 89% remained alive with no detected dementia, of whom 18,695 had dementia detected later, at mean age 77 (SD=4) years. Dementia detection during years 15+ was associated with baseline obesity (BMI 30+ versus 20-24 kg/m2: RR=1.21, 95% CI 1.16-1.26; p<0.0001), but not clearly with low BMI, low caloric intake or inactivity at baseline. The latter three factors were associated with dementia rates during the first decade, but these associations weakened substantially over time, approaching null after 15 years. Conclusions: Mid-life obesity may well be a cause of dementia. In contrast, behavioural changes due to preclinical disease could largely or wholly account for associations of low BMI, low caloric intake and inactivity with dementia detection during the first decade.