The Nurses Health Study (NHS) [1][2] is considered one of the best sources of long term data on factors affecting health due to its size. Regarding dietary fat

The Nurses Health Study (NHS) [1][2] is considered one of the best sources of long term data on factors affecting health due to its size. Regarding dietary fat intake and risk of coronary heart disease (CHD) [3] following 80K women over 14 years, they failed to find a significant correlation between total fat intake and CHD. The linked summary discusses the relative risks of different fats (limit saturated, and completely avoid trans), but it essentially comes down to calories being replaced by carbohydrates. Note that foods labeled low-fat or fat-free do tend to have more sugar. Another NHS study following 40K women over 8 years[4] showed only a weak correlation between fat intake and weight gain. The take away again is probably to care more about which fats. Unsaturated fats tend to be healthier than carbohydrates. Saturated fats tend to be more unhealthy, and transfats should be completely avoided. [1] https://en.wikipedia.org/wiki/Nurses%27_Health_Study https://en.wikipedia.org/wiki/Nurses%27_Health_Study [2] http://www.nurseshealthstudy.org/ http://www.nurseshealthstudy.org/ [3] https://www.ncbi.nlm.nih.gov/pubmed/9366580 https://www.ncbi.nlm.nih.gov/pubmed/9366580 [4] https://www.ncbi.nlm.nih.gov/pubmed/17426332 https://www.ncbi.nlm.nih.gov/pubmed/17426332