So I just saw a blurb about this study from this month’s American Journal of Clinical Nutrition:
Design: Men (n = 16) aged 22 ± 1 y (mean ± SE) were randomly assigned to 5 d of a high-fat, low-carbohydrate diet containing 75 ± 1% of calorie intake through fat consumption or to an isocaloric standard diet providing 23 ± 1% of calorie intake as fat. In a crossover design, subjects undertook the alternate diet after a 2-wk washout period, with results compared after the diet periods. …
Results: Compared with the standard diet, subjects who consumed the high-fat, low-carbohydrate diet had 44% higher plasma free fatty acids (P < 0.05), 9% lower cardiac PCr/ATP (P < 0.01), and no change in cardiac function. Cognitive tests showed impaired attention (P < 0.01), speed (P < 0.001), and mood (P < 0.01) after the high-fat, low-carbohydrate diet.
Conclusion: Raising plasma free fatty acids decreased myocardial PCr/ATP and reduced cognition, which suggests that a high-fat diet is detrimental to heart and brain in healthy subjects.
Really Oxford researchers? I guess they’ve never heard of induction flu, the temporary “mental fuzziness, fog and fatigue” many folks experience when initially going low-carb.
Here’s my lay take: if you want to test the response to a high fat diet, let the subjects acclimate to it first!
Weight Maven is written by Beth Mazur. Beth believes that obesity is more symptom than cause and that the real problem is our Western diet -- especially sugar, refined grains, and industrial oils. Beth writes about nutrition, ancestral health, & food policy. And cats! 
Hi Beth, I agree on the time - 2 weeks not long enough for adaptation. But interesting about the free fatty acids and the effect on the heart.
And it wasn’t even 2 weeks … that was the washout period! They only did the diet for 5 days. Re the heart, given that cardiac function didn’t change, seems unclear whether the lower cardiac PCr/ATP was likewise a temporary artifact or not.