This is true for two reasons. Not only are many fad diets low fat, but they are also low calorie. Your body is not stupid! It can see that you are not taking in enough energy to support your basal metabolic rate. Your basal metabolic rate is the number of calories that your body requires to run your heart, brain, liver, digestive system, lungs etc. This critical number is very responsive to the environment because back in the good old days food wasn’t widely available. If you weren’t able to find food for a few days then your whole system slowed down to require less calories and protect you from dying.
Dr Jacomien de Villiers qualified as a specialist physician at the University of Pretoria in 1995. She worked at various clinics at the Department of Internal Medicine, Steve Biko Hospital, these include General Internal Medicine, Hypertension, Diabetes and Cardiology. She has run a private practice since 2001, as well as a consultant post at the Endocrine Clinic of Steve Biko Hospital.
The blood vessels and blood are the highways that transport sugar from where it is either taken in (the stomach) or manufactured (in the liver) to the cells where it is used (muscles) or where it is stored (fat). Sugar cannot go into the cells by itself. The pancreas releases insulin into the blood, which serves as the helper, or the "key," that lets sugar into the cells for use as energy. http://www.sandysidhumedia.com/wp-content/uploads/2012/12/quotecaroline.jpg
[Guideline] Whelton PK, Carey RM, Aronow WS, et al. 2017 ACC/AHA/AAPA/ABC/ACPM/AGS/APhA/ASH/ASPC/NMA/PCNA Guideline for the Prevention, Detection, Evaluation, and Management of High Blood Pressure in Adults: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines. Hypertension. 2018 Jun. 71(6):e13-e115. [Medline]. [Full Text].