Intermittent Fasting works best when combined with the Low-Carbohydrate, High-Fat (LCHF) diet see:
Do we need Carbohydrates?
Diabetes and obesity are, to a large extent, caused by excess (refined) carbohydrate (and they tend to be different stages of the same condition). Depending on your medical history of Diabetes, Insulin Resistance, Obesity and non-communicable diseases like cancer, the "usual low-carbohydrate advice" would be:
No food (fasting)
No carbs (Keto is close to no carbs)
No refined carbs or sugar
Low carb or Low-Carbohydrate, High-Fat (LCHF)
Reduced Carbohydrates
No Grains
Whole grains only
Is sugar bad for you?
I eat a "moderate-carbohydrate" diet, including "no-grains" Buckwheat muesli, full-fat live set yoghurt, and an orange and a banana for breakfast.
Keto - or a very low carbohydrate diet is good for acute diabetes, epilepsy, cancer etc., but carbohydrates are a cost-effective source of energy and fibre, and I personally have problems with a high-fat IF diet as I have no gall bladder. I still fast 14:00-10:00 nearly every day.
Intermittent Fasting is not snacking all day, and, when you are not eating, your glycogen energy gets depleted, and you start to get energy from your fat reserves. (Intermittent) Fasting is about so much more than weight loss, but burning fat to lose weight and improve body-fat percentage the main reason most of us fast.
If you have plenty of glycogen and glucose, your body does not burn fat.
Carbohydrate quickly turns into blood glucose, which spikes insulin and prevents you from burning fat.
A Low-Carbohydrate, High-Fat (LCHF) diet (or the lower-fat keto diet) limits carbohydrate intake, so it lowers blood glucose and helps get the body to burn dietary and stored fat. Fat is sustaining, so it decreases appetite - the desire to snack. (I eat a medium-carbohydrate diet, partly as I have no gall bladder, and cannot digest large fatty meals.)
So IF and LCHF complement each other and they both help you burn fat.
A few endurance athletes have got well fat-adapted - and have performed very well - and some hunters can run down antelope after not eating for nine days!
It takes a while to transition into fat-burning mode, and that is why it is difficult to go straight into fasting, and LCHF will make this process quicker and easier.
If we were trapped somewhere for a week with water but no food, few of us would suffer any long-term consequences, and Dr Jason Fung puts some of his acute Diabetic patients straight onto a 14-day medically-supervised water-fast... but the prospect of losing one's sight or a limb can provide the required incentive!
This video advocates combining IF, keto and exercise and says that it takes weeks to get well fat-adapted - and you will be getting better at burning fat for years!
For more detailed information on "Low-Carbohydrates", See:
Further reading "My low-carb Journey" thank-you, moreless :
healthunlocked.com/nhsweigh...
LCHF what to expect, Thanks "TheAwful Toad":
healthunlocked.com/lchf-die...
Low Carbohydrate is now becoming mainstream (2019 PHC conference).
Net Carbs on Keto
Fasting and visceral fat (or at least waist size) 
Why do they rank the Keto diet worst for health?