Don’t try this at home
I suggest that you Intermittent Fast for weeks or months before attempting a fast of more than 24 hours (without medical supervision). Then gradually increase the length of your fasts. I suggest that you do not try anything beyond 72 hours until you have done several 72 hours fasts – at least a month apart.
If we were trapped, and could not eat for a week or two, few of us would come to any harm.
Dr Jason Fung puts many of his acute diabetes patients on an initial one or two week medically supervised fasts.
See Contra-indications for fasting:
healthunlocked.com/fasting-...
Month before
I am at my target weight... and I have been eating more to keep up to my target weight.
I was eating four jacket potatoes per day when I had lunch at home... but potato is carbohydrate, and was not helping me keep fat-adapted.
Cream is (marginally) cheaper per calorie than potato, and it is keto, gluten-free, cereal free, etc. ...so I have been consuming 600ml of cream each week - and fewer potatoes. 600ml of cream has about the same number of calories as 14 or 15 potatoes, so, if I eat at home five days a week, I need to eat 3 fewer potatoes a day... so now I am starting to eat only one potato a day when I eat at home.
Last month
I ate to get back to my target weight, ate too many treats and I was over my target weight most of the month. I ate less the last week or so... and just about or almost got back down to my target weight before the start of the fast.
Last meal
They say that you get into ketosis quicker if your last meal is Keto (very low carb) and I had raw carrot, broccoli and cheese for early lunch (finished by 12:30). I had just had breakfast: two bananas, an orange, a large bowl of “Eat Natural” Buckwheat (cereal free) Muesli, and a lot of double cream and natural bio yoghurt. I felt stuffed for most of the fast, but “fat satiates” - and I did not feel hungry.
Exercise
Fasted exercise is supposed to be very good for you – and, of course, it burns calories and blood glucose – so it helps get you into ketosis more quickly.
I walked 10 km on day one, and about five km the other four days - 30km in total.
I have heart problems – so feeling unenergetic and finding it difficult to put one foot in front of the other is normal (I was disabled for about five years, so I am glad to be able to walk at all) and I have to walk at a gentle pace when I am fasting.
Hunger
I did not feel hungry at any time before during or after the fast.
Phosphenes
Phosphenes are “seeing light that is not there”, and “seeing stars” is one example. At the end of my last fasting walk, I did see stars (without banging my head). Apparently, they can be caused by low blood pressure (hypotension)... which I get every day (I hope they will turn up my pacemaker a bit at my appointment next month). I wonder if low ketones or low blood sugar (hypoglycaemia) could contribute?
Heart Rate Variability
I have the HRV4Training app, which tells me if I am over-training, and how fit I am, and my score usually goes up after a fast – but, this time, it went up during the fast. I am just better than average fitness for males over 50 (and I am 71).
Sleep
I have the AutoSleep app, and during the fast, it was telling me I was not sleeping well (but it still told me I did not need to get to bed before 02:30!). If your Pulse Rate is high, it tells you you are not sleeping well – but fasting is stress for your body... and my pulse rate is often 10 BPM higher (24/7) when I am fasting – presumably, the heart has more work to do to get energy from one’s fat stores?
Re-feeding
I broke my fast “keto”... I had 500ml of natural yoghurt at 16:30, followed two hours later by cheese on broccoli... and I did not feel I needed any more – it is carbohydrate that makes you hungry (Carbohydrate stimulates the release of insulin, which lowers your blood glucose, which makes you hungry). I was back on my normal diet the following day.
Weight
Most of the weight initially lost during a fast is fluid and “contents of the digestive system”. When I was 125kg, and doing more exercise, my phone told me I was, sometimes, burning 3,500 calories per day – and that would be a pound a day fat loss in a fast. Smaller, less active people might lose half a pound of fat a day.
Last time I lost 7lb... This time, initially, I lost 2lb - but, the day after breaking my fast, I was down about 5 pounds – and I am still 3.5lb down the day after.
Dr Mindy’s benefit chart
The picture is Dr Mindy Pelz’s Fasting benefit chart (she told me I could share it). She does quote citations – but some of these parameters (e.g. autophagy) are difficult to measure.
It says that some of the benefits of fasting do not get to their maximums until you are 72 or 100 hours into your fast.
Autophagy is one of the main reasons I fast – and that is declining at 100 hours... but Stem cells and immunity are doing well.
Why 100 hours?
100 is a round number – and if you fast from breakfast to tea, and you normally IF 20:4, you are not eating for three days, but you can exercise fasted for five days.
A 72-hour fast from early lunch to late lunch is not eating for two days – and you could extend it to e.g. 76 hours... without having to eat too late in the evening.
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I have mentioned some things in passing without detailed explanation... there are many answers in my FAQ:
healthunlocked.com/fasting-...
If you have any questions or clarifications, please reply.
Psychological benefits of fasting?
Fasting and visceral fat (or at least waist size) 
Fasting Today - A Gentle Relaxing Experience, I Hope :-)