I (sort of) did it one day last week and found it really easy! I regularly don't eat breakfast, but will eat an early lunch around 11:30 to 12:00. But last week I had a hair appointment at 11, so I ended up eating my first meal of the day at 3:30pm. I found I was really hungry around noon, rumbling belly etc, but then it completely went away and I just felt good all afternoon - not even particularly hungry when I did eat at 3:30.

So my question is, does it 'ruin' the whole principle (physiologically I mean) of IF if you have a coffee in the morning with milk and sugar? I have one every day when I get up at 6:30, and honestly it's one of my fave 'meals' of the day - I SO look forward to a great coffee, but it has to be with the 'right' amount of milk, and sugar (not the same with sweetener to me).

Or, is IF more of a tool to control calories, and really is not dependent on how long between meals or whether there are a small number of calories taken in between meals?

From my limited amount of research on IFing, it's about the fast and hormone levels because of the fast, or something along those lines. How many calories are in your coffee? I think (not 100% sure), that if you have more than like 50 calories it is considered breaking the fast.

Well it's about 70 cals at present, but if I could have up to 50, I'm sure I could tweak it a little!

If your goal is weight loss, it doesn't matter if you have 50-100 calories in the morning. I'm sure not everyone would agree with me but IMO the whole point of doing IF or something like it is to control your intake and eat when you're hungry instead of because you're supposed to at X time.

There are two things that IFing does.

The first is helps to control calories.

The second is theoretical health benefits.

The first is easy to understand. The second is not well understood at this point but one of the key drivers seems to be insulin levels being at baseline for an extended period of time so the fewer calories you consume during the fasting period the better.

Martin Berhkan of "leangains" came up with the 50 calorie rule but as far as I know he just made it up based on his experience and it relates more to bodycomposition than the health benefits.

I personally try to keep my calories under 25 during the fasting period but the simple facts are:

We don't know how long one would have to fast to experience the theoretical health benefits of fasting nor do we know how many calories one would have to consume to spoil those benefits. (We also don't know how often one would have to fast to get these benefits)

Thus, I wouldn't worry about it. The most important health benefits are the ones we know about which come from reducing your weight and exercising at least 20 minutes a day 3 x a week so I would be primarily concerned with those and remaining compliant to your diet is what matters most. If drinking 100 calories in the morning helps you stay compliant the rest of the day I would argue that would be more important than the theoretical additional benefits that intermittent fasting offers.

I just take my coffee as an afternoon treat instead of a morning requirement and stick with tea, since the purpose of IFing for me is calorie control and a 'food' for breakfast can make me hungrier than not. And some days I reverse my fast and eat a ton earlier in the day and fast from 4 pm to breakfast or lunch the next day. It is whatever is convenient or works for you, rather than hard and fast rules.

The Leangains guy drinks like 50 cals worth of milk during his fasting period, I think.
He said that in the past, when he would have to eat at a cal deficit, he would struggle to make 5 tiny meals a day like msot diets recced so he preferred the bigger meal in an eating window.
For me, that is so much better for me mentally than tiny meals throughout the day so that is why I do it, but I don't always do IF (and manage to keep the cal intake). Results are the same.