Hi Gugu and welcome.
If you’ve read the book you will understand that Dr MM says low fat dairy products have more sugar in than full fat – and we are all trying to cut out sugar as one of the most dangerous refined carbs (the others are bread, potatoes, rice, etc)
This is why he says use full fat milk and yoghurt.
We have been told over the years that fat is bad for us but this is not necessarily true. Full fat fills you up and counter-Intuitively cutting the refined carbs and eating more fat stimulates the hormones to change to get energy from burning your fat cells.
You can do whichever diet programme you like, but following the BSD including full fat dairy, has been really successful for lots of people here as you’ve probably seen. (It was for me!)
I would just follow the guidelines as in the book. You will get used to full fat which tastes better and you probably need less of it in coffee.
If you like, you can get an app like MyFitnessPal which shows you how many more calories and carbs there are in low fat products.
And 800 calories per day is much easier on full-fat products as they fill you up as the book says.
Hope that helps 🙂