I want to cut for a while and lower my fat percentage. As everybody I want to maximize fat loss and minimize muscle loss. Are there any warning indicators that my body is burning muscles instead of fat? What is the best way to avoid it? Should I eatless proteins than in bulking phase? Should I keep doing the same strength exercises with the same weight?
The body can use about 31.4 Cal / lb of fat / day. This key statistic tells you a lot about your maximum rate of fat loss, because fat loss only happens when the body is using calories from your fat stores. There is thus a maximum rate of fat loss, and that rate is dependent on how much fat you have.
If you have 25 lbs of fat on your body, then you can metabolize about 31.4*25 = 785 fat Calories per day.
Meanwhile, a pound of human fat tissue is estimated to store about 3500 Cal. It might be a little higher or lower, but that is the number that is frequently thrown around.
If you estimate your weekly fat calories at 785*7 = 5495 Cal/week, and divide that by 3500 Cal/lb, then you get a maximum fat loss rate of about 1.57lb/week.
To avoid muscle loss, you should never run a deficit greater than what your body can make available from fat, or else you are guaranteed to lose muscle just to stay alive. Beyond that, you should train with weights and eat a high protein diet in order to prevent catabolism - the breakdown of muscle. Small and frequent meals to keep insulin levels constant is also believed to help. Another technique which is suspected to help but hasn't been proven is increasing your free testosterone, such as with a zinc supplement, or by abstaining from masturbation.
The obvious indicator that you're losing muscle is that your muscles get smaller and weaker. I don't know of any others.
One point I should mention is that the 'bulking & cutting' method to building muscle is actually quite an inefficient one. Not that it doesn't work, it does and has done for decades but it is far easier for the body for you to eat sensibly but not intensely strict for 12 months a year than it is to pig out for 6-8 months and then cut for 3-4 months.
The 'bulking' phase of muscle building as you are aware, usually involves ingesting extreme excesses of calories. Now, unless everything you eat is basically steak, eggs, chicken for example and wholegrains & veg, then it is going to be hard to not put on excess fat. Your body can only synthesis muscle so quickly. No matter how many calories you eat or protein you eat, there will be a point when the body physically cannot build anymore muscle and when this point is reached, it will convert excess calories as fat.
By bulking, you are also then losing the amount of muscle you can gain in a year as you then have to spend several months 'cutting' which is essentially dieting. Real strict eating restricting calories is going to hinder muscle building. It may help you burn a lot of fat, but it isn't beneficial to continue building muscle. You may still be ingesting more calories than you are burning but after a significant period of bulking, high insulin release, fat cell hyperplasia (the expansion of fat cells due to extreme eating) then it will be really hard to build muscle, whilst you cut enough calories and consume enough nutrients to encourage muscle growth. It's not impossible, not at all, but it makes it more difficult.
You're honestly really better just addressing your diet all year round and having treat food every now and then and a couple cheat meals a week and just maintaining an excessive calorie intake but not going 100% bulking and then 100% cutting. Two extremes are difficult for the body to adjust to and means you have to waste time burning fat when you could be building muscle. Unless you're a bodybuilder and looking to compete, then bulking and cutting isn't really the best way to get in shape. There is a host of scientific reasons you should avoid this method but I won't bore you with them.
But if you have bulked, then a very high protein diet is the best way forward (higher than you would usually consume). Protein contains the least calories per gram in 4 compared to 7 & 9 in carbs and fat respectively. Protein is also a food that is difficult to break down and digest in the body and has been shown to enhance the body's metabolism due to the effort it takes to break down etc. although that is a very simplistic look at the whole process. It's best to work out your lean body mass in pounds (use a bio-impedance scale for a good measure) and consume between 1-1.5g of protein per pound of lean muscle mass. Keep wholegrain carbohydrates to the size of a clenched fist and then pack up on veg. to get your vitamins etc. Obviously not forgetting water! Cut down your sugar intake a lot too and increase your fat intake through butter, whole milk, natural Greek yoghurt etc. consuming naturally occurring fats stimulates the release of Glucagon, a hormone that stimulates the body to BURN fat and things like milk and yoghurt are usually high in protein too! While it is true that Insulin is important to increase muscle growth, insulin is also the primary hormone that stimulates fat storing and seeing as insulin isn't the most important part of building muscle, I'd focusing on reducing the secretion of it.
One other tip is regular HIIT. Cardio is another inefficient way to burn fat etc. but mainly it trains slow oxidative muscle fibres which are the less bulky fibres such as on distance runners like Mo Farrah. By doing high intensity exercises like sprints etc. you are stimulating the Fast Oxidative/Fast Glycolytic fibres which are the bulkier muscle fibres associated with sprinters and bodybuilders! By doing sprints etc. you are building bulkier muscles and burning the same number of calories in a shorter space of time!
You can build muscle by bulking and cutting, but it is a lot easier for the body to just continuously build muscle year round! I've not gone into the hormone responses to exercise/bulking/cutting but if you want any info or any studies to back up what I said then feel free to ask! Sorry that turned into a bit of an essay!
Stumbled upon this late, but, there's actually an indirect way of guesstimating that muscle is being catabolized. I'm probably going to show my age in this question, but, back in the "old" bodybuilding days, we would use Ketostix to check the urine for the presence of Ketones. Ketosis indicates a high level of ketones in the body. Ketones appear in the urine when there is an insufficient supply of carbohydrates available for energy. Fat is then used to provide the energy slack. However, if fat stores are low, as would be the case for someone in a "cutting" phase, the next choice for energy would be lean protein (eg. muscle). It's a fine line to walk to make sure you have enough carbohydrate to maintain energy, while at the same time, trying to reduce calories to reveal muscle.