How will your body react if your norm is to eat around 2,500 calories per day, and only a little walking as your physical activity, and you suddenly cut your food intake by HALF and jump up to a "perfect" workout regimen? My body reacted like this, as would most: "Holy crud!! What the hell is going on?!! We must be starving, and running to try to find food! What apocalypse is going on out there? Oh man, this is it, this is the end. Emergency! Emergency! Going to code red- crisis management mode!" My body senses a sudden catastrophic world event on many January 1st's.
Now time for the science nerd in me to come out. What happens physiologically while on a low-calorie diet that your body is not used to, is that all food coming in as much as possible will be stored as fat for later in case the emergency gets even worse, and muscle gets broken down to be used for quick energy. Cause if you're in a crisis, keeping your brain well-fed, and planning for the worst is top priority. Your brain is not able to use energy from fat, and if you are starving your brain by eating a severely low-calorie diet, muscle is the quickest way to get energy to your brain. Your body doesn't really have a way to tell how much fat it already has stored so that it can "cap" it at a certain amount, which is why you can get very morbidly obese people. Your body will just keep making more fat when it is in these crises even though you already have a lot, or when there is a plain old excess of energy coming in. Losing that muscle lowers my energy and makes sustaining this plan much more difficult.
Your brain needs glucose. Your body does too. It's the energy that gets broken down into usable energy for your body, known as ATP. You can also use other simple sugars like fructose, but those actually just get converted to glucose before being used to make energy.
Glucose can be stored in a number of ways. The easiest way is as glycogen. That is a ginormous molecule of glucoses linked together in a network. Your liver and muscles store glycogen so that your body and brain have enough energy between meals. They can only store enough to last for a few hours (between meals). When there is no glucose in your blood from a recent meal and there is a need for energy, the glycogen gets broken down to glucose.
In the muscle glycogen, the glucose is used right there in the muscles so you can still walk around, type at your computer, chase your kids, etc. between meals. The liver is the bank account for your brain. The glycogen in the liver is broken down to send glucose into the blood for the brain to use. Brain always has priority. If the brain doesn't get enough, the body must supply.
When glycogen is gone, the body will go to the next source. Muscle. There are a lot of Amino Acids - building blocks of proteins - in your muscles, cause that's what muscles are made of- loads and loads of proteins. Those proteins can then be made back into glucose to send to the brain so it has energy. This process is called gluconeogenesis (gluco=glucose, neo= new, genesis= make; make new glucose).
When you are on a very low-calorie diet or starvation, your body will break down some muscle to feed your brain. In the absence of readily useable glucose in the diet, the same happens. This is why people on Atkins/ low carb diets lose a lot of weight quickly at the beginning. They are starving their brains of glucose, so the muscle gets broken down. Well, muscle weighs A LOT. It is much more dense and heavy than fat. It also takes a lot more water to metabolize muscle, so the majority of those pounds you are dropping on those diets are muscle and water. Great if all you care about is the number on the scale. Terrible if you care about your body composition, shape/ size, and actual health.
The other way for glucose to be stored is for PARTS of it to be put into fat. Glucose has 6 Carbons in it, but a bunch of 2-carbon subunits (Acetyl-CoA) can be put together into a long chain: a fatty acid. Then 3 of these chains can be attached to a glycerol and stuck in adipocytes- fat cells. A lot of energy can be tucked away this way. You get loads and loads of energy out of fat. You are probably familiar with this if you pay attention to nutrition labels. There are 9 Calories per gram of fat, but only 4 Calories per gram of carbohydrate or protein. That's equalizing the weight. Fat takes up a lot more space, so all that extra energy stored in your body also makes your body bigger than the same amount of energy stored in muscles or glycogen.
So, how and when does this fat energy get used? That's the tricky thing. Your brain lacks the proper gateways and enzymes to metabolize the subunits from fatty acids (acetyl CoA) directly. It can only take in glucose or ketone bodies. The glycerol from the triglyceride can be made into glucose, so those get sent off to the liver to do that. But the fatty acids are better used by the cells of the body cause they have the right enzymes and gates to allow that to happen.
But since fat is long-term storage, just like a trust fund, the body is not going to break into it at the first sign of trouble. It waits to see if it can get by with the cash on hand, the checking and savings account. (Glucose in blood from your meal, then glycogen, then muscle as explained already.) Then if it's dire enough it will go for those fatty acids IF the body is in need of it. So, from what I have heard from a Physiology professor about some research (but haven't located the research myself yet so I have no link, sorry), is that the mark for the fat getting mobilized is about 45 minutes of exercise. After that point you would start to break down the fat for your body to use. So the recommendation of 30 minutes of exercise most days doesn't even touch that. If I want to reduce body fat, the best thing is to go on hours-long hikes as much as possible. (The regular aerobic exercise IS crucial to your health in other ways though, and should still be maintained for your cardiovascular, respiratory, muscle and mental health. Benefits of regular exercise are nearly endless.)
I mentioned the brain can use ketone bodies. In severe starvation, the fatty acids can be made into ketone bodies, which can get into the brain and used for energy. But they are very dangerous because they turn the blood acidic and it is detrimental to your body. This is the last ditch effort during severe starvation, to keep your brain alive, cause without your brain, the show is over.