How many calories per day do I need to burn to lose weight?
A human body consists of fats (in the form of adipose tissue), carbohydrates (in the form of glucose and glycogen) and proteins (in the form of muscle). If I have to lose weight, I ll talk in terms of losing either fat, carbs or proteins. Below is a table lifted off Wikipedia for ready reference.As can be gathered from the table, a gram of 'fat' can release up to 9 kcal of energy, more than double the figure for proteins and carbohydrates. Therefore if you want to lose a pound of fat you will have to burn 0.453*9 kcals ~ 4082 cal. Similarly to lose a pound of carbohydrates you will have to burn 2041 cal. Therefore if you burn lets say a 100 cal, you will be burning a lot less fat (in terms of mass) compared to carbohydrates.Let me elaborate with the help of a story at the end of which you will understand the concept of losing weight.Ram is an athlete and runs regularly. Today he ran 5 km in 25 minutes. Running is the act of contracting a whole range of muscles and releasing them in a synchronous manner. While we may think that running involves only leg muscles, it is a combination of movement of many more areas of the body (arms, shoulders etc.). This requires energy to do so. Energy can be obtained readily from breaking down Adenosine triphosphate. All the substrates (fat, carbs, protein) that we mentioned earlier create energy packed ATP when required. But this conversion of substrates to ATP requires Oxygen (aerobic respiration). No wonder Ram breathes faster while running. But how can Ram ensure that he burns fat only and not the carbohydrates and protein. Ram can't. His brain decides what to burn and in what proportion. There is a way however by which Ram can measure what he is burning. The composition of CO2 while exhaling can tell us how much oxygen was used in burning fat, carbohydrates and protein (a little knowledge in chemical reactions and math will help). Therefore with careful observation, Ram can figure out how much oxygen he has consumed while running and in what ratio has it been used to burn the substrates. Once he has that he will know exactly how much fat and carbs he has utilized.Now the part which is missed by a lot. Ram is also hungry at the end of the run. He eats a peanut butter sandwich which is filled with carbohydrates and fat thereby restoring whatever he had lost as part of the workout. And so he neither loses nor gains any weight. The trick is to outgo more than you inlet energy substrates.Happy Workout
What if I only eat 300-500 calories a day a few months?
You won't make it for a few months. I have experience with this but nevermind that. The other answers are correct; they describe ketosis. But what you feel? The awful hunger is more than you might think. It's literally all-consuming, and it will keep you from sleeping until you get so tired you oversleep, like hibernate. That's what you will be craving because being awake is painful. 300-500? That's about what I had to go on. What little money I had would go to what I saw as the highest nutritive benefit per moneys: my daily diet would be something like 1 Powerbar and 1 nectarine. I decided that those senior drinks for people who are allergic to chewing, like Boost or Ensure, was better than a Powerbar, and the amount either fills you up is small enough that it doesn't matter.Later I could eat both lunch and dinner at the shelter. So we might be having 1000 calories a day, and I'm describing that, and that's twice as much as you are asking about! And the dinner wasn't guaranteed, so all this is in that calorie range.So, I described the gnawing hunger. After a while, your brain slows down. Your neurons don't fire as much as they used to, and you think "slower." you get confused and distracted and can't concentrate on one damn thing.You'll get constant headaches. When they are low-level it's tolerable, but they can be pounding.Your stomach will rebel. You won't be able to keep anything down. If you take pills, you'll puke them up. It's nausea, but not the type everyone thinks defines nausea--it's not queasiness. Instead, your gag reflex doesn't really stop. You will take a pill then feel that gag-reflex sensation for a few hours. You'll regurgitate your coffee. The pain in your stomach will "physicalize" and make you cry, so you'll become histrionic and cry a lot. Stomach hurt? Cry. Think of something sad? Cry. Get mad? Cry. Miss your bus? Cry. Can't do your homework? Cry.So yeah, you keep up like that, and your body will start to consume itself. That's why we have fat reserves, and you don't want to start dipping into those. I can only imagine the agony of people in famine.
What happens if I eat too many calories and then restrict my intake the day after?
There are some terrible answers to this. What you are talking about is a blend of intermittent fasting and overeating.In terms of weight loss and weight gain, it comes down to calories in versus calories out. Calories in being what you eat and calories out being what you spend in energy simply living, and through activity.If you ate 500 calories more than your total daily energy expenditure one day and 500 less the next you would essentially be level and would maintain weight.If you ate 1000 more calories than you burn one day and then eat 500 less than you burn the next you will have added 500 kcal to your net. This would equate to a few ounces of added weight if all other factors are controlled.Calorie restriction is not an inherently bad thing at all. Many top athletes use intermittent fasting to achieve their goals. I advise that you look up the term intermittent fasting on a quality site like bodybuilding (.com) and go from there.Ignore people saying you will “ruin” your metabolism, they are talking complete crapola!