Calories in calories out (cico) is like a budgetary point: losing weight means burning more calories than you consume.
Burning fat after x minutes of exercise is just a comment on your body switching from primarily carb power (glycogen) to primarily burning fat (ketones) for fuel. This distinction is not really relevant to casual exercisers or dieters.
To illustrate: A lb of fat is 3500 calories. So to lose a lb of fat a week you need a calorie deficit of 500/calories a day. You can have a 500/cal deficit without any exercise or without exercising for greater than 30 minutes. In other words, your body also burns fat stores when you eat at a caloric deficit, otherwise no one would lose weight from starvation.
Your immediate energy reserves consist of a bit of glucose, phosphocreatine etc. while glycogen is the medium-term energy reserve that provides more glucose when needed. Only once that's more or less exhausted will the muscles be burning fat directly. That's what hitting the wall is in a marathon or similar endurance event.
But it doesn't really matter to the overall balance whether your muscles are burning fat directly during exercise. What matters in the long run is the energy balance. If your body is in energy starvation more often (and/or more intensely) than it's in energy surplus, the insulin-glucagon balance will favor net breakdown of fat from adipocytes.
The thing is, different parts of the body use different energy sources in different situations. The brain is one of the less flexible organs - it uses glucose exclusively if at all possible, only under severe conditions (starvation or extreme carbohydrate restriction) will it start to use ketone bodies, derived from fat. So it's not so important whether the muscles are actually burning fat or glucose during exercise.
Isn't the simplest answer that the latter statement is incorrect? Or at least phrased poorly - maybe he meant to say that during exercise we won't burn fat until 30 mins in.
You could sit on your ass and do nothing. If you ate less than you needed, your body would eventually start breaking down fat for energy. That's what it is there for.
Your blood sugar is consumed as you exercise.
The pancreas will try to regulate this level by emitting glucagon, which stimulates cells into releasing trapped glucose into the bloodstream. This is a finite, short term resource.
Fat cells store far more energy. While we regularly break down amino acids for various reasons, this happens more frequently approaching ketogenic states (low blood sugar).
By this method, fat burns more easily after exercise. BUT calories-in must remain unchanged or lower to avoid replenishing those stores.
Carb loading raises blood sugar above normal levels temporarily. Doing so prior to or during exercise allows this fuel to be available to muscles promptly without being stored first. The body stores what it doesn't need immediately over several hours. It works to keep blood sugar levels at a "middle range".
If you normally eat 2000 calories a day and don't exercise, your body will seek an equilibrium baseline at this caloric level. It's easy to raise this baseline (the body doesn't resist calories beyond present stomach capacity) but it takes several months to shift this preferred state downward sustainably. That's why dietitians will often describe weight loss being 20% exercise and 80% diet... to describe where the focus should be. (Although you can achieve weight loss with 0% exercise, this breaks down more muscle you don't want to lose. And less muscle means lower caloric needs, which makes rebounding easier.)
Now, imagine you start burning 500 extra calories daily during exercise. Picture how much effort that takes. It's a lot for many. And then realize how little food that is: a moderate hamburger. Or a small milkshake.
The body will release hormones to make you hungry. After burning 500 calories during exercise, do you think you sustainably keep your increased hunger at bay with just a small milkshake?
There have been many lengthy responses so I'll just give you the TL;DR: The rule that you have to work out for enough time (or that walking is better if you want to burn fat for that matter) is bogus. If fact, the energy expended during a normal workout is small compared to the basic upkeep, so the thing that really matters for losing weight is intake.
Diet plays a more important role in weight loss than exercise does. The benefits of exercise in weight loss really come from adding muscle, which increases your metabolism, making better use of the food you eat.
Exercise actually has almost nothing to do with overall weight loss. If you look up
doubly labeled water “diet vs exercise”
you’ll see that a very reliable method of measuring overall calorie burn is telling is that daily activity levels make very little to no difference. Example article: https://well.blogs.nytimes.com/2012/08/01/dieting-vs-exercise-for-weight-loss/
Basically, the body seems to do a really thorough job at lowering rest metabolism to counteract any increased energy burn due to activity, such that the overall daily/weekly calorie burn rate is very consistent. This is what allowed our ancestors to survive while walking all over the place looking for food. Today, it means we spend the same energy whether we exercise every day or spend all our free time as a couch potato.
And when we get very little exercise, our body is likely spending extra energy on “overhead” activity— such as inflammation and other things that can be bad for us when sustained for too long. So getting exercise is still very important for our health, even if it doesn’t actually help with weight loss per se.
The notion about 20 or 30 minutes of exercise is probably really just about how long it takes for your metabolism to fully kick into higher gear due to exercise. I know that’s how long it takes for me to start feeling significantly warmer when I go on a walk. Yes you’ll be burning more calories per minute during that exercise, and maybe if you’re already close to ketosis it’ll finish the job and get you into pure fat-burning, but again your body is going to make up for that exertion later with a lower metabolism during rest.
Where available, your body will use free glucose in your blood as an energy source for cells (some cells, such as in your brain, can **only** use glucose as an energy source). A healthy adult will normally have a blood glucose level of between 4 and 8 mmol/l.
During gentle exercise, your blood glucose is used up, and then your body releases a hormone called glucagon, which releases stored glucose from the liver, maintaining a healthy blood glucose level. When this store is exhausted, or if the demand is high enough, the body will find other sources for energy. To do this, it breaks down proteins, and fatty acids, and forms glucose, and ketones. When people use the term "keto diet", it means they are deliberately running on a deficit of glucose, to force the body to break down fats. The ketones can sometimes be smelt on the breath (sickly sweet pear drop kind of smell).
It is an inefficient energy source for the body, and creates toxic byproducts (the ketones), which increase stress on the kidneys and liver, and can be harmful in excessive quantities.
In terms of the "calorie in minus calorie out" formula, that is true. If you eat at a calorific deficit, you will lose weight - but not necessarily all fat. In order to turn the fats into usable energy sources, the body needs amino acids, and it gets them from breaking down muscle cells, as well as from dietary protein intake.
None of this takes account of the proportion of body weight made up by water. You can change weight through retention of water completely independent of the calories in your diet, and one of the key factors in this is dietary sodium. If you have a high sodium diet, your body retains more water in order to keep a constant concentration in your extra-cellular fluid. Low sodium diets mean less water (and less weight).
I haven't seen anyone here mention what I think is the point of your question:
If you do a normal exercise and don't reach the direct fat-burn stage, you still lose fat because your body has to subsequently either convert fat to sugar to restore the reserves or use your next meal for the reserves instead of storing it as fat.
If your primary focus is on losing weight, aim for Calorie-In, Calorie-Out (CICO) Your body is constantly burning calories. Whether it is coming from fat cells or glucose reserves(don’t quote me on the correct term), your body will use whatever energy source it can find.
As you start eating less than what you burn, the body will start using fat cells to do the same activities.
Exercising at a fat-burn heart rate for longer time will expedite some of the burn. Because our glucose reserves can only store so much energy. Unlike fat cells they can’t store unlimited amount of energy.
But, if you don’t control CICO, all that exercise would not matter as fat cells are good at absorbing energy too. Any excess will immediately go to them.
Full Disclosure - not an expert, but these are statements I gathered reading through some books/comments while I was going through a similar phase.
What you learned is highly simplified and not always true.
The body has 3 sources of energy to utilize, called substrates.
These include both simple and complex carbs, like bread or sugar. The body uses this first for energy.
Some people that cut carbs are trying to trigger ketosis, the generation of ketones that burn more fat.
When carbs arent enough the body turns to long term energy stores, fat.
In emergency situations the body will start devouring muscle tissue to power critical systems.
Cico is a formula for weight loss. "ONLY bunring fat after...." is just false. Everything your body does burns calories, so you don't need to hit the treadmill, lift, or elevate your heart rate to lose weight. If you're in a deficit a healthy body will burn fat for energy regardless of added exercise.
The word only is what makes it a false statement rather than an incomplete one
Gonna answer this as simply as I can because a lot of these answers are overly complex. Your body is burning calories constantly just to keep you alive. If you eat fewer calories in a day than your body requires, you will lose weight. Some of this weight will be lean mass (such as muscle), but if you’re substantially overweight, most of it will come from fat stores which will break down and release energy for use by the rest of the body.
Forgot the part about only burning fat after 20 minutes of exercise. That’s incorrect.
Source: med student 2 months away from being a doctor.