When trying to lose weight, the majority of people believe that a person needs to burn more calories than they take in. While this is true to some extent, there are many factors that make this simple “calories in versus calories out” equation not work exactly like you would think.
For example, if you take two people who are the same height, weight, and gender, there are situations where one of them is working out more frequently and eating fewer calories and not losing weight, whereas the other is eating more and exercising less and losing more. But how can that be?
First, you need to look at style of training. Longer, steady state cardio is often thought of as one of the best ways to lose weight from fat, but in fact it is one of the least effective ways. Steady state cardio burns calories while actually doing the exercise, but once done expends very few recovery or “afterburn” calories. On top of this, when you exercise for long periods (over an hour, for steady state cardio, or 90 minutes, for a varied routine — though this is highly dependent on the individual), you increase your stress hormones, which can cause your body to retain or gain fat weight even when exercising heavily. Strength training and HIIT on the other hand are typically done for shorter periods of time, which means you have fewer stress hormones released into your body, so it does not actively try to work against fat loss. Strength training and HIIT also have an “afterburn” effect, which means that with less time exercising, you can actually burn more total calories as compared to steady state cardio.
Second, how a person eats can have a huge effect on fat weight as well. Most people think that the fewer calories you take in, the more fat weight you will lose — and that is also not necessarily true. When your body starts to take in fewer calories than it uses, you start to use your body's fat stores to make up the difference. When you start to pull from your fat stores too quickly (which is different for everyone), however, your body starts to intentionally slow its metabolism and goes into what's called “Starvation Mode” — where your body starts sending out hormones to slow your development of muscle tissue, your BMR (base number of calories your body burns doing nothing), and your physical ability/energy during the day and during exercise.
How many times you eat a day can also have an effect. With the same number of calories coming in, someone who only eats two meals a day (or three with only one of them being large) will not see the same fat loss results as someone who spreads out those calories over 4-6 meals with similar calorie content. This is because when you only eat a few meals a day, your body has to use your fat stores to get from one meal to the next and this sends a "feedback loop" to your body that you need your fat stores, so don’t let them go. On the other hand, if you eat frequent small meals, your body rarely uses fat stores (typically only when exercising), so your body does not see the need to keep those stores any more (since they are not being used as often), so it is more likely to let them go.
Finally, if you take one person who is eating very few calories, only eating a few times a day, and doing very significant steady state cardio, they could easily not lose (or even gain!) weight due to the stress reaction that this combination causes in the body. On the other side, if you take another person who is eating more calories, eating upwards of six times a day, and doing strength training and HIIT for shorter periods, they can easily maintain and even lose fat weight.
So, all of this is to say that losing weight is complicated, and every person's body reacts differently to diet and exercise. Doing research, listening to your body, and finding what works for you is key to weight loss and maintenance.
Related: My Experience: Cardio and Dieting Versus Fitness Blender and Clean Eating