Losing weight is as simple as following one rule: burn more calories than you eat. When you burn more calories than you eat, your body is forced to use fat stores for energy which over time, causes weight loss. The weight loss rule has two sides: how many calories you burn (exercise) and how many calories you eat (diet).
Energy balance is the relationship between how many calories you burn and how many you eat. It is the basis of weight management. The difference between how many calories you burn and eat will determine whether you gain, lose or maintain your weight.
Losing weight is achieved by burning more calories than you eat. A calorie deficit (state in which you burn more than you eat) forces your body to turn to body fat to make up for the shortfall created in your diet.
If you burn 2,500 calories per day but only eat 2,000, your deficit is 500 calories per day. That 500 calories comes from body fat which results in weight loss. Generally (read Metabolism below), a larger calorie deficit will lead to faster weight loss.
There are two sides of the energy balance equation: calories burned and calories eaten. You can increase the amount of calories your body burns by exercising more and decrease the amount of calories you eat by dieting. Taking either of these steps will help you lose weight. The variable that will determine which is more efficient is your metabolism.
Your metabolism can be described as your body's power plant. Your metabolism determines how much energy your body needs and is made up of the energy needs of all your organs, digesting/absorbing food and physical activity. You can estimate how many calories your body needs each day (by using the calorie calculator) but your metabolism isn't set in stone. Your metabolism is fluid and and adapts to the environment you create.
Diet or Exercise?
On paper, both calorie restriction and exercise will produce a calorie deficit. Dieting however, can be less effective when it comes to weight loss. When you cut calories, your body's metabolism reacts. There is generally a positive correlation between how much you eat and your body's energy expenditure. When you eat less, your body burns less. When you reduce your calorie intake, your body slows its metabolism in an attempt to preserve energy. The more calories you cut, the more your body will react by slowing its metabolism. You can largely avoid a slowing metabolism by cutting calories moderately (no more than 500 under what your body burns each day).
Exercise is a much more efficient way to create a calorie deficit. Exercising has the opposite effect on your metabolism than cutting calories. Exercising will speed up your metabolism. Exercise causes tears in your muscle fibers which get treated as injuries. These "injuries" are repaired during rest. This recovery process makes your muscle tissue bigger and stronger. Muscle tissue is metabolically active: it needs a constant supply of nutrients. The more muscle you have, the more energy your body has to use to keep that muscle "alive".
The best approach to take is combining a moderate reduction in your calorie intake (no more than 500 under what your body burns each day) with an exercise routine that is focused on both strength training and cardio. A moderate reduction in calories is best achieved by replacing junk with healthy food. Junk foods (fast food, potato chips, sugary drinks, refined carbs) are typically high in calories while healthy foods (fruits, vegetables, lean meats, nuts, beans, seafood) are low in calories. Eating a diet that is high in these healthier options will help you lower your calorie intake while improving your overall health.
The Bottom Line
The best permanent weight loss solution is to slowly incorporate more exercise into your lifestyle and replace unhealthy with healthy food options. Trying to make changes too quickly will lead to temporary weight loss.