Losing weight and burning fat are sometimes seen as two different goals but the steps needed to accomplish either are identical. At the core of both is calorie balance; the relationship between how many calories you burn and eat. During positive calorie balance, the body stores fat while a negative calorie balance forces the body to burn fat leading to weight loss. This calories in versus calories out relationship dictates whether the body stores fat causing weight gain or burns fat causing weight loss.
Weight changes based on how many calories you eat relative to how much you burn. Weight loss happens when you burn more than you eat; a calorie deficit. A calorie deficit is the "magic" or "secret" making weight loss possible.
For example, if you burn 3,000 calories in a day and only eat 2,500, you've only given the body 2,500 calories when it needs 3,000. This forces the body to find those extra 500 calories from non-food sources. The 500 non-food calories make up the calorie deficit. Typically, non-food sources of energy come from either stored body fat or muscle tissue.
Inversely, if you burn 2,500 calories and eat 3,000, you've given the body an extra 500 calories of energy; a calorie surplus. With exercise, part of this 500 calories is used to grow muscle tissue. The rest is stored as fat. Without exercise, all excess energy is stored as fat.
A calorie deficit always leads to weight loss while a calorie surplus always leads to weight gain. During a calorie surplus, the body stores extra energy. This extra, stored energy becomes non-food sources of calories during a deficit. In other words, a calorie deficit burns the energy stored during a calorie surplus.
When a calorie deficit forces the body to burn fat, all the disappearing fat causes weight loss and a decreasing body fat percentage. Weight loss and burning body fat are two ways of describing the same process.
During weight loss, it's ideal to lose as much body fat as possible without sacrificing any muscle. Unfortunately this is very hard if not impossible to do. Weight loss is achieved through a combination of fat and muscle loss. There are a few ways to keep weight loss from eating up too much muscle, but there is no way to completely avoid it.
Burn Fat, Keep [More] Muscle
A calorie deficit forces the body to burn non-food sources of energy. Though the desired outcome of weight loss is to reduce body fat percentage, the body does not always turn to body fat to make up for a calorie deficit. Remember, non-food sources of energy could either mean body fat or muscle mass.
The leading cause of muscle loss during weight loss is a large calorie deficit (drastically cutting calorie intake or excessive physical activity). If you force the body to look for too much non-food energy, it sometimes turns to muscle in addition to fat to make up for the shortfall. There are a few ways to keep the body from eating too much muscle mass during weight loss.
Branched Chain Amino Acids. Branched Chain Amino Acids (BCAAs) are a group of three amino acids - leucine, isoleucine and valine, (available in powder or pill form) believed to have an important role in preventing muscle breakdown. BCAAs taken before, during or after a workout prevent the body from using muscle tissue for energy and preserve muscle mass during weight loss.
Eat Small, Frequent Meals. Eating small frequent meals and snacks throughout the day allow the body to more efficiently utilize energy. The body doesn't always like using fat for energy. Fat is good for low to moderate physical activity but cannot provide enough energy for high intensity exercise. Furthermore, some cells in the body lack the capability to burn fat for energy forcing the body to look elsewhere for non-fat calories. Carbohydrates from food sources allow the body to avoid breaking down muscle tissue to provide energy for high intensity activities and carb-only powered cells.
Keep Calorie Deficits Moderate. Smaller calorie deficits allow the body to better deal with energy shortfalls. When the body deals with small rather than large calorie deficits, it burns more fat than muscle because energy needs from non-food sources aren't as high. The more energy the body needs to find from non-food sources (i.e. bigger calorie deficits), the more likely it turns to muscle for energy.
The Bottom Line
There's no need to rush weight loss. Bigger calorie deficits lead to temporary results because they're hard to maintain. Rather than striving for a huge calorie deficit, try a more moderate approach; a 500 calorie difference between the amount you burn and eat. Use the calorie calculator for guidance. A smaller calorie deficit is much easier to get accustomed to because the change required isn't as drastic. Small calorie deficits lead to longer term healthy lifestyle changes while large calorie deficits lead to temporary results.
Yes, a large calorie deficit leads to faster weight loss but with undesirable results. Large calorie deficits cause temporary rather than permanent body change and an increase in the amount of muscle lost during weight loss. Losing some muscle during weight loss is inevitable. This painful side effect of weight loss is partially mitigated through a moderate rather than extreme reduction in calorie intake accompanied with a balanced increase in physical activity.