How someone views weight loss and fat loss will have a huge bearing on their attempts to become leaner. To many, weight loss and fat loss are viewed as the same and often are used interchangeably in normal, every-day conversation without complication. But for many a distinction needs to be made.
Fat loss can be defined as a reduction in body fat only and can change even when total body weight remains the same. For example, when someone follows a resistance training program, their muscle mass may increase and their fat levels may decrease, but because one change offsets the other, overall body weight can remain virtually the same.
Muscle and liver storage of glycogen (carbohydrate) and water can affect body weight without effecting body fat levels. Following a bout of resistance training, and assuming adequate nutrition has been consumed with sufficient amounts of carbohydrate, the muscle and liver glycogen (carbohydrate) stores are filled to capacity. And for every 1 gram of glycogen stored, 3-4 grams of water is also stored. (This is why muscles appear to be bigger and fuller the day after a weights session. The muscle hasn't dramatically grown overnight; it's just full of glycogen and water). This storage explains why even though body fat levels haven't changed, total body weight can fluctuate on a daily basis.
When this process is manipulated, rapid weight loss is possible (and spot reduction - but that's another article). Training depletes the muscle of glycogen and water, and if not replaced, the body becomes lighter on the scales and rapid weight loss is reported, albeit without a reduction in actual body fat.
This brings us to our definition of weight loss - a reduction in total body weight whether it's from a reduction in body fat, muscle tissue, water stores, glycogen stored, liver glycogen stores or a combination of 2 or more.
Unfortunately, too many people fail to see the difference between fat loss and weight loss and mistakenly focus on total body weight, thinking that to reach their 'ideal size' their weight must be a certain number on the scales. This line of thinking has serious implications in terms of exercise adherence and motivation. For example, a minimal or non-existent reduction in total body weight can be seen as a failure even though a reduction in body fat has occurred. For those that fail, or simply refuse to distinguish between fat loss and weight loss, this may be enough to deter them from continuing with their exercise program.
Weight loss without an associated loss in fat is an unfavourable outcome. This usually means that muscle tissue is being lost and that's bad news for your metabolism. Your muscle mass drives your metabolic rate so any reduction makes it harder to for your body to lose fat and to avoid gaining fat.
Another body composition scenario that may occur is that total body weight may stay the, with an increase in body fat and a decrease in muscle mass. This is common amongst retired sports people who cease training, resulting in muscle atrophy (wasting), but continue to follow the eating habits they had when playing and training. Although muscle can't literally turn into fat, this is a common and reasonable description of what happens when people stop training and continue familiar eating habits.
So when someone is following an exercise program to lose fat, they must be aware of the distinction between fat loss, weight loss, and muscle mass. And realise that in all probability, if they include a resistance training component for muscle mass and metabolic maintenance, their body weight won't change as much as their size will.