Although it has been changing in the last decade or so, the reality is that most people (women or men) tend to focus only on changes in body weight. If weight goes up, that's usually bad (unless it is an athlete trying to gain muscle) and if weight goes down that's usually good. For the sake of example, let's say someone starts a diet and a few days later their scale weight has dropped by a few pounds. Most would consider that a success but I would ask the following question: What was lost? Was it water, stored carbohydrate, muscle, fat, bone, organs?
Perhaps the person just had a big bowel movement.
The scale can't answer this question in any meaningful fashion and this presents a rather large problem. And this becomes an enormous practical problem as many people, especially if they are dieting, not only focus solely on the scale to track their progress but often obsess over the changes (or lack thereof). They may weigh multiple times per day (or before and after going to the bathroom) and often over react completely to small day-to-day changes in body weight. A frequent pattern is that if weight goes down, that means it's time for celebration. Bring on the cake. But if weight goes up, it's time to reduce food intake even more and add an extra hour of exercise to the gym. As you'll see below, these types of short- term changes are relatively meaningless overall although daily weighing can still be useful so long as it is approached correctly.
Women, primarily the normally cycling female but also others, have an added problem here that I will discuss below.
The only bodily tissues worth worrying about when it comes to the above are water, the carbohydrate stored within muscle (which is actually related to water storage), digesting food, fat mass, and the part of total LBM that is represented by muscle.
Food residue, the undigested food moving from the gut through the colon before excretion can actually make up 3-7 lbs (~1.5-3 kg) depending on the diet (high-fiber diets tend to produce more food residue) and this can be a significant portion of a woman's total bodyweight in some cases. But over most realistic time frames (i.e. the months that most diets will last), those are really the only tissues that need to be worried about. And the basic bathroom scale can't differentiate between them.
That said, there are some general comments that can be made regarding the relative contribution that changes in food, water, carbohydrate, muscle and body fat may be making changes in body weight and over what time frame.
Almost without exception, very short-term changes in scale weight tend to represent changes in water, glycogen or food residue (a good bowel movement can cause a 1-2 pound weight loss in some cases). Even small changes in the diet can cause scale weight to change pretty significantly in a fairly quick period of time. Someone on a low-sodium diet who eats high-sodium meal may bloat up for a day or two, gaining several pounds of water weight. Chronic stress can also cause water retention due to the increase in cortisol which binds to the receptor involved in water retention.
Dietary carbohydrate intake can enormously impact on water weight. Through a variety of mechanisms, when carbohydrates are lowered, the body tends to lose a lot of water; losses of 1-15 pounds in a few days have been seen in studies of low-carbohydrate diets. Many diet books use this to their advantage. By lowering or eliminating carbohydrates from the diet, body weight drops enormously in a few days and this can be very rewarding to the dieter. It can also backfire when dieters get frustrated that the rapid losses don't continue indefinitely. They may lose 5 pounds in the first week due to water loss and then lose "only" 1-2 pounds per week after that.
This works in the opposite direction as well: someone who has been on a low- or even lowered- carbohydrate diet who eats a lot of carbs for one reason or another can see their weight spike fairly significantly (large individuals may gain 7-10 pounds in one or two days). Every gram of carbohydrate stores 3-4 grams of water with it which explains the big increase in body weight. This partially explains why women often "feel bulky" when they start weight training. Their muscles start storing more carbohydrate and this causes them to store more water. But it goes away within about a week.
While all of the above is true for both women and men but normally cycling women have the added issue of the menstrual cycle (recall that some forms of birth contr