Losing 13 pounds of fat is an accomplishment, no matter what the scale says. In fact, your body-fat percentage is more important than your weight in terms of overall health. If you truly are losing body fat, stick with a balanced diet and exercise plan and continue working toward your ideal body composition -- but don't get discouraged by a few pounds of weight gain.
Determining Fat Loss
Analyzing body fat is a complicated task, so the first step is to ensure the reliability of your results. Although handheld body-fat analyzers are readily available for consumers, they don't provide accurate readings, according to MayoClinic.com. The same is true of bathroom scales that claim to measure body fat, and even calipers used by professionals have a standard error of 3.5 percent with correct use. The most precise way to learn how much body fat you carry is to get tested by a doctor who uses X-ray technology, a density-measuring chamber, underwater weighing or other medically proven techniques.
If you've been getting regular cardio and/or engaging in strength-training exercises such as lifting weights or performing pushups, pullups and squats, you're probably gaining muscle mass. Muscle is denser than fat, meaning it weighs more per square inch. Plus, new muscle tissue may become inflamed and store extra sugars and water; therefore, people often gain weight after starting a new routine, even as they lose body fat.
Fluid retention throughout your body may also be a factor in weight gain. If you were light on water at your first weigh-in but were retaining water the last time you stepped on the scale, liquid gain could account for much of the difference. Dehydration leads to water weight as your body tries to reduce fluid loss. Sodium is another culprit: When your body retains an extra 400 milligrams of sodium, the amount in a gram of table salt, you gain 2 pounds in temporary water weight, according to Dr. Jack D. Osman of Towson University. Conversely, you may lose water when you sweat during activity or reduce your sodium intake.
Fat Loss and Health
If you truly are shedding fat and gaining muscle, you're on the right track health-wise, no matter what the numbers on the scale may be. Obesity actually refers to body fat, not weight, according to MayoClinic.com, and having too much fat increases your risk of heart disease, diabetes and other illnesses. To be considered fit, women should have 21- to 24-percent body fat while men should have 14- to 17-percent body fat, according to the American Council on Exercise. Female athletes may have as little as 14- to 20-percent body fat, while male athletes may have as little as 6 to 13 percent, but having less than that may cause health concerns.