While a proper diet and daily exercise are both components of a healthy lifestyle, each individual person has a different metabolic rate and different physiology. For this reason, a diet and exercise plan that promotes weight loss in one person might not work as well for another. The most important part of weight loss (commonly expected to be fat rather than water weight or muscle weight) is the ratio of caloric intake versus caloric expenditure; if a healthy person consumes fewer calories than are burned each day, weight will be lost in most cases.
However, if a person's metabolic rate is lower than the average, even a drastically reduced caloric intake may result in weight gains, most likely through water weight, which is partially absorbed and stored in adipose tissue. Many people find that their metabolic rate remains low even though they are dieting faithfully; in many cases, this is due to the body going into "starvation mode," and storing more calories as fat instead of burning them. This is an evolutionary response to times of famine, but remains in the body.
Many people find that diet and exercise together are helpful in maintaining a specific weight. Exercise activates muscular activity and growth, and muscles burn more calories in their resting state than fat does. In addition, while many people find that their caloric intake increases during the first few months of exercise, hunger decreases as their body's metabolic rate increases and their body adjusts to the new routine.
In all cases where weight loss is desired, consult a medical doctor. Fad diets often fail when the body's cravings overcome the rigid structure, and many people stop exercising when they don't see drastic results immediately. Patience is very important, as is persistence.