People who wish to change the shape of their body typically hope for a reduction in body fat, rather than an increase in it. If your legs are excessively skinny, however, you might wish to gain fat in this part of your body for aesthetic reasons. Because fat gain is largely unhealthy, you shouldn't attempt to gain fat unless your doctor specifically advises you to do so.
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Focus on Leg Strength
A healthier approach than attempting to gain leg fat is to focus on building the size of your legs through exercise. Your legs have several major muscles, including the quads, hamstrings and calves, each of which you can target through exercise. Cardiovascular activities such as jogging can help develop these muscles. Strength-training exercises such as squats and lunges can also be beneficial, as can yoga, which uses your body's weight as resistance.
Be Wary of Fat Gain
Before you attempt to develop a diet or lifestyle that leads to fat gain, familiarize yourself with the risks of becoming overweight or obese. Adding some body fat doesn't necessarily mean you'll be overweight or obese, but maintaining a healthy weight limits your risk of weight-related conditions. When you have excess body fat, you have a heightened risk of heart disease, high blood pressure, Type 2 diabetes, several types of cancer, sleep apnea and osteoarthritis.
Create a Caloric Excess
Although it's possible to add body fat, you can't specify the part of your body in which you gain fat -- just as you can't choose which fatty area you wish to reduce if you're trying to lose weight. To gain fat, your caloric intake must exceed your caloric burn. For example, if you consume 1,800 calories in a day but only burn 1,500 calories, you've created a caloric excess of 300 calories. Every caloric excess of 3,500 calories makes you gain 1 pound.
Make Diet and Exercise Changes
Making a daily stop at your nearest fast-food restaurant can help put your body in a caloric excess, but your overall health will suffer. A healthier way to boost your caloric intake is to eat foods high in polyunsaturated and unsaturated fats, which are healthy types of fat. These foods include fish, nuts, seeds and oils such as olive oil. Reducing your overall amount of physical activity can also lead to a caloric excess. Physical exercise, however, is an important way to keep healthy, and you should only limit your exercise if your doctor recommends doing so.