Because a number of factors can affect metabolism, there isn't a particular age when a teen's metabolism slows down -- it varies from one teenager to another. The ups and down in metabolism occur with growth spurts during puberty and can be affected by additional muscle growth, cutting calories in attempts to lose weight or certain medical conditions. Check with a doctor if you have questions about your teen's metabolism.
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Teen Metabolism Overview
During puberty, teenagers have growth spurts that cause them to use more energy, increasing their metabolism somewhat and requiring them to eat more calories. Once teenagers have finished growing, their metabolic rates may slow down slightly. Approximately 25 percent of growth in height happens during puberty, and during this time, the body needs extra calories to form the muscle, bone, fat and other tissues that make this growth possible. However, there's a wide range of "normal" when it comes to puberty. Some children start later than others or develop more slowly, which can mean their metabolisms slow down or speed up at different times than those of their peers.
Metabolism of Teenage Girls
Girls usually have growth spurts earlier than boys, often gaining approximately 1 pound each month from 11.5 to 12.5 years old. Much of the weight girls gain during puberty is in the form of fat, because higher body fat levels are necessary for reproduction. Girls typically reach adult height by the time they are 14 or 15, and metabolism will have slowed somewhat by then.
Teenage girls are often concerned about the weight they gain as they develop, and sometimes begin unhealthy dieting practices such as fad diets. Eating too few calories isn't healthy though, because the diet won't have adequate amounts of nutrients for proper growth and skeletal development. Girls between 11 and 14 years old need between 1,500 and 3,000 calories per day, and those between the ages of 15 and 18 need between 1,200 and 3,000 calories per day, depending on whether they're still growing and their weight and activity levels.
Metabolism of Teenage Boys
Boys usually grow taller quickly between ages 12 and 15, with growth slowing around age 16, although they may continue adding more muscle after this age. The average 14-year-old boy will gain about 14 pounds before he turns 15, then the weight gain slows down as he reaches his adult height. Most of this weight is in the form of muscle.
Teenage boys between the ages of 11 and 14 need anywhere from 2,200 to 3,700 calories per day, and boys from 15 to 18 years old need 2,100 to 3,900 calories per day. More muscular teenagers will need more calories, even after they stop growing, because muscle takes more calories to maintain than fat.
Other Causes for Decreased Metabolism
Certain medical conditions, including hypothyroidism and diabetes, can cause metabolism to slow down, so check with a doctor if this is a concern. People with hypothyroidism don't produce enough of the hormone thyroxine, causing decreased metabolism, weight gain, fatigue, constipation and a slow heart rate. Some teenagers who develop type 2 diabetes are overweight, which may contribute to the development of the disease. Teens with diabetes have increased blood sugar levels, because their bodies no longer respond properly to insulin.
If a teenager's metabolism hasn't already slowed down, it will most likely start to as he leaves the teen years behind. After the age of 20, metabolism usually slows by about 2 to 3 percent every 10 years. This amounts to burning about 150 fewer calories each decade.