same size as they were before puberty. The hormones produced by the thyroid gland regulate the rate of metabolism, controlling how quickly the body uses energy, and controls how sensitive the body should be to other hormones. Body fat distribution may change from time to time, depending on food habits, activity levels and hormone levels. When women reach menopause and the estrogen produced by ovaries declines, fat migrates from their buttocks, hips and thighs to their waists; later fat is stored at the abdomen. Body fat percentage recommendations are higher for females, as this fat may serve as an energy reserve for pregnancy. Males have less subcutaneous fat in their faces due to the effects of testosterone; testosterone also reduces fat by aiding fast metabolism. The lack of estrogen in males generally results in more fat being deposited around the waist and abdomen (producing an "apple shape"). Muscles Testosterone is a steroid hormone which helps build and maintain muscles with physical activity, such as exercise. The amount of testosterone produced varies from one individual to another, but, on average, an adult female produces around one-tenth of the testosterone of an adult male, but females are more sensitive to the hormone. The muscles most likely to be affected are the pectoral muscles, biceps and the triceps in the arms and quadriceps in the thighs. On the other hand, estrogens reduce muscle mass . Muscle mass changes over time as a result of changes in testosterone and estrogen levels and exercise, besides other factors. Changes to body shape The aging process has an inevitable impact on a person's body shape. A woman's sex hormone levels will affect the fat distribution on her body. According to Dr. Devendra Singh, "Body shape is determined by the nature of body fat distribution that, in turn, is significantly correlated with women's sex hormone profile, risk for disease, and reproductive capability." Concentrations of estrogen will influence where body fat is stored. Before puberty both males and females have a similar waist–hip ratio. At puberty, a girl's sex hormones, mainly estrogen, will promote breast development and a wider pelvis tilted forward for child bearing, and until menopause a woman's estrogen levels will cause her body to store excess fat in the buttocks, hips and thighs, but generally not around her waist, which will remain about the same size as it was before puberty. These factors result in women's