肥胖的营养治疗

第十三章代谢性疾病营养治疗

第一节肥胖症

Obesity is defined as an excess body fat content for stature. Ideally, the best assessment of obesity would be a measure of actual body fat. Among adults, the body mass index (BMI, expressed as

kg/m2) has been recommended internationally as a measure of obesity. A BMI between 25.0 and 29.9 defines overweight (24.0 to 27.9 for Chinese); a BMI ≥30 defines obesity (≥28 for Chinese). Unlike adult, no universal definition of the weight-for-height measures necessary to define obesity in youth exists, but definitions have been proposed. WHO expert committee recommended that for infants and children a child’s weight for height be compared with mean age- and sex-specific values in the National Center for Health Statistics (NCHS) and WHO reference data. Children with a

weight-for-height value >2 standard deviation scores (Z score) above the median are classified as overweight. For adolescents the committee recommended BMI as the best indicator; however, age and sex must still be considered. Adolescents whose age- and

sex-specific BMI is ≥85th percentile BMI of a reference population are classified as at risk for overweight.

Obesity can be viewed as both a disease and a risk factor for chronic disease and other morbidities. Increased fat in the abdominal region predicts physiological risk factors for cardiovascular disease (CVD), independent of body mass. Among adults, both ratio of waist circumference to hip circumference and the waist circumference alone have been used to assess body fat distribution. Waist-to hip

ratios >1.0 for men and >0.85 for women have been used to identify

people at increased risk for morbidity. For both adults and children, obesity is associated with a higher prevalence of risk factors for cardiovascular disease and type 2 diabetes. People with overweight or obesity are more likely than normal-weight people to experience elevated blood pressure; dyslipidemias, such as high levels of total cholesterol, triglycerides, and LDL and low levels of HDL; and insulin resistance. They are also more likely to experience 2 or more these risk factors.

Ultimately, overweight and obesity are caused by a chronic imbalance between energy intake and energy expenditure. Two behaviors are thought to contribute to energy imbalance; excessive energy intake and inadequate physical activity. Genetics also contribute significantly to obesity. Ultimately, any intervention to facilitate weight loss or prevent weight gain must result in an appropriate relation between energy expenditure and energy intake within the individuals: to facilitate weight loss, expenditure must exceed intake; to prevent weight gain, expenditure must equal energy intake.

The goal of dietary modification is to reduce energy intake. This strategy is an important and effective strategy to facilitate weight loss for both adults and children. Among overweight adults, low-calorie diets (3.35-6.28MJ[800-1500 kcal]/d) can reduce body weight by an average of 8% over 3-12 months. Increased physical activity may aid weight loss by increasing energy expenditure and by helping maintain lean body mass and thus resting metabolic rate. Increased physical activity contributes modestly to weight loss for both adults and children, but the effect of physical activity alone is generally smaller than that of energy reduction. Physical activity does appear to be important in long-term weight maintenance.