Rogers, P.T. & Coleman, M.
Medical care in Down syndrome: A preventative medicine approach, Chapter 17
Copyright © 1992 by Marcel Dekker, Inc.
|Reprinted with the permission of publisher, Marcel Dekker, Inc.|
The etiology of obesity in Down syndrome is multifactorial, so it is difficult to predict the specific factors in any single individual. Some general trends suggested by the information gathered to date suggest that if a person with Down syndrome is relatively tall, has good muscle tone, and lives at home with excellent preventive medical care, he or she may be at the least risk for obesity. Conversely, a short child living in an institution, somewhat hypotonic, and with the minimum of conventional medical care may be at greatest risk.
In a longitudinal assessment of overweight children with Down syndrome, Cronk et al. studied children living in institutions and at home, including children from a variety of ages from birth to 18 years . The institutions that were studied included one in Australia and one in the United States. The comparison data came from groups without mental retardation. These investigators report that children with trisomy 21 living in institutions tended to have larger mean weights for each stature interval than did those reared at home.
Whether males or females with Down syndrome are more likely to be obese is not clear. The study by Cronk et al. reported a greater tendency for obesity in girls than boys . These investigators raised questions about whether females were constitutionally disposed to greater overweight or whether their greater lack of opportunity for activity caused them to be relatively more overweight than the males. The study comprised 124 girls and 138 boys and is the largest study available. Another study of very young children by Cronk and Pueschel, in 63 boys and 51 girls, reported that the difference between control and Down syndrome children's mean weight was greater for boys, particularly past early infancy . A third study by Baer et al. of 190 children, ranging in age from birth to 18 years, found that 18% of the boys and 14% of the girls had a weight for height ratio greater than the 95 percentile . Apparently, both boys and girls are at considerable risk for obesity.
The key to treatment of obesity is the prevention of it in the first place. In the field in general, it has been shown that childhood obesity tends to be related to adult obesity; this is a consideration in patients with Down syndrome. Thus, an important question that needs to be considered is when does obesity first begin to show up in a population of children with Down syndrome. Apparently, the answer is very early indeed.
In the Cronk and Pueschel study, approximately one-third of the children were classified as being overweight at some time during the course of the 3-year study as follows: 22% became overweight during the first year of life, 55% between 13 and 24 months, and 23% in the third year . In other studies by Cronk et al., two different indicators were plotted out on the same raw data [5,8]. By one calculation, obesity began approximately between 30 and 37 months; by the other calculations, obesity became evident between 4 and 7 years. In another study, Baer et al. reported an increased risk for obesity between 25 and 48 months of age (10% were already above the 95th percentile), and they reported a tendency to gain weight with age .
So no matter what method of calculation is used or which study one looks at, it is clear that the tendency to obesity is present from early childhood in Down syndrome. Understanding that reality will help the development of preventive strategies in the early intervention programs.
Adolescence is another period when children in this patient group appear to be at risk for developing obesity. Children with Down syndrome have a pubescent growth spurt but the magnitude of the growth spurt is often smaller than in normal children (see Chapter 12). It is possible that these children have a normal pubescent fat spurt but that it occurs with a deficient pubescent spurt in stature . This combination could result in excessive weight relative to the limited increase in stature and lead to adolescent obesity. Detailed studies of this problem in this age group are not yet available.
Why are individuals with Down syndrome often fat? The first thought is that they consume excess calories and that they need to be on a diet. However, it is interesting to note that a careful examination of the studies available in the field does not confirm this concept.
To start with institutionalized children (i.e., the group that Cronk et al. found most likely to be obese), Culler et al. analyzed the caloric intake of 23 children 5-12 years old . Each child consumed adequate calories per centimeter of stature, and each child's total caloric intake was less than normal for that age. There also have been studies of home-reared children. In one study of children between 1 and 12 years of age, again more than half of these children had a caloric intake less than that of normal children . However, in this study, a small percentage of the children did exceed the recommended daily caloric intake by more than 50%. In another study of home-reared infants from 3 to 26 months of age, caloric intake was reported to be similar to control data . So it does appear that excessive appetite or overeating is not a major problem in the age groups in Down syndrome at which obesity begins.
Another approach to losing weight is to increase ones activity level. This raises the question of whether underactivity is a factor in the obesity found in Down syndrome individuals. In normal children, a reduced level of physical activity has been shown to be associated with excess weight [12,13]. In the case of children with Down syndrome, specific studies of the question of physical activity have not been done. However, the hypotonia characteristics of young children with Down syndrome could have several effects. Delay in the achievement of motor milestones may limit physical activity during infancy. Later in childhood, poor gross motor performance may limit the amount of sports activity and organized play. There may be an element of the vicious cycle at which decreased physical activity results in excess weight, which then leads to an even further decrease of activity. Hypotonia even is reported to result in decreased levels of physical activity that can affect later psychological evaluations .
Based on what is known so far, one culprit in the development of obesity in Down syndrome is the retardation of growth resulting in short stature. It is the relationship of weight to stature that reveals a disproportion in this patient group. Length in the newborn period is normal or very close to normal if one takes into account the age of gestation [6,15].
However, the velocity of growth in stature shows the greatest deficiency between 6 and 24 months of age (24% less than normal) [16,17]. If one includes total time from birth until 3 years of age, velocity of growth in stature is 14% less than normal. Regarding weight gain, there is also a comparable diminution of weight gain 20% less than normal from 6 to 18 months. However, between 18 and 36 months, weight gain does not differ from a comparison group. In other words, after 18 months, weight gain occurs at a normal (faster) pace than the delayed (slower) rate of height gain in young children with Down syndrome. This disparity of weight versus height percentiles can predispose to obesity. The study by Baer et al. also confirmed that growth retardation occurs early in life, with 59% of the boys and 63% of the girls having a height less than the normal percentile by 4 years of age . Skeletal age, or bone age, appears not to provide relevant information on this particular problem because the bone age in children with Down syndrome differs little from normal children or may in some cases exceed that of normal children . However, that does not change the reality of the height percentiles. A simple way of summing up the relationship of the height and weight percentiles for this patient group is that if many of the children with Down syndrome were not so short, they would not be so overweight.
A number of metabolic abnormalities, associated with obesity in normal individuals, have been reported in studies of Down syndrome. These include abnormal carbohydrate tolerance  and elevated blood lipid levels . Their relevance to the problem of obesity in Down syndrome is not yet defined. A study of resting metabolic rate in 18 home-reared children with Down syndrome is not definitive because it did not have matched controls, but the data indicate that the rate may be depressed . These investigators suggested that if energy expenditure is lower, a regular exercise program incorporated into the child's lifestyle would be the best antidote.
Serotonin, a neurotransmitter that has been linked to appetite control, is known to be low in the platelets (and probably functionally in the brain) of individuals with Down syndrome . It can be elevated by pharmacological doses of pyridoxine , a vitamin found diminished in the whole blood of children with Down syndrome living at home compared to normal age- and sex-matched controls . Again, the effect on obesity is not defined.
Finally, there is the problem of undiagnosed hypothyroidism in some dull, obese patients (see Chapter 12). This form of obesity is preventable and should not occur in any child with Down syndrome in a program conscientiously following the Down Syndrome Preventive Medicine Checklists.
WHAT CAN BE DONE?
There are no final answers regarding the underlying etiologies of obesity in children and adults with Down syndrome. However, it is already clear that there are things that can be done to make a difference in any individual child.
Above all, the importance of prevention cannot be stressed too much. Children with Down syndrome are no different than normal children; it is far easier to prevent obesity rather than struggle to take off the pounds once they are there.
There are many reasons why an infant should never go to an institution, and it is desirable that no person with Down syndrome ever live in an institution. In addition to these other reasons, home-rearing reduces the chances of obesity. This is one preventive measure that should be able to be accomplished.
Also, exercise is important to children with Down syndrome, especially those who have a lot of hypotonia. Whether it is the hypotonia, the shortness of stature, or the possibility of a depressed metabolic rate, it is exercise, more than dieting, that is likely to eliminate tendencies to obesity based on present knowledge. The types of playing that involve a lot of physical exercise are both fun and healthy for these children.
Exercise is part of any infant learning program (see Chapter 2). The fight against obesity may need to start as early as 18 months of age. It is not a good idea to place very young children on diets; they need the fat in the whole milk for crucial myelinization of the central nervous system and elsewhere in the body. This dictum is particularly true of infants with Down syndrome in whom there is no evidence that excessive caloric intake is a factor in potential obesity.
In patients with cardiac disease, any exercise program may need medical supervision.
Another essential part of an antiobesity strategy is an excellent preventive medical program. Today, no individual with Down syndrome should suffer from hypothyroidism untreated.
For the sake of both health and attractiveness, the amelioration of the development of obesity in persons with Down syndrome is a priority.
|Revised: July 27, 2000.|