In this study we demonstrated that infants born to overweight and obese biological mothers exhibited lower extrapolated 24-h EE than those of lean mothers. In other studies parental obesity was associated with a lower resting metabolic rate in children [25
], though others did not find any decline in the resting metabolic rate in already obese adolescents [26
]. We also found that 3–6 month old infants born to obese mothers had a greater percentage of body fat along with greater weight-for-length and weight-for-age percentiles. Moreover, the BMI of the infants was related to that of their mothers. We utilized maternal BMI for classifying the infants since it is a well established criterion for determining obesity in adults [16
] and is highly correlated with body fat in both children and adults [27
The Infants in the obese group spent the least amount of time asleep during the metabolic test. This might be due to them spending more time feeding as evidenced by the greater amount of energy consumed. Furthermore, the metabolic test was conducted between the hours of 9:00 AM and 1:00 PM where sleeping and feeding habits might differ in comparison to that during evening or night periods. Moreover, the lack of differences in the respiratory quotient and the amount of physical activity among the three groups of infants suggest that other metabolic or physiologic components might account for the reduction of extrapolated 24-h energy expenditure. In a previous 24-hour metabolic study, infants born to obese mothers slept more, consumed a greater portion of their energy intake as carbohydrate and tended to have a greater respiratory quotient than their lean counterparts [14
]. It is possible that over a 24-hour period, greater sleeping time and more carbohydrates being oxidized to lipids might be contributing to a lower overall metabolic rate. Moreover, it is possible the greater carbohydrate intake might reduce the thermic effect of food thus also contributing to a reduced metabolic rate [28
]. However, it is very difficult to measure this parameter accurately in infants. It is possible that the shorter duration of this study might have masked these contributions to a lower metabolic rate.
It is possible that the lower extrapolated 24-h EE found in infants of obese mothers might be an underlying contributing factor to the relationships between childhood and adult BMI's found in previous studies [3
]. Moreover, two additional reports described that if children were in the upper percentiles (85–95th
) for BMI, there was a probability of greater than 80% that they would be obese at age 35 [27
]. Finally, the association between infantile and maternal BMI was found in a large cohort study where over 52,306 infants were followed up after birth [30
]. The recent studies of Kral et al [8
] regarding the prevalence of obesity of children from mothers who underwent weight reduction through bariatric surgery suggest an abrogation of the expression of genetic factors in offsprings in post severely obese mothers and that external factors may be more significant in determining weight gain of their children.
In our previous study [7
] where maternal obesity and infant feeding interactions were recorded during 24-hour metabolic measurements in the EMTAC, infants born to obese mothers were shown to consume more energy in a shorter period of time than their leaner counterparts. Obese biological mothers also spent less time interacting with their babies than their normal weight counterparts. The 24-hour measurements of food intake are evidently more accurate and reliable indicator of total energy intake than the four-hour measurement performed in this experiment, though the infants of obese mothers appeared to ingest more energy and were spending more time awake even in a brief period of observation.
We have found in a previous study, utilizing infants from similar ethnic backgrounds, that at least four-hours are necessary to obtain an accurate measurement of infant metabolic rate [14
]. Due to employment obligations and other commitments, the parents of the infants in our study were unable to stay for a longer period of measurement, hence we utilized the minimum time of four-hours that provided valid results [14
]. To date we have studied over 150 infants with various clinical conditions in the EMTAC [14
]. All of the mothers reported feeling comfortable with utilizing the hand access ports when caring and feeding their infants. This probably reduced, or eliminated, the separation anxiety and/or discomfort of the infants that was associated with previous measurements of metabolic rate [11
In regards to energy metabolism, extrapolated 24-h EE and sleeping metabolic rates were both highly positively correlated with fat-free mass. This has been shown in many previous studies in adults [9
] and infants [14
]. This is suggestive that fat-free mass is the main metabolic component of the body. However, none of these relationships had an intercept equal to zero; therefore, this necessitated the adjustment of the energy expenditure data for differences in body composition and age utilizing regression analysis, a mathematical methodology that has been validated in adults [9
] and in infants [21
]. Moreover, body weight alone cannot be used to adjust 24-h EE because the relationship between the two parameters does not pass through zero. This is due to different proportions of metabolically active muscle mass in relation to the assumed non-metabolically inactive fat-mass in same sized individuals, thus causing errors in the adjustment of 24-h EE [9
Actual feeding practices and the relationship to obesity have been studied for decades. In a recent review of 3600 publications in the area of infant feeding and related cardiovascular risk factors [34
], it was found that breast feeding, especially for a longer duration, was found to be protective against future obesity. The infant feeding method and obesity in the Avon longitudinal study of parents and children showed that there was a protective association with attenuation of fat-free mass when breast feeding was prolonged for over six months [35
] while maternal feeding restriction was an important factor determining the effects of breast feeding on future overweight [36
]. Other studies [37
] found that obese mothers of a low social class were more likely to formula feed thus leading to obesity in their offspring. In our study, all infants were formula fed, and the resulting increase in body fatness seems to coincide with the results of previous studies [33
Most of our study population was from Hispanic origin. This is in contrast to the mainly Caucasian subjects in some of the previous studies of the relationship between metabolic rate and obesity [25
]. It has been reported in another previous study that resting metabolic rate is lower in Afro-American boys when compared to their Caucasian counterparts. Moreover, Afro-American girls had lower total daily and activity energy expenditures than Caucasians of similar age and sex [39
]. It is possible that infants from certain ethnic backgrounds begin to show metabolic characteristics that might predispose them to future childhood obesity right from the time of birth.
The onset of obesity can occur in any infant, regardless of ethnic background, that has a lower than average metabolic rate. Knowing which factors play a role in determining which infant is prone to excess body weight gain may allow early lifestyle interventions in order to prevent the future onset of childhood obesity. Future studies with a larger number of infants from Caucasian, and other ethnic groups, need to be conducted in order to determine which infants show metabolic changes that might predispose them to future childhood obesity. Finally, our results indicate that lifestyle interventions should begin right from the time of birth in those individuals predisposed to future obesity.