Table of Contents
In the last 3 lessons I discussed information regarding various aspects of feeding infants and toddlers. Here I will discuss other relevant aspects such as:
- infant growth and how tracking growth can give insight regarding future obesity risk
- exercise in infancy and toddlerhood
- other considerations regarding environmental exposures and antibiotics
Rapid weight gain (“RWG”) and catch-up growth (“CUG”)
RWG has been defined in different ways but a common definition is to have an increase in weight-for-age of 0.67 standard deviations (crossing two percentile lines) in the first 2 years of life.(Arisaka, 2020) CUG describes when infants who are born small for gestational age (“SGA”) undergo rapid growth to “catch-up” to their peers on the growth curves. Thus, RWG can occur in the setting of CUG but it can also occur in infants who are born an appropriate size. It is of great interest to determine what impact these initial phases of growth have on the future risk of developing overweight and obesity, as if they have a significant impact and we detect this early then it is possible we can intervene and adjust the infant’s nutrition to help improve their growth trajectory. I’ll discuss what some of the research suggests here.
Here’s an example growth curve showing RWG taken from the CDC website here:
On average infants will lose weight the first few days of life, gain back to their birth weight by 7-14 days of life (some will take longer), and then gain weight relatively quickly until they are ~6 months old with continued slower weight gain after that. In parallel to this, the initial body fat percentage after birth may be 11-15% in full-term infants but then increase to 25-30% by 6 months of age, decrease to ~20% at age 2 years., decrease further to ~15% and ~17% at age 5 years in males and females, respectively, and then change to ~14% and ~19% at age 10 years in males and females, respectively.(Gallagher, 2020)
Linear growth is also relatively fast in the first few months; these aspects collectively lead to a body mass index (“BMI”) peak between age 6-12 months prior to a decline thereafter. The BMI will typically reach a lowest point at age 5-6 years and then increase after this; the point in time when it begins this second increase is called the “adiposity rebound” (one could argue that “BMI rebound” is a more appropriate term). Of note, BMI is generally not used under age 2 years as the length measurement is less accurate (remember, BMI is proportional to the inverse of the length squared); weight-for-length (proportional to the inverse of the length without squaring) is thus preferable. Of interest, race and ethnic disparities are more apparent with BMI at age 2-5 years than with weight-for-length at age 0-2 years, though the growth curves linked below do not make distinctions by race or ethnicity.(Gallagher, 2020)
Looking at the WHO growth charts for age 0-2 years, which are compiled here, it is clear that infants typically gain ~30 grams of weight daily the first 2 months of life, which then tapers off to 15-20 grams per day by 3-5 months of life, and then further decreases to ~10 grams per day for much of the remainder of the first year. Length increases more readily the first 3-4 months of life (~4-5 cm the first month, 3-4 cm in each of the next two months) tapering to 1-2 cm per month by month 6 and closer to 1 cm per month thereafter.
Note: It is important to understand that it is normal for infants to lose weight the first week of life. One study found that when infants actually gain ≥100 grams (~3 ounces) the first week of life, which was more common if they were formula-fed, they had a higher risk of having an overweight BMI at age 2 years.(Feldman-Winter, 2017) While more research is needed to replicate this finding, there is no reason to emphasize early weight gain, and in particular there is no need to supplement breastfeeding with formula unless the actual weight loss is excessive or the weight gain is very delayed.
The impact of rapid weight gain
A 2018 systematic review and meta-analysis (“SR/MA”) including 17 studies found that RWG up to age 2 years associated with an odds ratio of 3.66 for developing overweight or obesity later in life; 3 of the studies that specifically looked at associations in adulthood found the odds ratio was 2.02.(Zheng, 2018) Most of these studies showed that the associations remained after adjusting for the infant’s birth weight. There seems to be a greater risk when considering RWG throughout the first 2 years of life as opposed to just the 1st year.(Arisaka, 2020) Thus, it is likely the trends that are established during infancy that are subsequently carried forward lead to increased long-term risk.
A 2019 SR evaluating BMI trajectories in childhood included 14 studies.(Mattsson, 2019) For the most part the studies found the BMI trajectories were either:
- Normal BMI throughout
- Low BMI throughout
- High BMI throughout
- RWG at some point in the 1st or 2nd year
Of the studies that examined longer term obesity and body composition outcomes there were not many differences between the stable high and RWG trajectories. Thus, starting at an elevated BMI and remaining elevated (ie, a child born large for gestational age (“LGA”) who does not regress to a smaller size) or undergoing RWG in the first two years and then remaining at an elevated BMI both seem to contribute to similar health outcomes.
A 2021 SR on the impact of infant growth on subsequent obesity risk in childhood included 24 studies.(Halilagic, 2021) Almost all found associations of RWG with subsequent development of overweight, obesity, increased waist circumference, or less healthy body composition, with significant variability in the strengths of the associations as well as the impact of rapid growth in the early or later portions of the first 2 years of life.
Note: There is some evidence that an earlier age of adipocyte (fat cell) expansion and a faster rate of increase in adipocyte number can lead to a persistent elevated adipocyte cell counts into adulthood.(Spalding, 2008) This may be one of the reasons why increased weight gain at an early age is associated with elevated obesity risk in the future. However, this is speculative, and a recent publication indicates that adipocyte number can increase in adulthood as well.(Arner, 2022)
Separately, one study demonstrated that infants with greater levels of fat-free mass in the first several months of life continue to have higher levels of fat-free mass at 4 years of life, and the same also applies to fat mass.(van Beijsterveldt, 2021) When you look at a growth curve and determine if an infant has rapid weight gain the growth curve does not give any information regarding underlying body composition. Perhaps further research will help determine what impact, if any, an infant’s underlying body composition influences any association of rapid weight gain with long-term body composition and health status outcomes.
More research is needed to determine why RWG in childhood is associated with a future increased risk of obesity.
The impact of catch-up growth
In a 2019 narrative review regarding feeding interventions targeting weight status in young children, the authors noted that less is known about how to manage feeding in babies born at low birth weight who are undergoing CUG; accelerated growth may have cognitive benefits while it may increase the likelihood of future cardiometabolic risks.(Koplin, 2019) A 2020 SR/MA evaluating the influence of prematurity and RWG on the risk of obesity found from 4 studies that accelerated weight gain in preterm infants associated with 1.87 greater odds of obesity at age 8-11 years.(Ou-Yang, 2020)
It has been speculated that this increased risk may result from poor programming of neuroendocrine circuits, cellular aging, and/or epigenetic mechanisms, as well as increased energy conservation secondary to suppressed thermogenesis and associated insulin & leptin resistance (this can additionally increase hunger) that contributes to the storage of glucose in adipose tissue as opposed to the oxidation of glucose in skeletal muscle.(Arisaka, 2020; Martín-Calvo, 2021) In particular, RWG in infants born SGA (compared to infants born SGA who do not undergo RWG) has been associated with adverse cardiometabolic outcomes in early adulthood including lower insulin sensitivity, lower HDL-cholesterol (the “good” cholesterol), and higher triglycerides; these findings were potentially mediated by increases in visceral fat deposition.(Arisaka, 2020)
Thus, it is unclear how to proceed with CUG in infants born SGA to balance the neurocognitive benefits with cardiometabolic risks. Perhaps undergoing CUG in the first few months to somewhat close to the 30th percentile and then attempting to slow the rate of increase towards the 50th percentile curve over the next few years would be a good compromise to balance neurocognitive benefits with future cardiometabolic risks, but we will need more studies to determine if this is a useful strategy.(Hong, 2018)
Note: It is commonly recommended to provide various supplements to infants born preterm. For example, Neosure is a formula that is frequently given to infants born preterm as it is made of a composition that helps to supplement the nutrients that are typically provided throughout the 3rd trimester of pregnancy (infants born preterm will not get all of these nutrients during pregnancy due to being born early).
A 2020 SR/MA included 42 studies to determine the impact of macronutrient supplements on growth and bone development in preterm and SGA infants, but only 3 of the studies included SGA infants with the rest including preterm infants.(Lin, 2020) Regarding these preterm studies, the authors found no consistent and meaningful difference in BMI, weight, length, head circumference, fat mass, or lean body mass between infants who were or were not provided the supplements. Greater bone development and bone mineral density was seen in toddlers who were previously supplemented (with standardized mean differences of 0.40-0.43, thus a relatively small effect).
Therefore, there seems to be a benefit to this supplementation at least for bone health, and there are potentially other health or neurocognitive benefits not detected when looking solely at anthropometric outcomes. Importantly, there was no indication of an increased risk of obesity due to these supplements. It thus seems possible to appropriately provide supplements without causing excessive weight gain.
Exercise in young childhood
There is a general exercise course on this website that discusses exercise in much more detail, but that is designed for older children/adolescents/adults. Here I’ll discuss basic recommendations for younger children. While this may not aid weight management directly at this age, increasing physical activity generally has many health benefits and building up habits and a lifestyle that includes a lot of physical activity now will pay dividends if this is continued into later years.
When infants are first born they will squirm around a lot; you can encourage movement by allowing them to be free as opposed to being in a swaddle but it’s completely acceptable to swaddle your infant for comfort if they are fussy otherwise.
Starting at age ~2 months you will be in a better position to help your infant with exercise; this is the age when you can more effectively engage in tummy time with your infant. You can think of this as your baby’s first exercise. A 2020 SR of tummy time found a positive association with tummy time and overall development and ability to move, with goals of 30 minutes daily when <6 months old.(Hewitt, 2020) Remember, an infant should be placed on the back when sleeping but while awake and in your presence being placed on the tummy for this purpose is healthy and safe. Working up to at least 30 minutes daily (not consecutively, this can be spread throughout the day) is a good goal.
In 2020 the American Academy of Pediatrics published guidance for the assessment and counseling of physical activity in clinical settings.(Lobelo, 2020) The report is free and available here. Throughout the first year of life interactive floor-based activity is recommended. You can provide infants toys to hold on to and examine when they are 3-4 months old and able to grasp. When infants progress to the point they can sit on their own (usually at ~6-7 months) they should be allowed to do so and given toys for entertainment. You can converse with your infant while they play. Closer to 9 months they may begin crawling or otherwise moving around the room; this is great for development as long as the infant’s environment is safe (staircases blocked off, all small objects that could be choking hazards should be put away, there should be no sharp objects or strings/cords that would be strangulation risks, outlets should be safely covered, etc). You can place toys out of their reach so they have to move to get to them.
In the toddler age range (once reaching age 1 year) it is recommended to be physically active at least 3 hours daily. This is because most toddlers will not be continually active for 3 hours in a row, rather they will start and stop a lot during this time so the full 3 hour session will yield perhaps ~60 minutes of actual physical activity. Once toddlers begin walking (which may occur prior to 12 months or may not occur until 15-18 months with typical development), there are many more possibilities for physical activity. As long as toddlers have a safe environment with open space and sufficient stimulating toys, they should be able to keep themselves busy in a fun and productive manner. You can perform interactive activities such as walking outside, playing together in the backyard, using a playground designed for toddlers, or setting up small indoor games such as kicking or throwing a ball into a basket.
Tip: A 2019 SR/MA examined interventions designed to increase physical activity in children from 0-5 years of age and found most interventions were not successful or only yielded small changes.(Hnatiuk, 2019) However, the interventions that did show greater success generally led to positive changes in parents and caregivers. Thus, it seems role-modeling is imperative to help young children become more physically active; many children will want to move around more if they get to do so with their caregivers.
Various environmental exposures have been found to associate with increased risks of obesity.
A 2020 study found potential contributing factors to an increased childhood BMI include increased population density, indoor and outdoor air pollutants, and higher blood levels of some heavy metals (copper, cesium), while some factors (organochlorine pollutants & higher blood levels of cobalt and molybdenum) associated with a lower childhood BMI.(Vrijheid, 2020) A 2021 SR/MA found there are associations between traffic-related air pollutants and street intersection density with childhood obesity.(Malacarne, 2021) Some of the above factors such as heavy metals may affect various enzymes in the body and potentially lead to changes in metabolism, hormone levels, or other aspects of health. Factors related to population density and street intersection density may associate with the ability to exercise and play outside and may also associate with air pollution.
Specific concerns regarding air pollution
A 2021 SR/MA included 8 articles examining the impact of air pollutants found various measures of these were associated with small increased risks of obesity (odds ratios 1.06-1.13). The authors note that air pollutants have variably been associated with adipose tissue inflammatory responses, oxidative stress, poor sleep, decreased physical activity, and worsened mental health, all of which may contribute to increased weight gain.(Parasin, 2021)
As recently reviewed in the context of an interaction between the exposome (this refers to all of the various environmental exposures that are experienced throughout one’s life) and physical activity, air pollution may be particularly harmful for younger children due to(Gorman, 2021):
- Younger children have faster respiratory rates, a more permeable respiratory epithelium, and greater lung surface are per unit body weight.
- Younger children also more frequently exercise at higher intensities, leading to even faster respiratory rates that result in increased mouth-breathing, decreased mucociliary clearance, and raised pulmonary diffusion capacity.
- These factors contribute to increased particulate accumulation in younger children. In addition to the negative health associations mentioned above, this also associates with decreased production of beneficial brain-derived neurotrophic factors that are typically released during exercise.
Thus, while the impact of air pollution on obesity risk may be relatively small, it may negatively impact a child’s health in other ways as well. While I do not think that the risk of air pollution outweighs the benefits of regularly exercising outdoors, for children who are acutely ill with a respiratory virus or who chronically have asthma where air pollution is a trigger for worsening symptoms, if you are in an area where air pollution is severe it may be wise to preferentially engage in indoor activities.
A 2018 SR/MA of 17 articles found that greater broader-spectrum antibiotic exposure in childhood associated with greater odds of overweight and obesity, though the authors do note that one study found infants with untreated infections had the same odds as infants who were treated so it is possible that underlying infections lead to the greater risk.(Miller, 2018) A 2021 dose-response MA of 10 articles found that with increasing antibiotic exposures in childhood there were increasing odds of developing overweight or obesity, with odds ratios of 1.09/1.16/1.24/1.30/1.35/1.40 for 1/2/3/4/5/6 exposures, respectively.(Meng, 2021)
Thus, it is not crystal clear if it is underlying infections or the antibiotics themselves that increase the risk of obesity given the one study that compared both found it was the infections that increased the risk. Any increased risk from antibiotics may stem from a negative effect on the gut microbiome. While I do believe children with underlying infections that require antibiotic treatment should receive the antibiotics, this may be another reason why antibiotics should not be utilized unless they are actually indicated.
In this lesson I summarized typical growth in infants and toddlers and how rapid growth, whether born an appropriate size or small, can impact future obesity risk. This information can be considered if growth curves indicate rapid weight gain is occurring. I then covered other pertinent topics including exercise, environmental exposure, and antibiotic utilization. In general, promoting safe physical activity is great for general health and to help establish healthy habits, environmental pollutants can have negative health effects but generally should not preclude outdoor physical activity (barring significant underlying health conditions and/or very hazardous air quality), and antibiotics should only be used when indicated.
Together, this and the prior 3 lessons regarding various aspects of feeding infants and toddlers provide most of the relevant information for young children as it pertains to current and future risk of acquiring excess body weight. In the next several lessons I will discuss what the literature suggests for older children and adolescents.
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