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Rapid Responses: Rapid Responses published:
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Vicious cycle?
- Rahul Potluri (26 November 2007)
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A possible confound: genetics
- Michael Meadon (29 November 2007)
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Author's reply to Michael Meadon
- Calum G. Mattocks, Andy Ness, Kevin Deere, Kate Tilling, Sam Leary, Steven N. Blair and Chris Riddoch (5 December 2007)
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Should early determinants be the main focus?
- Mariane Héroux (5 December 2007)
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Re: Author's reply to Michael Meadon
- Michael Meadon (12 December 2007)
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Rahul Potluri, Final year medical student The Medical School, University of Birmingham, United Kingdom
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This study underlines an important point. Whilst it is indeed an important finding that early life influences only modestly correlate with physical activity in 11-12 year olds, the suggestions that active parents tend to raise active children is very important. Whilst we tend to highlight rapidly increasing levels of obesity in children and adults, there isnt much research on the psycho-social and behavioural interplay between adults and children leading to obesity. It is well known that during child development, children "pick up" numerous traits and behaviours from adults around them. Behaviours such as little physical exercise, over-eating and drinking may indeed be learnt behaviours by children from parents and may in-turn become a vicious cycle when these children become adults inspite of all the education, health warnings and other interventions in place to tackle obesity. Learnt behaviours in formative years of early life are not easy to change. It has been suggested that people who have been bullied/abused as children carry psychological scars that often cannot be cured by therapy. Whilst these are drastic examples, perhaps eating behaviours and attitudes towards the role of physical exercise may similarly be difficult to change in later life in-spite of all obesity management strategies currently in place such as the ban of "Junk Food" in schools and ban of "Junk Food" advertising on children's television channels. Therefore, it may be important to formulate future childhood obesity strategies targetting both children and their parents/guardians whilst the child is still in their formative age and learning life-long behaviours. For example, such strategies could involve parents being routinely educated by a multi-disciplinary team including a dietician regarding the role of healthy eating during the first few years of a child's life. It is imperative to break this vicious cycle. Competing interests: None declared |
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Michael Meadon, Graduate student, Cognitive Science University of Kwa-Zulu Natal
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This study is unfortunately significantly undermined by a lack of methodological rigour: it proceeds as if the entire field of behavioural genetics does not exist. Children share 50% of their genes with each parent, and since all human behavioral traits are heritable (the so-called First Law of Behavioral Genetics, Turkheimer, 2000), genetic factors are always possible confounds when relating parenting style (or other parental behavior) to outcomes in the children. As Turkheimer explains: "It is no longer possible to interpret correlations among biologically related family members as prima facie evidence of sociocultral causal mechanisms. If the children of depressed mothers grow up to be depressed themselves, it does not necessarily demonstrate that being raised by a depressed mother is itself depressing. The children might have grown up equally depressed if they had been adopted and raised by different mothers, under the influence of their biological mother's genes" (2000: 162). The exact same problem holds for Mattocks et. al.'s study: one can't simply assume parental physical activity (or lack thereof) influences children to be active (or inactive) because it's possible sedentary children inherit sedentary genes from their sedentary parents and active children inherit active genes from their active parents. Or, to put it differently, the fact that the physical activity of parents when the children were young is correlated with the children's degree of activeness later on simply does not constitute evidence of a socialisation effect. To be clear, I'm not claiming children are not socialized in this way; my point is we cannot tell one way or the other from the data presented because it fails to distinguish between the relevant causal hypotheses. In its current form, I'm not quite sure what the point of this study is: basically it tells us 'active parents raise active children OR active parents have active children OR some combination of the two OR some other factors are at play'. (Note: I have also written a blog entry about the study, it can be found here: http://ionian-enchantment.blogspot.com/2007/11/peer-reviewed- nonsense-active-parents.html) References Mattocks, C., Ness, A., Deere, K., Tilling, K., Leary, S., Blair, S. N., & Riddoch, C. (2007) "Early life determinants of physical activity in 11 to 12 year olds: cohort study," BMJ (British Medical Journal). doi:10.1136/bmj.39385.443565.BE Turkheimer, E. (2000) "Three Laws of Behavior Genetics and What They Mean," Current Directions in Psychological Science, 9(5): 160-164. Competing interests: None declared |
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Calum G. Mattocks, Research Associate Department of Social Medicine, University of Bristol, BS8 1TQ., Andy Ness, Kevin Deere, Kate Tilling, Sam Leary, Steven N. Blair and Chris Riddoch
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Dear Editor We thank Michael Meadon for his comments, however, there appears to be some misunderstanding on his part regarding genetic epidemiology and the nature of our study. It is not possible, at present, to control for “genetic factors” as a confounder in studies such as ours, as might be done with, say, socioeconomic status. The role of genetics in physical activity is poorly understood due to a lack of evidence. (1) Twin and family studies have reported a wide range in heritability estimates. (2) The relative contributions of genes and environment to sports participation in youth has been reported to change with age. (3) To date, no single “physical activity gene” has been discovered (although a few have been implicated). (1) Research into the identification of the genetic determinants of physical activity in children and adults is in its infancy and the ability to control for “genetic factors” in observational studies such as ALSPAC is some way off, as a familiarity with the rather large literature on the determinants and correlates of physical activity would show. Mr Meadon also seems to have misunderstood the aims of our study. (4) As part of an ongoing programme of physical activity research in the Avon Longitudinal Study of Parents and Children (ALSPAC), we had the opportunity to examine the associations between physical activity and some of the early life factors measured in ALSPAC. To state that “This study is significantly undermined by lack of methodological rigour” seems a somewhat exaggerated statement in light of the fact that parental physical activity amounted to five of the 29 early life variables analysed in our paper. As stated above, it is not currently possible to control for “genetic factors” and in our study, would require identification of the genes associated with physical activity (this is probably some years away) and measurement of those genes in ALSPAC DNA samples, before adding those variables to a model. 1. Cai G, Cole SA, Butte N, Bacino C, Diego V, Tan K, et al. A quantitative trait locus on chromosome 18q for physical activity and dietary intake in Hispanic children. Obesity 2006;14(9):1596-604. 2. Stubbe JH, Boomsma DI, Vink JM, Cornes BK, Martin NG, Skytthe A, et al. Genetic Influences on Exercise Participation in 37.051 Twin Pairs from Seven Countries. PLoS ONE 2006;1(1):e22. 3. Stubbe JH, Boomsma DI, De Geus EJ. Sports participation during adolescence: a shift from environmental to genetic factors. Med Sci Sport Exerc 2005;37(4):563-70. 4. Mattocks C, Ness A, Deere K, Tilling K, Leary S, Blair SN, et al. Early life determinants of physical activity in 11-12 year olds: a cohort study. BMJ 2007;doi:10.1136/bmj.39385.443565.BE. Competing interests: None declared |
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Mariane Héroux, Graduate Student Queen's University K7L 3N6
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Although there may be factors in a child’s early life that influence his/her physical activity patterns as an 11 or 12 year old, it is important to consider many immediate environmental and social factors. These immediate factors likely play a more dominant role on a child’s physical activity behavior than do early determinants. In this study the predictive factors that occurred before the age of 5 (such as the exercise patterns of parents) may be confounded by more recent circumstances and socio-economic variables. For example a child who views his/her primary care giver as a source of inspiration is more likely to imitate their behavior and follow their physical activity patterns. In Swedish schoolchildren, physical activity and obesity were both linked to caregiver influences1. A study by Thompson and colleagues also found similar results2. Many studies have also found that school setting, and peer influences are key factors123. At this age, children are highly influenceable, not only by older role models, but also by their peers. A study looking at obesity patterns in social groups found obesity to be related to social networks. The study showed that a person’s chances of becoming obese increased by 57% (95% CI 6 to 123) if he or she had an obese friend4. The study also suggested the possibility of applying the observed network phenomena to spread positive health behaviors such as physical activity4. Another important factor is play space. Lack of safe play environments is negatively associated with childhood physical activity. In their study of inner- city children in the US, Farley and colleagues showed that when provided with a safe play space, physical activity of children increased significantly suggesting that a child’s living environment influences physical activity patterns5. The examples presented above are few of many immediate factors that must be considered. Therefore, although the information revealed by this study is important, it is very likely that current factors in a child’s life have a greater effect on his/her physical activity patterns than do factors that occurred before the child’s 5th birthday. References: 1. Villard, L., Ryden, L., & Stahle, A. (2007). Predictors of healthy behaviors in Swedish school children. European Journal of Cardiovascular Prevention and Rehabilitation 2007;14:366-372. 2. Thompson, A., Rehman, L., & Humbert, L. Factors influencing the physically active leisure of children and youth: A qualitative study. Leisure Sciences 2005; 27:421-438. 3. DiLorenzo, T., Stucky-Ropp, R., Vander Wal, J., & Gotham, H. (1998). Determinants of exercise among children. II. A longitudinal analysis. Preventative Medicine, 1998;27:470-477. 4. Chrisakis, N., & Frowler, J. The spread of obesity in a large social network over 32 years. The New England Journal of Medicine 2007;357(4): 370-379. 5. Farley, T., Meriwether, R., Baker, T., Watkins, L., Johnson, C., & Webber, L. Safe play spaces to promote physical activity in inner-city children: results from a pilot study of an environmental intervention. American Journal of Public Health 2007;97(9):1625-1631. Competing interests: None declared |
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Michael Meadon, Graduate student University of Kwa-Zulu Natal
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I thank Mattocks et. al. for taking the time to reply to my criticisms. A couple of points in their response bear commenting upon. Mattocks and his co-authors seem to make two major points in their reply: (1) that it is currently impossible to control for genetic confounds (their first paragraph) and (2) that not all the variables they examined are confounded by genetics (their second paragraph). Let's grant, for argument's sake, that Mattocks et. al. are correct with respect to (1): that, at present, it's not possible to control for genetic confounds. What follows? Does assuming this proposition at all support the truth of their finding concerning the link between active parents and active children? Clearly not - the fact that we cannot control for a confound manifestly (and unfortunately) does not make it causally inert. It is one thing to say we cannot control for some variable; quite another to say we need not control for some variable. Since genetics clearly is a possible confound (something Mattocks et. al. do not dispute in their reply), we simply can't answer many questions with their data. I note, parenthetically, that Mattocks et. al. (a) do not mention genetics in any part of their paper despite the existence of a section called "Possible confounders", and (b) draw causal conclusions - without caveat or proper comment - despite there being an obvious possible confound. In reply to (2): Mattocks et. al. are certainly correct that not all the variables they considered are possibly confounded by genetics. I note, however, that the authors themselves emphasize the correlation (what they regard as causation) between maternal physical activity during pregnancy and early life and children's physical activity later in life (see the conclusion of the abstract). Furthermore, it seems a majority of the study's positive findings are at least possibly confounded by genetics, even if in some cases a particularly plausible causal mechanism is absent. In short, genetics is a possible confound (a fact that remains unaltered whether or not it's possible to control for it), but, despite this, the authors did not even try to control for it and readily drew causal conclusions in the absence of controls. My criticisms stand. Competing interests: None declared |
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