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  1. All, You may have seen the recent story in the popular press about the possibility of "mysterious" causes of the obesity epidemic with titles like: Obesity is about More than Diet and Exercise: Why It’s Harder to Stay Healthier Now More Than Ever and: Not mom's weight loss: For millennials, more than diet and exercise at play Here is the sensational way the reporter in the first story describes it: According to research, a person with the same diet and exercise habits in the 80s would have a 2.3 point increase in their BMI in 2006 without changing anything at all — simply by existing 30 years in the future. That’s right — existing in our modern environment has contributed to a greater body mass among the population. Wow - that sounds bizarre, scary, and oddly comforting at the same time. Maybe it's not our lifestyle choices that are making us fat, so maybe it's not our fault. I've got to read more! ... They are reporting on a new study [1] that they (and the authors, to some degree) claim shows that over the last 30 years Americans have gotten fatter for reasons other than eating too much food and/or engaging in too little physical activity. The reporters speculate about a range of causes, including: We are more stressed and sleep less. We are exposed to more pesticides and industrial chemicals. Because of our changing diet we have less healthy gut microbiomes. We take more medications associated with weight gain than we used to, such as antidepressants. We increasingly live in climate-controlled worlds that don't require us to burn calories to maintain our body temperature. So I decided to look at the actual study to see if it is sound, and what the authors actually said. The paper [1] analyzed the NHANES data from 1971 to 2008, in which every few years researchers went to the homes of about 3000 Americans (men and women) to take measurements (e.g. height and weight) and ask them questions about their diet and lifestyle. Here is the key table from the paper showing how the various measures changed over the years (note - they are interviewing different people every few years, this isn't a longitudinal study in which they follow the same people over time): The first thing they observed is that BMI has gone up about 10% (3 units) in both men and women since the early '70s - the population has obviously gotten fatter. They also observed that self-reported calorie intake has gone up by 10-14% over that same period. So people are eating more, and getting (nearly proportionally) fatter - where is the discrepancy? The authors say that self-reported leisure time physical activity (PA) rose by 47% and 120% in men and women, respectively, over the latter part of the time period (physical activity wasn't recorded prior to 1980 in the NHANES protocol). So, the authors conclude, if people say they are only eating 10-14% more, and are exercising a ton more, their weight shouldn't have gone up by 10%, so something else besides diet and exercise must explain their weight gain. No potential flaws in this logic, no siree ... Seriously. The NHANES data was interviewing different people, using different questioning techniques and different questions over the years. In NHANES I (1971—1975) and NHANES II (1976—1980), in-person interviews were used to obtain self-reported dietary information via a 24- h dietary recall questionnaire that assessed food and beverage intake for weekdays only. In NHANES III (1988—1994), dietary information was obtained through a self-reported 24-h dietary recall using a computer-assisted, automated, interactive method for any day of the week. In NHANES 1999—2002, a multiple-pass computer-assisted dietary interview format was used to collect detailed self-reported information about all foods and beverages that were consumed the day prior to the in-person interview (weekday or weekend). In NHANES 2003—2008, 24-h self-reported dietary recalls were performed twice (3—10 days apart) using an automated multiple pass method. Right off the bat, people may have been more reluctant fudge the numbers on what / how much they ate during the face-to-face in-person interviews from 1971-1980, relative to the impersonal computer-based data collection used after 1980, which could easily have resulted in subjects underestimating calorie intake during the later years. Plus, it's well known that self-reported diet recall is a crappy source of information about what people actually eat. More importantly, the heavier people are, the more likely they are to underreport how much they eat, and overreport how much physical activity they engage in. So as people got fatter over the years, the "mystery" of why they are gaining weight when not eating that much more, while exercising a lot more, may simply be that they lie more about their eating and exercise habits as they get fatter. In fact, the authors suggest this effect as the first possible explanation for their findings: Whether self-reported dietary intake accurately reflects an individual’s true dietary intake has been questioned [34]. Indeed, doubly-labelled water studies typically show that individuals underreport their energy intake, and that the magnitude of the underreporting may be larger in people who are obese [35]. <snip> This finding is in line with those of several other studies in which individuals with obesity reported consuming similar or fewer daily calories than those who are normal weight [7,11]. While this has frequently been attributed to underreporting [19,20], several additional possible explanations must be considered... The authors then go on to speculate about a few of the 'mystery' causes of weight gain mentioned in the popular reports and listed above. This seem to me to be a clear case of the authors of the study, and especially the reporters writing about the study, downplaying the most likely explanations (i.e. under/over reporting of diet and exercise by the obese, changes in protocol skewing results) in favor of speculative explanations that appeal to people's desire to avoid personal responsibility for their weight gain, e.g. "it isn't your fault - you've gained weight because of the mysterious obesity-promoting chemicals in our food or environment these days". To be fair, the second (CNN) article linked above does acknowledge at the end the possibility that the explanation could be a mundane combination of misremembering and/or intentional underreporting. But it nonetheless stresses the significance of mysterious causes, likely to gain more eyeballs for their story. Pretty sad... Note: I'm not saying (definitively) that these other possible factors haven't contributed at all to the recent dramatic weight gain among the American population. All I'm saying is that this study provides extremely weak evidence to support such speculations, and that the mundane explanation of a positive energy balance due to eating too much and exercising too little is likely the cause of the vast majority of the observed weight gain. --Dean ---------- [1] Obes Res Clin Pract. 2015 Sep 14. pii: S1871-403X(15)00121-0. doi: 10.1016/j.orcp.2015.08.007. [Epub ahead of print] Secular differences in the association between caloric intake, macronutrient intake, and physical activity with obesity. Brown RE(1), Sharma AM(2), Ardern CI(1), Mirdamadi P(1), Mirdamadi P(1), Kuk JL(3). Full Text: http://www.obesityresearchclinicalpractice.com/article/S1871-403X(15)00121-0/pdf BACKGROUND: To determine whether the relationship between caloric intake, macronutrient intake, and physical activity with obesity has changed over time. METHODS: Dietary data from 36,377 U.S. adults from the National Health and Nutrition Survey (NHANES) between 1971 and 2008 was used. Physical activity frequency data was only available in 14,419 adults between 1988 and 2006. Generalised linear models were used to examine if the association between total caloric intake, percent dietary macronutrient intake and physical activity with body mass index (BMI) was different over time. RESULTS: Between 1971 and 2008, BMI, total caloric intake and carbohydrate intake increased 10-14%, and fat and protein intake decreased 5-9%. Between 1988 and 2006, frequency of leisure time physical activity increased 47-120%. However, for a given amount of caloric intake, macronutrient intake or leisure time physical activity, the predicted BMI was up to 2.3kg/m(2) higher in 2006 that in 1988 in the mutually adjusted model (P<0.05). CONCLUSIONS: Factors other than diet and physical activity may be contributing to the increase in BMI over time. Further research is necessary to identify these factors and to determine the mechanisms through which they affect body weight. PMID: 26383959
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