8.8: What causes obesity? What causes the diseases associated with obesity?
- Page ID
- 116461
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Our genes determine much about how we store fat, how we burn it, what our food cravings are, etc. On an individual level, genetics is probably the most important factor in determining body size and composition.
However, since the genetic pool of an entire population does not change quickly, genes cannot explain the population change in average weight over the last 30 years.
Much of the research on obesity focuses on the "big two" factors that are modifiable: changes in diet and physical activity. These have changed significantly over time in the U.S. - for example, many fewer children bike or walk to school than in the past. A movie like Super-Size Me highlights how portion-size, especially in the fast food industry, have increased. On average, people eat the same AMOUNT of food as in the past, yet that food contains many more calories, salt and fat than in the past. The book Salt, Sugar, Fat does a great job of showing how the food industry engineered food to be more delicious, less healthy, and possibly addictive.
Yet, several other important factors that contribute to obesity that get less attention:
- Sleep debt - not getting enough sleep changes both metabolism and hunger signals. Check out this article about sleep and obesity: 'Abnormal' sleep linked to obesity risk - http://www.bbc.co.uk/news/uk-scotland-glasgow-west-39127601
- Changes in breastfeeding - babies who are breastfed are much less likely to grow up to be obese - yet due to mothers working and other social factors, fewer children are breastfed. This is a major focus of the CDC's work against obesity -- promoting breastfeeding.
- The average maternal age is increasing - that is, the age at which a woman has a child - and having an older mother increases the chance that a child will be obese. (Obviously, there is nothing the child can do about this particular risk factor.)
- Epigenetics - in particular, how the environment in utero affects what genes do. Epigenetics is a growing field of study. Interventions during pregnancy have shown some promise for decreasing rates of diabetes, as the birthing parent's diet, stress and medications during the nine months of pregnancy can influence not the actual GENES of the baby, but how the genes get turned on and off.
- Heating and cooling - the body spends less energy to obtain the optimal temperature when the furnace or the air conditioner do it for us. This might sound minor, but actually, we burn an enormous number of calories simply to maintain body temperature.
- Certain pharmaceuticals may contribute to weight gain as a side effect - and Americans use many more prescription drugs now than they did 30 years ago.
- Cyclical dieting can also contribute to higher average weight - as anyone who has "gained it back and more" can testify. (Me included!)
- Discrimination. As mentioned before, the discrimination that obese people face in hiring, dating, shopping, casual comments on the street, etc. also contributes to stress, strain on the heart, insulin function, and even the metabolic tendency to store fat. (That is, teasing someone for being fat could actually make them fatter!) Criticism of one's weight tends to lead to less motivation to do physical activity, not more.
These underlying factors have an independent effect on the body's inflammation response and the rate of chronic diseases. Therefore, the focus on creating healthier environments does not only benefit the obese (or potentially obese) - such changes could improve population health overall.
It should be noted that many of these risk factors appear in people who are not fat -- and can have serious health effects, regardless of weight.
Due to underlying genetics or behaviors that off-set the "obesogenic" environment, not everyone who is exposed to a negative environment will become obese - yet the environment may still make them sick.
Egger & Dixon make this point with the following diagram (Figure 2 in their article, cited below):
Figure 2: Environmental and lifestyle 'inducers' of metaflammation, showing both independent and dependant effects through obesity
Source: Egger, G. G., & Dixon, J. J. (2009). Should obesity be the main game? Or do we need an environmental makeover to combat the inflammatory and chronic disease epidemics?. Obesity Reviews, 10(2), 237-249. doi:10.1111/j.1467-789X.2008.00542.x