I have long thought that there is more to the obesity epidemic than merely a change in diet, although I do believe that the large increase in the consumption of carbs in general and fructose specifically are exacerbating forces. I’m always on the lookout for something that happened around the late 1970s/early 1980s that might be a major driving force behind the explosion of obesity that we’ve experienced since then.
The epidemiology of the sudden increase in fat deposition in so many people looks like an infectious disease, but I have a problem in buying into that idea whole hog. If it were an infectious disease, I don’t see how the switch to a low-carb diet would make it go away. I suspect that it is probably some sort of environmental contaminant that underlies the situation. The difficulty is in discovering what the factors are and how they work to make us all struggle to keep our weights in check. I doubt that there is one specific catalyst for the entire obesity problem; I would think that a combination of ingredients are probably at fault. But what are they?
I’ve posted on a few previously. In addition to those, a paper recently published online in the journal Environmental Health Perspectives implicates phthalates as another possible agent.
Phthalates are a class of chemicals that are plasticizers. In other words, they turn hard plastics – particularly PVC – into softer, more flexible, ones. Phthalates are also used in cosmetics, shampoos, soaps, lubricants, pesticides, paints, and, strangely enough, rubberized sex toys. (Due to the phthalates in this last, some manufacturers of these objects recommend the use of a condom on the device prior to use.)
Although phthalates are oil soluble like PCBs and dioxins and the other pollutants I’ve posted about before, they are also apparently fairly quickly metabolized and their metabolic by products are excreted in the urine. But while they hang around – and they do hang around because most of us are constantly being exposed – they cause some problems. More than 75 percent of Americans have measurable detectable levels of phthalate metabolites.
One of the actions of phthalates is to act as an anti-testosterone.
Phthalates are known antiandrogens in experimental animal models, with consistent results dating back several decades. Testicular steroid hormone synthesis and reproductive system development in males have been adversely affected by exposure, especially neonatal exposure, to certain phthalates…
And, in males at least, since a lack of testosterone or low testosterone levels have been associated with obesity and insulin resistance, anything that reduces testosterone or negates its effects should cause insulin resistance and/or obesity. The authors of this paper looked for such a correlation.
The researchers used 1999–2002 data from the CDC National Health and Nutrition Examination Survey (NHANES) to look for a correlation between phthalate exposure and obesity and/or insulin resistance in adult men. They compared urine concentrations of six phthalate metabolites to the participants’ waist circumference and measures of insulin resistance while controlling for a variety of potential confounders, including age, ethnicity, fat and calorie consumption, physical activity, and smoking status.
The data show a significant correlation between four of the phthalate metabolites and a larger waist circumference and three metabolites and increased insulin resistance. When subjected to more rigorous statistical analysis taking into consideration liver and kidney function, the researchers found that the associations decreased a little but were still significant for all the metabolites but one.
The paper is available in free full text if you would like to read it in its entirety and look at the graphs showing a dose-response curve effect for these metabolites.
I want to add that this paper in no way ‘proves’ that phthalates cause obesity and/or insulin resistance. What it does show is that there is a correlation between large levels of phthalate metabolites (and presumably phthalites) and an increased waist circumference and insulin resistance. But it is axiomatic that correlation does not imply causation. Actually it does imply causation, but there is really no proof that the one causes the other.
If you look outside and you see a lot of umbrellas you can be pretty sure it’s raining. You look out a little later and see no umbrellas and it’s not raining. After you make these observations a number of times, it would be reasonable to say that umbrellas cause rain because every time you see an umbrella it’s raining. In this case there is a correlation but there is no causation. In fact, there is causation, but it works the other way: the rain causes people to pop out their umbrellas.
The next step to see if there is causation is to give lab animals phthalates to see what happens. In the case of rodents, the effect is smaller than that found in humans. As the authors point out
Associations between certain phthalate metabolites and antiandrogenic effects have also been found in humans at much lower exposure levels than those used in rodent experiments. Suspected metabolites include mono-benzyl phthalate (MBzP), mono-ethyl phthalate (MEP), mono-isononyl phthalate (MiNP), mono-methyl phthalate, and mono-butyl phthalate (MBP). Urinary phthalate metabolites in pregnant women have been found to correlate with subtle genital changes in their infant males, and breast-milk phthalate metabolites have been correlated with shifts in reproductive hormones in infant males.
Although there is no absolute proof that phthalates are a driving force, much less THE driving force, in the development of insulin and obesity, there is enough correlative evidence for me to make me avoid them as much as possible. There are way too many natural products out there that are cosmetics, soaps, shampoos, and even sex toys for all I know to keep me from having to use those products containing phthalates. I would encourage readers of this blog to do the same. Here is a Wikipedia entry on phthalates along with a list of the most common ones so that you can identify them on the labels of products you may be considering. All have phthalate on the end – dimethyl phthalate, for example – so they’re not tough to identify.