“Today we’re going to learn about odds ratios and relative risk.”
A week ago I watched part of a documentary called “Plastic Planet” on Current or Al Jazeera TV. It was alarming. Apparently chemicals are leaking out of plastics into the environment (or into foods contained by plastic), making us fat, diabetic, impairing our fertility, and God knows what else. The narrator talked like it was a sure thing. I had to go to work before it was over. A couple mentioned chemicals I remember are bisphenol A (BPA) and phthalates. I sorta freaked my wife out when I mentioned it to her. I always take my lunch to work in plastic containers.
A few days later I saw a report of sperm counts being half of what they were just half a century ago. (It’s debatable.) Environmental contaminants were mentioned as a potential cause.
So I spent a couple hours trying to figure out if chemical contamination really is causing obesity and type 2 diabetes. In the U.S., childhood obesity has tripled since 1980, to a current rate of 17%. Even preschool obesity (age 2-5) doubled from 5 to 10% over that span. In industrial societies, even our pets, lab animals (rodents and primates), and feral rats are getting fatter! The ongoing epidemics of obesity and type 2 diabetes, and our lack of progress in preventing and reversing them, testify that we may not have them figured out and should keep looking at root causes to see if we’re missing anything.
Straightaway, I’ll tell you it’s not easy looking into this issue. The experts are divided. The studies are often contradictory or inconsistent. One way to determine the cause of a condition or illness is to apply Bradford Hill criteria (see bottom of page for those). We could reach a conclusion faster if we did controlled exposure experiments on humans, but we don’t. We look at epidemiological studies and animal studies that don’t necessarily apply to humans.
Regarding type 1 diabetes and chemical contamination, we have very little data. I’ll not mention type 1 again.
What Does the Science Tell Us?
For this post I read a couple pertinent scientific reviews published in 2012, not restricting myself to plastics as a source of chemical contaminants.
The first was REVIEW OF THE SCIENCE LINKING CHEMICAL EXPOSURES TO THE HUMAN RISK OF OBESITY AND DIABETES from non-profit CHEM Trust, written by a couple M.D., Ph.D.s. I’ll share some quotes and my comments. My clarifying comments within a quote are in [brackets].
“It should be noted that diabetes itself has not been caused in animals exposed to these chemicals [a long list] in laboratory studies, but metabolic disruption closely related to the pathogenesis of Type 2 diabetes has been reported for many chemicals.”
“In 2002, Paula Baillie-Hamilton proposed a hypothesis linking exposure to chemicals with obesity, and this is now gaining credence. Exposure to low concentrations of some chemicals leads to weight gain in adult animals, while exposure to high concentrations causes weight loss.”
“The obesogen hypothesis essentially proposes that exposure to chemicals foreign to the body disrupts adipogenesis [fat tissue growth] and the homeostasis and metabolism of lipids (i.e., their normal regulation), ultimately resulting in obesity. Obesogens can be functionally defined as chemicals that alter homeostatic metabolic set-points, disrupt appetite controls, perturb lipid homeostasis to promote adipocyte hypertrophy [fat cells swelling with fat], stimulate adipogenic pathways that enhance adipocyte hyperplasia [increased numbers of fat cells] or otherwise alter adipocyte differentiation during development. These proposed pathways include inappropriate modulation of nuclear receptor function; therefore, the chemicals can be termed EDCs [endocrine disrupting chemicals].”
Don’t assume mouse physiology is the same as human’s
Literature like this talks about POPs: persistent organic pollutants, sometimes called organohalides. The POPs and other chemical contaminants that are currently suspicious for causing obesity and type 2 diabetes include BPA, arsenic, pesticides, phthalates, metals (e.g., cadmium, mercury, organotins), brominated flame retardants, DDE (dichloro-diphenyldichloroethylene), PCBs (polychlorinated biphenyls), trans-nonachlor, dioxins.
Another term you’ll see in this literature is EDCs: endocrine disrupting chemicals. These chemicals mess with hormonal pathways. EDCs that mimic estrogen are linked to obesity and related metabolic dysfunction. Some of the chemicals in the list above are EDCs.
The fear—and some evidence—is that contaminants, whether or not EDCs, are particularly harmful to embryos, fetuses, and infants. For instance, it’s pretty well established that mothers who smoked while pregnant predispose their offspring to obesity in adulthood. (Epigenetics, anyone?) Furthermore, at the right time in the life cycle, it may only take small amounts of contaminants to alter gene expression for the remainder of life. For instance, the number of fat cells we have is mostly determined some time in childhood (or earlier?). As we get fat, those cells simply swell with fat. When we lose weight, those cells shrink, but the total cell number is unchanged. What if contaminant exposure in childhood increases fat cell number irrevocably? Does that predispose to obesity later in life?
The authors note that chemical contaminants are more strongly linked to diabetes than obesity. They do a lot of hemming and hawing, using “maybe,” “might,” “could,” etc. They don’t have a lot of firm conclusions other than “Hey, people, we better wake up and look into this further, and based on the precautionary principle, we better cut back on environmental chemical contamination stat!” [Not a direct quote.] It’s clear they are very concerned about chemical contaminants as a public health issue.
Here’s the second article I read: Role of Environmental Chemicals in Diabetes and Obesity: A National Toxicology Program Workshop Review. About 50 experts were empaneled. Some quotes and my comments:
“Overall, the review of the existing literature identified linkages between several of the environmental exposures and type 2 diabetes. There was also support for the “developmental obesogen” hypothesis, which suggests that chemical exposures may increase the risk of obesity by altering the differentiation of adipocytes [maturation and development of fat cells] or the development of neural circuits that regulate feeding behavior. The effects may be most apparent when the developmental [early life] exposure is combined with consumption of a high-calorie, high-carbohydrate, or high-fat diet later in life.”
“The strongest conclusion from the workshop was that nicotine likely acts as a developmental obesogen in humans. This conclusion was based on the very consistent pattern of overweight/obesity observed in epidemiology studies of children of mothers who smoked during pregnancy (Figure 1) and was supported by findings from laboratory animals exposed to nicotine during prenatal [before birth] development.”
I found some data that don’t support that conclusion, however. Here’s a graph of U.S. smoking rates over the years since 1944. Note that the smoking rate has fallen by almost half since 1983, while obesity rates, including those of children, are going the opposite direction. If in utero cigarette smoke exposure were a major cause of U.S. childhood obesity, we’d be seeing less, not more, childhood obesity. I suppose we could still see a fall-off in adult obesity rates over the next 20 years, reflecting lower smoking rates. But I doubt that will happen.
“The group concluded that there is evidence for a positive association of diabetes with certain organochlorine POPs [persistent organic pollutants]. Initial data mining indicated the strongest associations of diabetes with trans-nonachlor, DDT (dichloro-diphenyltrichloroethane)/DDE (dichloro-diphenyldichloroethylene)/DDD (dichloro-chlorophenylethane), and dioxins/dioxin-like chemicals, including polychlorinated biphenyl (PCBs). In no case was the body of data considered sufficient to establish causality [emphasis added].”
“Overall, this breakout group concluded that the existing data, primarily based on animal and in vitro studies [no live animals involved], are suggestive of an effect of BPA on glucose homeostasis, insulin release, cellular signaling in pancreatic β cells, and adipogenesis. The existing human data on BPA and diabetes (Lang et al. 2008; Melzer et al. 2010) available at the time of the workshop were considered too limited to draw meaningful conclusions. Similarly, data were insufficient to evaluate BPA as a potential risk factor for childhood obesity.”
“It was not possible to reach clear conclusions about BPA and obesity from the existing animal data. Although several studies report body weight gain after developmental exposure, the overall pattern across studies is inconsistent.”
“The pesticide breakout group concluded the epidemiological, animal, and mechanistic data support the biological plausibility that exposure to multiple classes of pesticides may affect risk factors for diabetes and obesity, although many significant data gaps remain.”
“Recently, the focus of investigations has shifted toward studies designed to understand the consequences of developmental exposure to lower doses of organophosphates [insecticides], and the long-term effects of these exposures on metabolic dysfunction, diabetes, and obesity later in life. [All or nearly all the studies cited here were rodent studies, not human.] The general findings are that early-life exposure to otherwise subtoxic levels of organophosphates results in pre-diabetes, abnormalities of lipid metabolism, and promotion of obesity in response to increased dietary fat.”
In case it’s not obvious, remember that “association is not the same as causation.” For example, in the Northern hemisphere, higher swimsuit purchases are associated with summer. Swimsuit sales and summer are linked (associated), but one doesn’t cause the other. Swimsuit purchases are caused by the desire to go swimming, and that’s linked to warm weather.
In at least one of these two review articles, I looked carefully at the odds ratios of various chemicals linked to adverse outcomes. One way this is done is too measure the blood or tissue levels of a contaminant in a population, then compare the adverse outcome rates in animals with the highest and lowest levels of contamination. For instance, if those with the highest contamination have twice the incidence of diabetes as the least contaminated, the odds ratio is 2. You could also call it the relative risk. Many of the potentially harmful chemicals we’re considering have a relative risk ratio of 1.5 to 3. Contrast those numbers with the relative risk of death from lung cancer in smokers versus nonsmokers: the relative risk is 10. Smokers are 10 times more likely to die of lung cancer. That’s a much stronger association and a main reason we think smoking causes lung cancer. Odds ratios under two are not very strong evidence when considering causality; we’d like to have more pieces of the puzzle.
These guys flat-out said arsenic is not a cause of diabetes in the U.S.
Overall, the authors of the second article I read were clearly less alarmed than those of the first. Could the less-alarmed panelists have been paid off by the chemical industry to produce a less scary report, so as not to jeopardize their profits? I don’t have the resources to investigate that possibility. The workshop was organized (and paid for, I assume) by the U.S. government, but that’s no guarantee of pure motivation by any means.
You need a break. Enjoy.
My Conclusions
For sure, if I were a momma rat contemplating pregnancy, I’d avoid all those chemicals like the plague!
It’s premature to say that these chemical contaminants are significant causes of obesity and type 2 diabetes in humans. That’s certainly possible, however. We’ll have to depend on unbiased scientists to do more definitive research for answers, which certainly seems a worthwhile endeavor. Something tells me the chemical producers won’t be paying for it. Universities or governments will have to do it.
You should keep your eyes and ears open for new evidence.
There’s more evidence for chemical contaminants as a potential cause of type 2 diabetes than for obesity. Fetal and childhood exposure may be more harmful than later in life.
If I were 89-years-old, I wouldn’t worry about these chemicals causing obesity or diabetes. For those quite a bit younger, taking action to avoid these environmental contaminants is optional. As for me, I’m drinking less water out of plastic bottles and more tap water out of glass or metal containers. Yet I’m not sure which water has fewer contaminants.
Humans, particularly those anticipating pregnancy and child-rearing, might be well advised to minimize exposure to the aforementioned chemicals. For now, I’ll leave you to your own devices to figure out how to do that. Good luck.
Why not read the two review articles I did and form your own opinion?
Unless the chemical industry is involved in fraud, bribery, obfuscation, or other malfeasance, the Plastic Planet documentary gets ahead of the science. I’m less afraid of my plastic containers now.
Additional Resources:
Sarah Howard at Diabetes and the Environment (focus on type 1 but much on type 2 also).
Jenny Ruhl, who thinks chemical contaminants are a significant cause of type 2 diabetes (search her site).
From Wikipedia:
The Bradford Hill criteria, otherwise known as Hill’s criteria for causation, are a group of minimal conditions necessary to provide adequate evidence of a causal relationship between an incidence and a consequence, established by the English epidemiologist Sir Austin Bradford Hill (1897–1991) in 1965.
The list of the criteria is as follows:
- Strength: A small association does not mean that there is not a causal effect, though the larger the association, the more likely that it is causal.
- Consistency: Consistent findings observed by different persons in different places with different samples strengthens the likelihood of an effect.
- Specificity: Causation is likely if a very specific population at a specific site and disease with no other likely explanation. The more specific an association between a factor and an effect is, the bigger the probability of a causal relationship.
- Temporality: The effect has to occur after the cause (and if there is an expected delay between the cause and expected effect, then the effect must occur after that delay).
- Biological gradient: Greater exposure should generally lead to greater incidence of the effect. However, in some cases, the mere presence of the factor can trigger the effect. In other cases, an inverse proportion is observed: greater exposure leads to lower incidence.
- Plausibility: A plausible mechanism between cause and effect is helpful (but Hill noted that knowledge of the mechanism is limited by current knowledge).
- Coherence: Coherence between epidemiological and laboratory findings increases the likelihood of an effect. However, Hill noted that “… lack of such [laboratory] evidence cannot nullify the epidemiological effect on associations”.
- Experiment: “Occasionally it is possible to appeal to experimental evidence”.
- Analogy: The effect of similar factors may be considered.
