Fat cells are metabolically and hormonally active cells. They burn calories and secrete hormones that affect energy expenditure and satiety. There are a number of toxins which are stored in fat, so it has been hypothesized that these toxins may have negative effects on weight by negatively affecting such processes. I looked at this theory, specifically looking at the effects of PCB’s, organochlorines, bisphenol-A, synthetic estrogens, and phthalates on weight.
The first study  involved baby rats, so obviously it’s not the best one to look at but it had interesting findings. Two groups of rats were formed, one group was exposed to PCB (which is a toxin with a strong affinity for storage in fat tissue) and measured the effects on leptin and thyroid hormone, both of which (especially leptin) are hugely important in preventing weigh gain:
“Leptin concentrations were significantly depressed in 15-day-old animals exposed to PCB when compared to same-aged control animals”. “We speculate that the accumulation of fat-soluble PCB in adipocytes may be sufficient to cause these alterations”.
One study  examined the correlation between apparent PCB exposure and adiponectin levels in obese women. Adiponectin is secreted mainly by fat tissue which has been clearly shown to benefit insulin sensitivity and possibly obesity. This was observational, so no causation can be determined, but the researchers reported the following:
“Plasma levels of total adiponectin…significantly negatively correlated with plasma levels of PCB 153 in OB (OB=obese women). Our results may suggest suppression of adiponectin by PCB 153 in obese women under non-energy-restrictive regime”.
Perhaps unsurprisingly, PCB exposure has been linked to diabetes.
One study , utilizing mice and in vitro trials, concluded that “PCB-77 may contribute to the development of obesity and obesity-associated atherosclerosis”. Another study found a relationship between PCB-189 and
Finally, another study found a correlation between PCB exposure and waist circumference .
So certain PCB’s, which are stored in fat tissue, seem to promote weight gain, though I’m aware these studies aren’t slam dunk evidence. But they’re interesting.
Moving to organochlorines, another toxin which is stored in fat; this review paper  had some really interesting things to say, including this:
“Most of us have been exposed to organochlorines found in pesticides, dyes, solvents, etc., and we contain residues in our adipose tissue, where they are preferentially stored. Thus, the obese tend to have increased organochlorine concentrations compared to lean individuals …organochlorine concentration has been correlated with decreases in triidothyronine (T3) concentration and resting metabolic rate . This is also associated with a reduction in activity of the skeletal muscle oxidative enzymes that normally are involved in fat oxidation ”.
This paper explains that organochlorines are stored in fat and during weight loss are released into the blood where they exert negative effects on metabolism.
Another study  followed 90 people for 20 years, measuring their markers of health and apparent exposure to persistent organic pollutants (POP’s), finding a correlation between exposure to POP’s (including certain organochlorines and PCB’s) and future BMI. They also discussed (but did not conclude) possible mechanisms:
“Chemicals may cause obesity by altering homeostatic metabolic set-points, disrupting appetite controls, perturbing lipid homeostasis to promote adipocyte hypertrophy, or stimulating adipogenic pathways that enhance adipocyte hyperplasia during development or in adults,”
These studies are not the strongest evidence, but suggest organochlorines can cause weight gain and inhibit weight loss, though this seems to be dependent on the number of chlorines contained in the compound.
Bisphenol-A is a fairly well known chemical found in canned foods and the like and has an affinity for storage in fat. A recent study  reported the following:
“BPA at environmentally relevant doses inhibits the release of a key adipokine that protects humans from metabolic syndrome. The mechanism by which BPA suppresses adiponectin and the receptors involved remains to be determined”
In mouse studies, BPA exposure may cause or increase diabetes  and obesity . BPA may also increase the inflammatory adipokines interleukin-6 and Tumor Necrosis Factor-alpha , which evidence is beginning to casually link to insulin resistance , as well as other negative health effects.
So BPA may have some negative effects on weight gain, but the evidence is a bit scant as of yet.
Well, I didn’t get to the synthetic estrogens or phthalates, but I will edit this post to add them in if and when I find convincing evidence to support their connection. I think this makes a clear case that fat stored toxic chemicals can hinder weight loss. Some evidence suggests their release into the blood during weight loss is largely what causes this and that being stored in fat is actually protective, but results are mixed. It’s all hard to say, but either way such compounds are worth being aware of. More research into how to avoid and deal with such chemical exposure and fat storage will likely be good for aiding in weight loss efforts
: Newbold RR, Padilla-Banks E, Jefferson WN (2009) Environmental estrogens and obesity. Mol Cell Endocrinol 304: 84–89.
: Grun F, Blumberg B (2009) Endocrine disrupters as obesogens. Mol Cell Endocrinol 304: 19–29.
: Pelletier C, Despres JP, Tremblay A: Plasma organochlorine concentrations in endurance athletes and obese individuals. Med Sci Sports Exerc 2002, 34: 1971-1975
: Pelletier C, Doucet E, Imbeault P, Tremblay A: Associations between weight loss-induced changes in plasma organochlorine concentrations, serum T(3) concentration, and resting metabolic rate. Toxicol Sci 2002, 67:46-51.
: Imbeault P, Tremblay A, Simoneau JA, Joanisse DR: Weight loss-induced rise in plasma pollutant is associated with reduced skeletal muscle oxidative capacity. Am J Physiol Endocrinol Metab 2002, 282:E574-9