Is it safe to eat canned foods? 

That’s a question worth asking after a recent study in human volunteers found a huge spike in urinary levels of the chemical bisphenol A – commonly known as BPA – in a group of volunteers who ate canned vegetable soup for several days. BPA, which has been linked to a variety of health disorders, is used in the lining of many food and beverage cans.

The results suggest BPA is being absorbed by the canned food and then ingested by consumers.

The study, published recently in the Journal of American Medical Association (1), represents one of the first attempts to measure BPA urine levels soon after the consumption of canned foods.

Title of Study (Research Letter)
Canned Soup Consumption and Urinary Bisphenol A: A Randomized Crossover Trial. Journal of the American Medical Association 2011;306:2218-2220.

Study Protocol
The experiment involved 75 participants. Half of them were asked to eat a 12-ounce bowl of canned vegetable soup at lunch for five consecutive days. After a two-day break, they consumed the same-sized serving of fresh vegetable soup for five lunches in a row. The other volunteers did the experiment in the reverse order – starting with five days of fresh soup, followed by five lunches of canned soup.

Five kinds of canned vegetarian Progresso soups (including tomato and minestrone) were used in the study, as well as five similar homemade soups. Participants were not aware of whether they were consuming canned or fresh soup during any given period.

Urine samples were collected on several occasions, usually a few hours after the noon-time meal.

Study Results
About 25% of urine samples from the homemade soup periods contained no detectable bisphenol A, all study participants were found to have bisphenol A in their urine after consuming canned soup for 5 days.

The analysis revealed that when participants ate the canned Progresso soup, their urinary BPA concentrations rose to levels 12-times higher than when they dined on fresh soup.

Study Discussion
The results of this study (1) suggest that the endocrine disruptor BPA is being absorbed by the canned food and then ingested by consumers. Because BPA is used in the liners of most canned foods and beverages it can be hard to avoid.

Although the team investigated only canned vegetable soup, the researchers suspect they would find similar results with other canned food and possibly canned beverages (e.g., soda) as well.

Scientists have warned that BPA could interfere with numerous biological processes because its structure resembles the hormone estrogen. It has been shown to interfere with reproductive development in animals and it has been linked to cardiovascular disease, diabetes, thyroid disease, and obesity in humans. Aside from cans, it’s also used to make some hard, clear plastics, dentistry composites and sealants, and some cash register receipts.

This study has reported that consuming canned soup increases urinary bisphenol A (BPA) excretion to levels that would be considered potentially very harmful to health if they were sustained.

My Comments

Bisphenol A (BPA) is a key building block of the epoxy resins commonly used for lining metal cans.  This thin epoxy coating on the interior surface of almost all metal food cans helps prevent corrosion of the can and makes it possible for food products to maintain their quality and taste, while extending shelf life (2-4).

Exposure to the endocrine disruptor BPA is thought to occur primarily through ingestion (2). BPA is a chemical of concern because it is an endocrine disruptor and has been associated with various adverse health effects, including thyroid disorders (5-7). It is well established that residual BPA monomer migrates into can contents during processing and storage (9,10), and evidence of BPA contamination of canned foods for human use has been widely reported (3,10).  Similarly, in two studies which measured the levels of BPA in pet foods, most dog and cat foods were found to contain measurable levels of BPA (10,11).  In one of the studies, investigators confirmed that the BPA had originated from the can coating, which had leached into the food (11).

Does BPA Ingestion Contribute to Feline Hyperthyroidism?

One large study of control and hyperthyroid cats demonstrated an association between hyperthyroidism and cats fed food from “pop-top” cans (12). Results of that study suggested that overall consumption of pop-top canned food at various times throughout a cat's life was associated with greater risk of developing hyperthyroidism. In female cats, increased risk was associated with consumption of food packaged in pop-top cans or in combinations of pop-top and non-pop-top cans. In male cats, increased risk was associated with consumption of food packaged in pop-top cans alone. Although these investigators suggested that only the lids of the pop-top cans are lined with the BPA-containing epoxy resins, it appears that most cans (even those that require a can opener to open them) are coated with BPA (10,11). Nevertheless, it appears that feeding canned cat food may pose a greater risk than feeding food from pouches or sachets.

Blood or tissue levels of BPA have not yet been measured in cats, but life long, continuous daily exposure to even the relatively low doses of this chemical found in commercial canned cat foods would likely result in potentially harmful effects (13).

In one of the major proposed actions, BPA acts as a thyroid hormone receptor antagonist, which might work at the pituitary level to increase circulating TSH concentrations (14-16). This could lead to thyroid hyperplasia, goiter formation, and hyperthyroidism in susceptible cats (17).

References

1. Carwile JL, Ye X, Zhou X, Calafat AN, Michels KB. Canned soup consumption and urinary bisphenol A: a randomized crossover trial.  JAMA 2011;306:2218-2220. 
2. Tsai WT. Human health risk on environmental exposure to Bisphenol-A: a review. J Environ Sci Health C Environ Carcinog Ecotoxicol Rev 2006;24:225-255.
3. Noonan GO, Ackerman LK, Begley TH. Concentration of bisphenol A in highly consumed canned foods on the U.S. market. J Agric Food Chem 2011;59:7178-7185.
4. Vandenberg LN, Maffini MV, Sonnenschein C, et al. Bisphenol-A and the great divide: a review of controversies in the field of endocrine disruption. Endocr Rev 2009;30:75-95
5. Boas M, Main KM, Feldt-Rasmussen U. Environmental chemicals and thyroid function: an update. Curr Opin Endocrinol Diabetes Obes 2009;16:385-391.
6. Patrick L. Thyroid disruption: mechanism and clinical implications in human health. Altern Med Rev 2009;14:326-346.
7. Meeker JD, Ferguson KK. Relationship between urinary phthalate and bisphenol A concentrations and serum thyroid measures in U.S. adults and adolescents from the National Health and Nutrition Examination Survey (NHANES) 2007-2008. Environ Health Perspect 2011;119:1396-1402.
8. Goodson A, Robin H, Summerfield W, et al. Migration of bisphenol A from can coatings--effects of damage, storage conditions and heating. Food Addit Contam 2004;21:1015-1026.
9. Cabado AG, Aldea S, Porro C, et al. Migration of BADGE (bisphenol A diglycidyl-ether) and BFDGE (bisphenol F diglycidyl-ether) in canned seafood. Food Chem Toxicol 2008;46:1674-1680.
10. Schecter A, Malik N, Haffner D, et al. Bisphenol A (BPA) in U.S. food. Environ Sci Technol. 2010;44:9425-9430.
11. Kang JH, Kondo F. Determination of bisphenol A in canned pet foods. Res Vet Sci 2002;73: 177-182.
12. Edinboro CH, Scott-Moncrieff JC, Janovitz E, et al. Epidemiologic study of relationships between consumption of commercial canned food and risk of hyperthyroidism in cats. J Am Vet Med Assoc 2004;224:879-886. 
13. Welshons WV, Nagel SC, vom Saal FS. Large effects from small exposures. III. Endocrine mechanisms mediating effects of bisphenol A at levels of human exposure. Endocrinology 2006;147(6 Suppl):S56-69.
14. Moriyama K, Tagami T, Akamizu T, et al. Thyroid hormone action is disrupted by bisphenol A as an antagonist. J Clin Endocrinol Metab 2002;87:5185–5190.
15. Kitamura S, Jinno N, Ohta S, et al. Thyroid hormonal activity of the flame retardants tetrabromobisphenol A and tetrachlorobisphenol A. Biochem Biophys Res Commun 2002;293:554–559.
16. Zoeller RT, Bansal R, Parris C. Bisphenol-A, an environmental contaminant that acts as a thyroid hormone receptor antagonist in vitro, increases serum thyroxine, and alters RC3/neurogranin expression in the developing rat brain. Endocrinology 2005;146:607-612.
17. Peterson ME: Feline hyperthyroidism – Risk factors and diagnosis. North American Veterinary Conference (NAVC) Conference 2012: Small Animal & Exotics Proceedings. pp. 865-872, 2012.

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