Canned Foods, Much Better Than Their Reputation: Issues W/ Heavy Metals, Plastics, and Nutrient Losses Revisited

While generalizations are problematic, it is true that the average canned product is better than its reputation would suggest.

As a SuppVersity-reader, you will know that frozen vegetables and fruit are much healthier than their reputation would suggest (learn more). In previous articles, I have even pointed out that for some foods, such as spinach, the frozen variety is superior to the "fresh" one with respect to almost all relevant nutritional aspects.

Now, this raises the question: If frozen fruits and veggies are so much better than most people believe, could the same be true for canned fruits and vegetables?
While you're devouring the results of my research, keep in mind that the take-home messages are generalizations - generalizations, though, which confirm: "Canned foods are better than their reputation."

Are non-coated cans a problem? Heavy metals are a potential problem, but... 

It is correct that studies have consistently found increased levels of heavy-metals non-insulated canned products, i.e. fruits and veggies in cans that don't have the (likewise problematic) plastic coating on the inside that's in direct contact with the canned foods. With copper, zinc, manganese, iron, selenium, aluminium, chromium, nickel, and cobalt, being the most prevalent "extras" you may be getting in your canned foods, the heavy metals that are significantly elevated in canned vs. fresh products are yet by no means as detrimental to your health as lead, mercury, cadmium, or arsenic, i.e. the heavy metals we usually think about when somebody talks about heavy metal residues.

Over time, especially high-nitrate foods like lettuce, beets, carrots, green beans, spinach, parsley, cabbage, radishes, celery, and collard greens have the potential to be contaminated with heavy metals from the alloy their cans are made of. Hence, you will usually find these products being canned in coated containers and/or canning jars (glass).

Figure 1: Relative increase in lead content of selected canned products over 3-day storage in the opened can (all but the tomato juice in non-lacquered cans | Capar 1978) - It is important to remember that modern cans don't contain the same amount of lead as those that were used in this 1979 study - still, storage in the opened cannot be recommended.

Especially when it comes to examples like the Capar study (see Figure 1) it is important to remember that the content of heavy metals has constantly been declining over the past decades.

There are at least two reasons for the declining heavy metal levels in canned foods: Firstly, modern cans are being made of lead-free and, more generally, largely heavy-metal "free" materials; and, secondly, and maybe even more importantly, many cans are insulated, these days. As long as the insulation stays intact, there's thus no heavy metal leakage from the mostly tin- and/or aluminum-based metal-alloys the cans are made of (e.g. for non-lacquered tin cans, the amount of tin in the product is as high as 77mg/kg, with the protective linking it's only 3.2mg/kg - that's a reduction of 96%!).
With an increasingly non-significant contribution of heavy metals from the packaging, the heavy metal content of today's canned fruits and vegetables is mostly present way before the foods are canned.

Figure 2: The unfortunate truth is that especially larger fish are high in mercury even before they're canned and stored. This illustration from Edward Zilliioux may help you find the fish with the lowest mercury content - before canning.

If you're a canned fish lover, for example, the original heavy metal content of the fish is way more important than the amount of heavy metal that may have leached from the rarely non-insulated can into the fish.


Meant to protect, but potentially harmful!? There're BPA and co in canned foods, but...

While they do protect you from heavy metal leakage, the plastic and resin coatings that are used in an increasing percentage of the cans pose a potential health threat - and that's despite the fact that BPA and co don't accumulate as easily in the body as the previously discussed heavy metals. Overall, studies such as Völkel et al. (2002) confirm that...

"the efficient glucuronidation of bisphenol A and the rapid excretion of the formed glucuronide result in a low body burden of the estrogenic bisphenol A in humans following oral absorption of low doses" (Völkl 2002). 

Or, in other words, as long as you don't get way too much of them, a healthy liver + kidney will get rid of these chemicals in your diet quite easily. Compared to the alternative of having the ingredients in direct contact with the metal alloys the cans are made of, lacquered cans are thus probably the safer choice - and that's despite the fact that it it is true that the same layer that's protecting you from heavy metal exposure will leak xenoestrogens into canned foods (Brotons 1995).

Figure 3: The amount of the xenoestrogen BPA that leached from the lining into the food is obviously not the only determinant of the estrogenic and hence proliferative effect of canned foods on breast cancer cells (Brotons 1995).

Unless you like injecting green bean dripping water into your breast tissue, it is, due to the previously mentioned rapid metabolism of BPA and co, yet very unlikely (I should say impossible) to achieve the concentration of plastics in your breast tissue of which the study by Broton et al. (see Figure 3) showed that it will increase the growth of MCF7 cancer cells.

Figure 4: Welshson et al. (2006) highlight and illustrate in their paper, there has been evidence of potential ill health effects for the comparatively small amounts of BPA humans are exposed to on a daily basis... what is often forgotten, though, is that the "human exposure" is not adequately modelled in rodents, let alone in in-vitro (cell line) studies.

What is less certain, however, is the potential ill effects of low dose exposure on your and even more so the undeveloped endocrine system (e.g. testosterone, estrogen, thyroid hormone levels) of un- and newborns and toddlers (Vandenberg 2009).


Significantly less nutritious? It's a prejudice that canned foods contain almost no nutrients.

On paper, many of the canned products the supermarket next door has to offer are "health-" if not "super-foods"... if you buy them fresh. The question many of you may have been asking themselves is thus: "Are these foods health-/superfoods even after they've been canned and stored for months, if not years?" Well, the answer to your question is, as it is so often: "It depends!"

• For vitamin C, for example, the loss is significant for broccoli (-84%), carrots (-89%), green peas (-73%), spinach (-62%), and green beans (-63%), for beets (-10%) and even more so for corn (-0.25%), however, the loss is negligible.
  
  Overall, the vitamin C content of canned foods is - nobody will argue that - suboptimal compared to fresh local and/or produce that has been frozen right after being harvested. Compared to fresh produce that has been stored chilled for weeks during transportation and frozen produce that remains in your freezer for several months and hence contains >50% less vitamin C than when it was initially harvested, they are not that bad, though. The ongoing loss of vitamin C in unopened cans is, after all, comparably small (15% | Rickman 2017).
• For B vitamins, the inconsistencies in terms of methodology and data reporting make it difficult to make definitive conclusions. As Rickman, et al. point out, "most data suggest that the B vitamins are sensitive to thermal processing, storage, and cooking" (Rickman 2017). Against that background, it appears reasonable to assume a significant B-vitamin loss in canned vs. fresh products... This wouldn't be nutrition research, though, if things were that easy. So, let's take a look at Figure 5, shall we?
  

  Figure 5: As always in nutrition science, there are exceptions to the rule. While most canned products contain significantly fewer b-vitamins than their fresh varieties, the opposite is the case for tomatoes (USDA data).

  Yes, you're right: Even though the previously touted loss of B-vitamins seems to occur in green bean, green peas, spinach, and peaches, it does not occur in tomatoes, where the canned product has more thiamin, riboflavin, niacin, and pyridoxine, and only the folic acid content is reduced by -38% - coincidentally this is exactly the one B-vitamin beans don't lose during the canning process; to generalize observations from one produce to others is thus highly unwarranted and partly responsible for the bad rep of canned foods.
• For phenolics, you know, the stuff that turns a health- into a superfood, it's similarly complicated as it was for the B-vitamins: while green beans, cherries, and even more so mushrooms lose statistically significant and practically relevant amounts of phenols when they are canned, the phenol content of beets is even increased (albeit by only 5%) and the relatively small phenol-loss of corn and tomatoes is neither statistically significant nor practically relevant (at least you get some phenols ;-).
  
  In contrast to the produce-specific loss of phenols, generalizations are warranted when it comes to average quantitative changes in phenol content, which is significantly smaller than the loss of the previously discussed nutrients, vitamin C, and certain volatile B-vitamins.
• For fatty acids, specifically omega-3 in canned fish, there are three crucial parameters that will determine the omega-3 content and degree of oxidation of fatty acids in fish.
  
  The first and most obvious of these parameters is the cooking temperature. In that, increasing temperatures are associated with lower omega-3 to omega-6 ratios - despite reduced cooking times (0.92 for canning at 110°C for 90 min; 0.6 for canning at 115°C min for 70 min, and worst 0.26 for canning at 121°C for 40 min. | Steven 2010). Similar detrimental effects were observed for the oxidation of fatty acids.
  

  Figure 6: The TBA levels (mg/kg malonaldehyde) of canned tuna are generally lower for tuna in oil vs. water/brine. What you must not forget, though, is that only 3 months of extra storage will double the amount of lipid oxidation products in canned tuna - tip: look at the manufacturing, not the BBE date (Medina 1988).

  In this context, it is yet worth mentioning that the oxidation of omega-3 and other fats is reduced when tuna & co are canned with different oils. Medina et al., for example, showed in their 1988 study that extra virgin olive oil provides the maximal reduction in TBA levels (Medina 1988) - at least within the first 2 months of storage.

• The second and surprisingly unknown determinant of the fatty acid content, ratio, and quality is a surprising one: the storage time. As the data in Figure 6 already suggests the lipid quality of tuna deteriorates relatively quickly (note: the degree of oxidation with storage differs between brands with two out of three tuna cans that were tested by Medina et al. having a profile comparable to the one in Fig. 5 and one with generally lower TBA values).
  
  More recently, Siriamornpun, et al. (2008) have shown an almost linear increase in both peroxide and TBA values in tuna (in brine) during 12 months of storage. In that, it's also noteworthy that the fatty acid make-up changed significantly over the course of the 12 months study period, too: The amount of omega-3 fatty acids in the tuna, for example, decreased from 293 mg/100g to 219 mg/100g - that's a reduction of whopping -25%!

So what's the verdict then? Well, overall, Rickman et al. (2017) are right to point out that canned foods are not, like the prevalent prejudices would suggest, so significantly less nutritious than fresh or frozen products that they don't contribute to the health benefits of increased fruit and vegetable intakes in your diet. Without studies that follow the same product from harvest through processing, intermediate storage, canning/freezing/transportation, storage at store/home, and cooking, i.e. all the way from the soil to the plate, it's however very difficult to quantify the existing disadvantages.


A few concluding remarks before you go

Before I get to the overall conclusion of today's article and you're closing this browser tab, I think it's important to emphasize (once again) that the number, quality, and comparability of papers about canned foods is, by no means, comparable to the data base that exists for frozen foods. There's thus a relative scarcity of data to address the prevalent prejudice that canned foods are low in nutrients and high in toxic substances. Based on what we know, there is, however, sufficient evidence to make the claim that a diet that's rich in canned fruits and vegetables is still healthier than one that relies heavily on pre-processed (frozen or even freeze-dried) foods.

When it comes to actually eating canned foods, there are yet two issues I've previously not addressed remain: Firstly, 90% of the canned fruits are a nutritional supernova, because they are canned with syrup and have tons (almost literally ;-) of simple sugars in them - often much more than their fresh equivalents. Secondly, taste is an issue in both fruits and vegetables - and let's be honest: everyone who has prepared both freshly cooked fresh or frozen as well as canned peas will know the eye- and tongue-catching difference in both the optics (greyish, shriveled, and mushy vs. green, firm, and almost crisp) and the taste of the peas.
References:

• Bassioni, Ghada, et al. "Risk assessment of using aluminum foil in food preparation." Int. J. Electrochem. Sci 7.5 (2012): 4498-4509.
• Brotons, José Antonio, et al. "Xenoestrogens released from lacquer coatings in food cans." Environmental health perspectives 103.6 (1995): 608.
• Capar, Stephen G. "Changes in lead concentration of foods stored in their opened cans." Journal of Food Safety 1.4 (1978): 241-245.
• Jaishankar, Monisha, et al. "Toxicity, mechanism and health effects of some heavy metals." Interdisciplinary toxicology 7.2 (2014): 60-72.
• Karbouj, Rim, I. Desloges, and Patrice Nortier. "A simple pre-treatment of aluminium cookware to minimize aluminium transfer to food." Food and chemical toxicology 47.3 (2009): 571-577.
• Medina, Isabel, et al. "Effect of packing media on the oxidation of canned tuna lipids. Antioxidant effectiveness of extra virgin olive oil." Journal of Agricultural and Food Chemistry 46.3 (1998): 1150-1157.
• Reilly, Conor. Metal contamination of food: its significance for food quality and human health. John Wiley & Sons, 2008.
• Rickman, Joy C., Diane M. Barrett, and Christine M. Bruhn. "Nutritional comparison of fresh, frozen and canned fruits and vegetables. Part 1. Vitamins C and B and phenolic compounds." Journal of the Science of Food and Agriculture 87.6 (2007): 930-944.
• Stephen, Nimish Mol, et al. "Effect of different types of heat processing on chemical changes in tuna." Journal of Food Science and Technology 47.2 (2010): 174-181.
• Sugita, Minoru. "The biological half-time of heavy metals." International archives of occupational and environmental health 41.1 (1978): 25-40.
• Tuzen, Mustafa, and Mustafa Soylak. "Evaluation of trace element contents in canned foods marketed from Turkey." Food chemistry 102.4 (2007): 1089-1095.
• Völkel, Wolfgang, et al. "Metabolism and kinetics of bisphenol A in humans at low doses following oral administration." Chemical research in toxicology 15.10 (2002): 1281-1287.
• Zillioux, Edward J. "Mercury in fish: history, sources, pathways, effects, and indicator usage." Environmental Indicators. Springer, Dordrecht, 2015. 743-766.