This page is a resource for people who are interested in delving into the scientific aspects of weight control in greater depth. Dr. Barbara Berkeley will post interesting articles about obesity, related medical issues, weight maintenance and research.
You can also read Dr. Berkeley's blogs related to science by clicking here.
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Canned Fish - What Should You Do?
By Loren Cordain, Ph.D. and Wiley Long, M.S.
Fish and seafood, in general, closely resemble the nutritional characteristics of humanity's original staple food (lean game meat). These are quite high in protein, low in total fat and typically contain high levels of omega 3 fats. Given these characteristics, you should not be surprised to find that regular fish and seafood consumption is one of the best strategies you can take to improve your health.
Numerous well-controlled scientific studies have shown time and again that regular fish consumption reduces total and LDL cholesterol (the "bad" cholesterol) and triglycerides while simultaneously increasing HDL cholesterol (the "good" cholesterol). The end result of these beneficial changes in blood chemistry and heart function translates into a reduced risk of heart attack, stroke, type II diabetes and death from all causes combined. By eating fish regularly, you can significantly reduce your risk of dying from the number one killer of all Americans － heart disease.
Canned tuna is by far America's favorite seafood, but the canning process causes a number of problems - the least of which is a loss of fresh flavor.
To make most canned tuna, the fresh fish is first pre-cooked for 45-180 minutes. This makes it easier to de-bone, but unfortunately causes a loss of omega-3 fatty acids. It is then sealed in a can containing vegetable oil or water, and usually salt is added, and it is then cooked again (retort cooking) for 2-4 hours in a pressure-cooking environment to prevent botulism and destroy other pathogens.1
This process significantly reduces the content of all heat labile vitamins including B vitamins in canned fish and meat.2-6 The canning process removes 99% of vitamin A, 97% of vitamin B1, 86% of vitamin B2, 45% of niacin and 59% of vitamin B6 found in fresh tuna.7
Canning also increases the level of oxidized cholesterol in fish, specifically increasing a molecule called 25 hydroxycholesterol that is extremely destructive to the linings of arterial blood vessels. This is so destructive, in fact, that oxidized cholesterol is routinely fed to laboratory animals to accelerate the artery-clogging atherosclerotic process in order to test theories of heart disease. In animal models of atherosclerosis and heart disease, only 0.3 % of the total ingested cholesterol needs to be in the form of oxidized cholesterol to cause premature damage to arterial linings.
The canning process also significantly reduces the amount of omega-3 fatty acid in the tuna. Specialty brands of tuna are available in which the tuna is packed into the can raw and cooked only once in its own juice. This results in a fresher tasting product that may contain up to six times as much omega-3.
One final concern with consuming tuna is exposure to mercury. Mercury is a toxic metal that can damage the brain and nervous system, and it gets into the waterways by the burning of fuels and through household and industrial wastes. Once in the water, bacteria convert mercury into the toxic compound methyl mercury. Fortunately, the amount of mercury that you can potentially accumulate by eating fish three or four times a week is infinitesimally lower than amounts that you can get by industrial, occupational exposure.
You can limit your dietary exposure by selecting fish species from less polluted geographical areas such as the Pacific Ocean and Alaska; by choosing specialty brands that test for mercury content; and by choosing "light" tuna over white albacore tuna, which may contain up to three times as much mercury.
However, the larger risk to your health is not from environmental pollutants, but from heart disease, diabetes, obesity, stroke and associated health disorders of metabolic syndrome. Fish consumption not only protects you from these diseases, but from all causes of death combined, including cancer. Because fish is one of our best sources of omega 3 fats, it will tend to prevent depression and improve your mood, as my friend and colleague Dr. Joseph Hibbeln of the National Institute of Health in Bethesda, Maryland has shown. Consequently, regular fish and seafood consumption is healthful and should be part of your diet.
If you have the choice, always choose fresh fish or frozen fish over canned fish. If you eat canned tuna, try to find specialty brands that are packed in water only (no salt added), or in more healthful oils such as canola or olive oil. Most water-packed tuna contains added salt, which can be removed by soaking the tuna in a shallow pan filled with tap water and then rinsing the fish in a colander under a running tap.
References are available at http://www.ThePaleoDiet.com/v4n21.shtml.
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Many scientific studies have been done to figure out whether weight loss permanently lowers metabolism. Most of them have shown that metabolism after loss is normal. Recently, however, a paper by some well-known researchers at Columbia cast doubt on this finding. It’s a tough read, so I will interpret.
Basically, the researchers put seven groups each with three people into a hospital unit where they could be observed. In each group was one person who was at their usual weight, one who had lost weight recently, and one who had lost the same amount but had kept it off for a long time. The three people were matched so that their weights and amount of muscle were very close. They were then fed a liquid diet which was fine tuned to keep them at exactly what they currently weighed. Their exercise was kept as identical as possible. Measurements of metabolism were then performed.
The result showed that in both recent dieters and longer term maintainers, the amount of calorie burn per day was lower than would be expected for their size, age and sex. In other words, there did seem to be a persistent problem with metabolism that was caused by the weight loss. Remember that this is only one study and that many others have failed to show this effect. Nevertheless, this study was well controlled and reputably done.
Perhaps most interesting was the area of metabolism that was slowed. The problem did not appear to be in the number of calories burned by the resting body. Instead, it appeared that when muscles worked, they burned less calories than would have been predicted. This effect would be camouflaged in people who exercised a lot. Because they were forcing the muscles to burn large amounts of extra calories, they were counteracting the muscle problem.
If these results turn out to be true, they will go a long way in explaining why exercise seems to be such a vital component of successful weight maintenance.
Here's the study in a PDF file:
Download maintenance_study.pdf
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Don’t be discouraged by the technical nature of this article. Skip to the end where the author posts his “discussion.” This will give you a general sense of the results. I promise that future posts will be less technical!