Belin RJ, Greenland P, Martin L, et al. Fish intake and the risk of incident heart failure: The Women's Health Initiative. Circ Heart Fail. 2011 May 24. [Epub ahead of print]
This retrospective study compared reported dietary intakes of various types of fish and fish oil against incidence of heart failure over the course of 10 years. Intake of baked or broiled fish, fried fish, omega-3 fatty acids, and trans fatty acids (TFAs) was determined from food frequency questionnaires.
Data collected from 84,493 participants in the Women's Health Initiative Observational Study (WHI-OS) were analyzed in this research. The participants were all female, aged 50–79, and were of diverse ethnicity and background. The participants had been followed for an average of 10 years ending in August 2008. They did not have heart disease at the start of the study.
Study Methods: Baked or broiled fish consumption was divided into 5 frequency categories: less than 1 serving/month, 1–3/month, 1–2/week, 3–4/week, and ≥5/week. Fried fish intake was grouped into 3 frequency categories: less than 1 serving per month, 1–3/month, and ≥1/week. Associations between fish or fatty acid intake and incident heart failure were determined using Cox models, adjusting for other heart failure risk factors.
Cox-proportional hazards models with 95% confidence intervals were constructed to examine the associations between consumption frequency of baked or broiled fish, fried fish, omega-3 fatty acids, and trans fatty acids and risk for incident heart failure.
Eating 5 or more servings of baked or broiled fish per week lowered the risk of heart failure by 30% (HR 0.7; 95% CI: 0.51, 0.95). This effect was attributed primarily to the consumption of dark fish—particularly salmon, but also mackerel, and bluefish. Eating fried fish 1 or more times a week was associated with a 48% increase in risk for heart failure (HR 1.48; 95% CI: 1.19, 1.84). No association was seen between consumption of omega-3 fish oil supplements or trans fatty acid intake with heart failure.
The results of this study change 2 fundamental assumptions we tend to make. The first is that eating fish is good for the heart. This idea needs to be modified to, “If cooked right, fish is good for your heart. If fried, fish is bad for your heart.”
The second fundamental assumption in need of examination is that fish oil supplements will provide the same benefit as eating fish. This large trial could not find any significant reduction in cardiac failure associated with taking omega-3 supplements.
While this study confirmed the common knowledge that fish is good for the heart, the magnitude of the fried fish effect is surprising and begs explanation. The data from this study suggest that a single serving fried fish per week increases the risk of heart failure by almost 50%, and 5 servings of baked or broiled fish lower the risk by only 30%. Women who eat less than 1 serving of baked or broiled fish per month but have more than 1 serving of fried fish per week had a 2.3-fold higher risk for heart failure than women eating one serving of baked or broiled fish per week and less than 1 serving of fried fish per month (HR 2.28, 95% CI: 1.38, 3.75).
The current paper isn’t the first study to report a negative impact from fried fish. In 2005 Mozaffarian et al from Harvard Medical School reported a similar risk from eating fried fish. They had analyzed data from 4,738 adults older than 65 who had been tracked for 12 years. During that period 955 developed congestive heart failure. Subjects consuming baked or broiled fish 5 times per week had a 32% lower risk for heart failure. Consumption of fried fish 1 or more times per week was associated with 35% higher risk.1 Neither of these percentages is far off from those in this new Belin et al study.
The current paper isn't the first to report a negative impact from fried fish.
In 2006, Mozaffarian’s group published a second study related to fish and heart disease. Assuming that fish intake was associated with improved cardiovascular health, they looked for the physiologic effects behind the cardiovascular benefits. They compared dietary fish intake in 5,073 older adults against measurements of cardiac structure, function, and hemodynamics. Eating canned tuna or broiled or baked fish was associated with lower heart rate, lower systemic vascular resistance, greater stroke volume, and healthier diastolic function. In contrast, fried fish or fried fish sandwiches were associated with left ventricular wall motion abnormalities, a reduced ejection fraction, lower cardiac output, a trend toward a larger left ventricular diastolic dimension, and higher systemic vascular resistance. While “nicely cooked” fish was associated with desirable characteristics, fried fish was associated with structural abnormalities suggestive of systolic dysfunction and potential coronary atherosclerosis. As far as the authors could prove, “The observed inverse associations were independent of major lifestyle factors and potential dietary and non-dietary confounders.”2
Although a number of plausible theories could explain the positive benefits of fish consumption, only weak theories exist to explain the negative impact of fried fish.
Consumption of fried fish, but not other fish, was inversely related to levels of a particular soluble cell adhesion molecule called sICAM-1. High levels of this molecule are considered a predictor of cardiovascular disease independent of other traditional risk factors. Again, consider the contradiction: High levels of sICAM-1 are bad, and eating fried fish lowers the levels. This offers us no explanation for why fried fish increases risk.
Fried fish, especially prepared away from home, is likely cooked in oil that is used multiple times. Reusing frying oil may cause chemical changes in the oil that are harmful. In a 2000 study, mice were fed mackerel fried in coconut oil. Rats fed fish fried in oil that had been reused multiple times developed higher cholesterol levels and were more likely to show signs of kidney and liver damage.3
If it were the TFAs or other chemicals formed in the fish fry oil, one would expect consumption of other fried foods, such as French fries would also increase risk. So far these studies have not revealed an association between cardiovascular disease and consumption of other fried foods.
It is possible that the explanation for these effects is from unmeasured dietary and non-dietary factors such as mercury or other contaminants in the fish.
Guallar et al from Johns Hopkins reported in 2002 that high mercury levels doubled risk of myocardial infarction (MI) and theorized this counter-action to the cardiovascular benefits of fish might complicate the analysis of fish and heart disease data.4
The data have certainly not been consistent. More is not always better when it comes to fish. Researchers from University of Alabama analyzed data from a Swedish cohort of 36,234 women and found that moderate consumption of fatty fish (1–2 times per week) was associated with a 30% lower heart failure (HF) risk, but more frequent servings were not as beneficial, lowering HF by only 9% for 3 servings/week or less.5 Could it be that with higher fish consumption, mercury overload comes into play?
The studies have been inconsistent in finding a correlation between mercury in fish and cardiovascular disease. Plante and Babo responded to Guallar’s assertion with data from a Japanese city of 50,000 where most of the population appear to have very elevated mercury, yet do not show higher MI risk than the general population that has acceptable levels of mercury.6
The fish/mercury/heart disease theory sunk a bit further earlier this year. Wennenberg et al in a January 2011 paper looked at the relationship between low or high fish consumption, methyl-mercury, selenium, and omega-3 levels with the risk for MI. This was a case-control study nested within the northern Sweden cohort, in which data and samples were collected prospectively. Data from 431 cases of MI were compared against 499 matched controls. While low selenium, high mercury, and low omega-3 fats were correlated with increased risk for MI, no correlation was seen for fish consumption.7
What is it about fried fish? Could it be the fried potatoes that are almost always served alongside fried fish? In the current Belin 2011 study, consumption of other fried foods was tracked, and independent associations were looked for but not found for total fried foods, French fries, fried potatoes, fried rice, fried cassava, fritters, or fried chicken intake. Fried food did not increase cardiovascular risk in the Belin data, yet fried fish did.
Perhaps it is the combination of specific chemicals found in the reused frying oil that trigger chemical changes in the polyunsaturated fatty acids (PUFAs) found in the fish. Thus it might be neither the PUFA in the fish nor the frying, but the combination of the 2 that together raise the risk.
The explanation for the fried fish effect that seems the most plausible is that it isn’t the fish per se that is at fault. Rather fried fish consumption may simply be a marker of a relatively unhealthy lifestyle. According to the Belin analysis, people who chose baked or broiled fish were younger than those who ate fried fish. The also had less diabetes, less atrial fibrillation (AF), less coronary artery disease (CAD), lower systolic blood pressure (SBP), lower body mass index (BMI), more physical activity, higher education, greater more fruit and vegetable consumption, higher DHA/EPA intake, and lower saturated and TFA intake. Those who consumed more fried fish were pretty much the opposite; they were more likely to have AF, CAD, SBP, higher BMI, less exercise, lower education, lower fiber intake, lower fruit and vegetable intake, higher total energy intake, higher intake of saturated fats and TFA. They were more likely to smoke. Study participants who had high fried fish consumption were more likely to have dyslipidemia.
Thus it may not be that the fried fish itself is at fault. Instead eating fried fish may be a litmus test that indicates an unhealthy life style, which is more likely to lead to heart disease.
On the other hand, the studies that could support this idea have not. For example, a 2009 paper that reported omega 3 intake from non-fried fish is inversely associated with interleukin-6 and C-reactive protein levels found that these associations were “independent of age, body mass index, physical activity, smoking, alcohol consumption, and dietary variables.”8
Fish Oil Supplements
Belin et al found no association between consumption of fish oil supplements and heart failure. In searching for possible explanations, the researchers wonder if “it may be that fish, as a complete food, exerts a more robust impact … than intake of omega-3 fatty acids.” This whole-foods approach sounds almost naturopathic. It may not be time for patients to throw out their fish oil pills yet, though. Measurements of omega-3 fats in blood and serum are still considered strong predictors of future heart disease, so this research alone does not entirely discredit the belief that fish oil supplements are beneficial.9
If our patients eat fried fish often, this is a bad sign; they are at higher risk for heart disease. Whether this is from the fried fish itself, or because the patient flunked the “fried fish litmus test” for healthy lifestyle is not clear. In case it is the fried fish that is to blame, it would be wise to eat it less often. For those patients who will only eat fish when it is fried, you should counsel them that fried fish is not good for their hearts. Fish oil supplements may not work as well as fish, a belief that is congruent with our naturopathic belief that food is the best medicine.