Science is as a quest for knowledge. Sometimes it ends with a clear-cut result, often not. The Earth goes around the sun. Water is composed of hydrogen and oxygen. The speed of light is 299,792,458 meters per second. These are facts and will never change. But knowledge is often fluid, conforming to new information as it comes to light. We no longer think maggots spontaneously arise from decaying flesh or that heart attack patients have to be kept on six weeks bed rest. These beliefs, though, were reasonable based on the knowledge available at the time.
Scientists are sometimes accused of being slipshod, too profit-motivated, or even incompetent for having made a decision that eventually turned sour. The prescribing of thalidomide, the introduction of DDT and the routine use of hormone replacement therapy at menopause are commonly cited as scientific errors, yet these were all backed by appropriate peer-reviewed studies. It often takes years for problems to emerge, and not having foreseen them does not mean that researchers erred or were motivated by interests other than the honest pursuit of science. Of course, when unexpected consequences do occur, it is important to recognize them. Making a change is not an admission of failure, indeed it is one of the hallmarks of science.
When back in the middle of the last century research revealed that cholesterol levels in the blood were associated with a risk for heart disease, it made sense to suggest that eggs, because of their significant cholesterol content, should be consumed in a limited fashion. That, though, was just an educated guess. Science dictates that such a guess be followed up and be properly evaluated. But it can take years of studies to determine if a hypothesis is correct, and such studies, particularly in the are of nutrition, are not easy to carry out. Food frequency questionnaires are notoriously unreliable, and there are all sorts of confounding factors such as genetics, age, weight and other dietary components that have to be taken into account.
Furthermore, no decision can be made based on any single study; a preponderance of evidence is required to arrive at a conclusion. With eggs, sufficient data has now been collected to indicate that their consumption does not have a significant effect on blood cholesterol levels. Indeed, the expert panel in the US that every five years makes dietary guideline recommendations has concluded that “cholesterol is not a nutrient of concern because cholesterol from foods doesn’t cause higher blood cholesterol levels.”
As far as scientific questions go, the effect of eggs on blood cholesterol was a relatively easy one to answer. But this is not always the case. Consider the question of chemicals that are suspected of causing cancer. Obviously people cannot be given suspected carcinogens and in any case, such trials would take decades to reveal an effect. Therefore, “knowledge” emerges from human epidemiological evidence along with studies in animals and cell cultures.
Various organizations classify carcinogens into different categories, with the most widely referenced one being the International Agency for Research on Cancer (IARC). Its Group 1 substances are definitely known to cause cancer based on human evidence and include tobacco, the combustion product benzopyrene, asbestos, benzene, formaldehyde, dioxin and ultraviolet light. Aflatoxins from molds, infection by helicobacter pylori bacteria, herbal remedies containing Aristolochia species, x-rays, occupation as a painter and alcoholic beverages are also on this list.
Group 2A is a compilation of “probable” carcinogens as determined by strong evidence from animal studies but limited human evidence. Lead compounds, acrylamide in baked goods, emissions from frying foods, hairdressing as an occupation, shift work, the herbicide glyphosate and insecticides such as diazinon and malathion are in this category. “Possible” carcinogens for which there is insufficient animal evidence and limited evidence in humans fall into Group 2B and include coffee, pickled vegetables, radiofrequency fields, titanium dioxide and DDT.
Combining all three groups, there are more than 450 substances or processes that are classified as known, probable or possible carcinogens. What do we do with this information? We can limit alcohol consumption, take care with sun exposure, treat helicobacter infection, avoid charred foods and of course shun smoking. As far as the other agents are concerned, sometimes we have to be satisfied with knowing that not everything can be known. Outside of occupational exposure, there just isn’t enough known to make sweeping recommendations. But if you are offered a job as a chimney sweep, don’t take it.
