I often say that in studies comparing one group to another, you won’t detect a harmful effect of _____ if both groups are exposed to a level of ______ that is risk-producing. I said this recently about that saturated fat study (meta-analysis) that got so much attention in the media, and that spurred proclamations such as “Eat Butter” and “Saturated Fat Vindicated”:

“One way you can arrive at all your groups showing similar risk, which this study found, is when there isn’t much difference in consumption among your comparison groups. … Or when most of your participants are consuming saturated fat above a threshold where risk for CVD increases.”

As Jeff Novick pointed out:

“One major problem with this study is they did not look at any studies where the saturated fat intake was less than 7%, which is the level recommended by the [American Heart Association]. … Most of the diets had saturated fat intakes in the range of 10-15% (or more).”

I also said this in reference to the PREDIMED study. That was a big trial studying effects of the Mediterranean diet. It had three groups: “low-fat,” olive oil, and nuts. All the groups ate high-fat, even the “low-fat” group which ate 37% of their calories from fat! Not even the American Heart Association considers that low-fat. For perspective, a whole-food plant-based diet gets less than 15% of its calories from fat. The PREDIMED study found eating lots of olive oil was similar in CVD risk to eating “low-fat.” All groups suffered heart attacks at similar rates. All groups were exposed to a level of fat that was risk-producing.

I just saw a video that Dr. Greger posted about a month ago where he says essentially the same thing:

He cited this study:

Sick Individuals And Sick Populations, International Journal of Epidemiology, 2001

“If everyone smoked 20 cigarettes a day, then clinical, case-control and cohort studies alike would lead us to conclude that lung cancer was a genetic disease; and in one sense that would be true, since if everyone is exposed to the necessary agent, then the distribution of cases is wholly determined by individual susceptibility.”

Within a population, that is, within a group with similar characteristics, you may not see a big range of exposure to something, e.g. in the US, most people eat around 30-35% fat, not a big range. But when you compare populations with different characteristics…

But at the level of populations it is a different story: it has proved easy to show strong associations between population mean values for saturated fat intake versus serum cholesterol level and coronary heart disease incidence, sodium intake versus blood pressure, or energy intake versus overweight.

The reason we know that smoking increases the risk for lung cancer is because:

In the case of cigarettes and lung cancer it so happened that the study populations contained about equal numbers of smokers and non-smokers, and in such a situation case/control and cohort studies were able to identify what was also the main determinant of population differences and time trends.

In sum:

But to identify the causal agent by the traditional case-control and cohort methods will be unsuccessful if there are not sufficient differences in exposure within the study population at the time of the study.

There are some real gems in this paper…

Grateful patients are few in preventive medicine, where success is marked by a non-event.

That’s so true! You don’t really know if your behaviors are adding years to your life.

And this. I can’t believe he said this:

Harder to overcome than any of these, however, is the enormous difficulty for medical personnel to see health as a population issue and not merely as a problem for individuals.

Just about 100% of the people I know, even those in the public health community, harp on about individual behavior. You will never, ever be successful in disease prevention if you only intervene at the individual level. Social norms, peer pressure, environmental pollutants, occupational hazards, there are just too many things an individual is up against, and doesn’t control. You have to change the environment in addition to, perhaps more than, changing individuals’ behavior.

Greger is right when he says:

This is one of the most famous papers ever written in preventive medicine. It should be required reading for all medical students.

Back to the point I was making at the beginning of this post… If you want to know if something is harmful – smoking cigarettes, eating saturated fat, watching television – make sure you have a comparison group that isn’t doing it, or isn’t doing very much of it. Something that’s red is invisible against a red background.