Diet and Observational Studies

Woody Allen, Sleeper:

Dr. Melik: This morning for breakfast he requested something called "wheat germ, organic honey and tiger's milk."
Dr. Aragon: [chuckling] Oh, yes. Those are the charmed substances that some years ago were thought to contain life-preserving properties.
Dr. Melik: You mean there was no deep fat? No steak or cream pies or... hot fudge?
Dr. Aragon: Those were thought to be unhealthy... precisely the opposite of what we now know to be true.
Dr. Melik: Incredible.

Two more pieces of traditional dietary advice take a beating.

Article #1: Trans fats, but not saturated fats like butter, linked to greater risk of early death and heart disease

Contrary to prevailing dietary advice, a recent evidence review found no excess cardiovascular risk associated with intake of saturated fat. In contrast, research suggests that industrial trans fats may increase the risk of coronary heart disease. 

Saturated fats come mainly from animal products, such as butter, cows' milk, meat, salmon and egg yolks, and some plant products such as chocolate and palm oils. Trans unsaturated fats (trans fats) are mainly produced industrially from plant oils (a process known as hydrogenation) for use in margarine, snack foods and packaged baked goods. 

To help clarify these controversies, de Souza and colleagues analysed the results of 50 observational studies [ed., emphasis added; see below] assessing the association between saturated and/or trans fats and health outcomes in adults.

Or: Butter Good, Margarine Bad

Article #2: The science of skipping breakfast: How government nutritionists may have gotten it wrong

Researchers at a New York City hospital several years ago conducted a test of the widely accepted notion that skipping breakfast can make you fat. 

For some nutritionists, this idea is an article of faith. Indeed, it is enshrined in the Dietary Guidelines for Americans, the federal government’s advice book, which recommends having breakfast every day because “not eating breakfast has been associated with excess body weight.” 

As with many nutrition tips, though, including some offered by the Dietary Guidelines, the tidbit about skipping breakfast is based on scientific speculation, not certainty, and indeed, it may be completely unfounded, as the experiment in New York indicated.

The scientific speculation was based on some concept of how metabolism might work.

This is not some moldy, old-wive's tale piece of advice that has been hanging around for decades (like needing to drink 8 glasses of water a day). This was only added to the guidelines in 2010.

The advice was based on "observational studies." That is, researchers simply observe people behaving naturally. The self-selected "fasters" where more likely to obese than those who ate breakfast.

However, if "observational studies" are replaced by actual experiments, with randomized controlled trials and all the rest of the modern scientific method, the experiments disprove the conclusions of the observational studies 90% of the time.

[Note: the study in the first article was an "observational study".]

This may be due to "confounders."

One of the primary troubles in observational studies is what scientists refer to as “confounders” — basically, unaccounted factors that can lead researchers to make mistaken assumptions about causes. For example, suppose breakfast skippers have a personality trait that makes them more likely to gain weight than breakfast eaters. If that’s the case, it may look as if skipping breakfast causes weight gain even though the cause is the personality trait.

And moralizing the dietary guidelines creates problems.

Allison attributes the widespread adoption of the breakfast hypothesis at least in part on researchers who read too much into observational studies, and wrongly ignore the stronger evidence from the randomized controlled trials. In addition, he speculates that there “maybe some sense that eating breakfast is ‘moral’ and ‘upstanding,’” and that makes people more willing to believe it’s good for you.

For the record, most of the hypothesis of catastrophic anthropogenic global warming is based on observational studies that have been moralized.