If we were to conduct what is called an observational study on your eating habits in the year 2014, you would complete a questionnaire that asked what you ate over the preceding five days. We would ask you, “What did you eat for breakfast on Monday?” You reply, “Well, that was the day I was running late for work, so I just grabbed a Yoplait and a banana.” “What did you have for lunch?” You say, “I believe I ate my lunch at my desk that day and had a beef sandwich with horseradish on two slices of rye bread and a little cottage cheese.” “What did you eat for dinner?” You get the idea. We repeat these questions to chart your dietary intake over 5 days,

We follow the same exercise in several hundred or several thousand other people, also sharing their eating habits over a 5-day period. We then report aggregate patterns such as whole grain vs. white flour intake, calories, saturated and polyunsaturated fat, protein and fiber intake, etc. We may contact you every few years and repeat this exercise.

In the ensuing years, you get divorced. A stressful few years follow in which you eat erratically, skipping meals, resorting to frequent fast food and drive-thru window lunches and dinners. You drink too much alcohol, having occasional weekends of binge drinking. Maybe you indulge in some pot smoking. You take an all-inclusive vacation in Mexico, a Caribbean cruise a year later during which you drank and ate far too much. You start a new relationship, trimming down 10 pounds. You resume an exercise program, having neglected physical activity for a couple of years. You engage in a “liver cleanse” or two, move to another city, start a couple of different jobs with different degrees of stress, attend the wedding of your daughter. You’re diagnosed with mild high blood pressure and “high cholesterol” and prescribed a thiazide diuretic, ß-blocker, and statin cholesterol drug. You’ve gained a net of about 20 pounds since 2014 and you suspect you’ve developed sleep apnea because of the snoring your partner complains about.

We re-contact you in 2024 and ask you questions about your health: “Have you been diagnosed with colon cancer, coronary disease, hypertension, dementia, etc.?” We repeat this with the other study participants. We then try to draw connections between the food you ate during that 5 day period 10 years earlier and the development of your current health struggles. We don’t ask you about divorce, binge-drinking, vacations, job changes, etc. We then, after analyzing the data, draw conclusions such as:

  • Consuming whole grains reduces cardiovascular risk
  • Consuming less total fat reduces weight gain
  • Eating eggs increases risk for cardiovascular events

I hope that you see the absurdity in these exercises. Drawing associations from a 5-day snapshot of eating habits while ignoring the many ups and downs of human life, and then declaring that this or that dietary habit over a 5-day period was responsible for a disease is ridiculous. Yet that is the basis for dietary guidelines, the beliefs of most doctors and others who offer dietary advice, guidelines on how to avoid conditions such as coronary heart disease, colon cancer, and type 2 diabetes.

Admittedly, a superior method of determining cause-effect is difficult to perform, i.e., a double-blind placebo-controlled clinical trial. We cannot “blind” participants to what they are eating, nor can we expect people to adhere to a specific dietary style for a decade. This superior form of clinical study is therefore rarely conducted. One of the few exceptions, the Women’s Health Initiative, or WHI, in which 48,000 postmenopausal females were enrolled and randomized to a control diet vs. a diet reduced in dietary and saturated fat, increased consumption of vegetables and fruits, and increased whole grain consumption. After 8 years of follow-up, there was no difference in cardiovascular disease, breast cancer, or colorectal cancer among the two groups.

Bottom line: Observational evidence of the sort generated by a 5-day diet questionnaire is garbage data that has no value in the vast majority of instances. Imagine I surveyed your financial habits over a 5-day period—would that predict your eventual financial status 10 years later? Of course not. But the absurdity of this exercise doesn’t stop researchers at the Harvard School of Public Health, bureaucratic scientists at the USDA, or the media from trumpeting headlines such as “eating eggs cause heart disease” or “healthy whole grains reduce risk of colon cancer.” Dietary Guidelines have been wrong so often about so many issues that I believe that they have painted themselves into a corner,  increasingly irrelevant and not worthy of your attention. It’s their dictates that have contributed to the epidemic of overweight, obesity, type 2 diabetes and other “lifestyle” diseases worsened, of course, by the proliferation of ultra-processed foods and predatory marketing practices. The next time you hear some headline about the effects of diet, recognize that those headlines are nearly always based on a house of cards of “evidence.”