By Mark Mahoney
Much of the advice we hear from the experts on diet and health is often attacked as being flawed or faulty.
Critics will denounce expert dietary advice as being based on weak “observational” studies that cannot prove cause and effect (thanks, in part to the food industry). Those types of studies are not necessarily weak, and most advice is also based on the gold standard of scientific research, randomized clinical trials. Here is a review of two of the five ‘game-changing’ studies that support expert dietary advice. In Part 2 I’ll cover the final three.
The DASH studies
The initial DASH (Dietary Approaches to Stop Hypertension) study was published in 1997. It’s still the standard for today’s advice from the American Heart Association (AHA), American College of Cardiology, and others. Focus on consuming a diet rich in vegetables and fruits; include low-fat dairy, poultry, fish, beans, whole grains, oils, and nuts. And limit sweets, sugar-sweetened beverages, and red meats.
Observations were noted within other cultures where there’s very little high blood pressure and very little rise in blood pressure with age.
Today, in contrast, roughly one out of two U.S. adults now have high blood pressure or hypertension, which is a major risk factor for strokes and heart attacks.
A study undertaken by scientists at the National Heart, Lung, and Blood Institute (NHLBI) randomly assigned 459 adults (most had higher than normal blood pressure) to eat one of three diets:
- A typical American diet
- A typical American diet with extra fruits and vegetables (instead of snacks or sweets)
- A DASH diet (a typical American diet with extra fruits, vegetables and less saturated fat and cholesterol)
Researchers prepared all the food for the participants thus ensuring the study was “well-controlled.” Results after eight-weeks noted that the fruits and vegetables group had lowered blood pressure significantly, but only about half as much as the DASH diet lowered blood pressure.
A second study, DASH-Sodium was undertaken by scientists to see what might happen if sodium was reduced in both the DASH diet and the typical American diet.
A random assignment was made to 412 individuals to eat one of those two diets with high levels of sodium (3,300 milligrams a day), with intermediate levels (2,400 milligrams a day), or with low levels (1,500 milligrams a day).
Results indicated a substantial reduction in blood pressure, particularly for older people. A switch to a DASH diet produced a huge drop in blood pressure. It appears the DASH diet alone can lower blood pressure as much as taking a blood-pressure-lowering drug, according to Frank Sachs, professor of cardiovascular disease prevention at the Harvard T.H Chan School of Public Health.
A third study known as OmniHeart looked at replacing some of the carbohydrates with protein or unsaturated fat which lowered blood pressure, cholesterol and triglycerides even more than the original DASH diet. It focused on the same healthy DASH dietary pattern.
Information on the DASH diet can be accessed at:
The Saturated Fat Trials
Early observational studies like the Seven Countries Study reported higher rates of heart disease in people who ate more saturated fats.
Two series of parallel trials were also underway testing which fats raised or lowered blood cholesterol levels.
According to Martin Katan, a cardiovascular disease expert and emeritus professor of nutrition, by 1965 it was beyond a reasonable doubt that if you replace saturated fats with polyunsaturated fats, there would be a subsequent substantial lowering of total cholesterol.
LDL (low-density lipoproteins) began to be looked at separately by researchers in the early 1970s and by 2016; a World Health Organization had looked at 91 trials. These well-organized experiments all showed that replacing saturated fats with polyunsaturated fats would lead to a lowering of the LDL. “…The effect of LDL on heart disease risk is one of the best-established facts in the whole of medical science,” according to Katan.
Several randomized clinical trials from the 1950s ‘60s and ‘70s looked not just at LDL, but also at heart attacks and strokes. Looking at the four highest quality trials together concluded that replacing a diet high in saturated fats with one high in polyunsaturated fats prevents heart attacks and strokes, according to Sachs.
Information on recent evidence can be accessed at Harvard’s Medical School at:
Note on Study Types
The two major types of studies are observational and randomized clinical trials.
Most observational studies are “prospective cohort” which ask people what they eat and typically wait 5 or 10 years to see who gets heart disease, diabetes, cancer, etc.
Pro: One sees a decades-long impact on health.
Con: It’s not possible to tell if unknown confounders explain the results.
Randomized clinical trials assign individuals to eat one or two (or more) diets. The best trials provide all the food. Upon completion of a few months, the scientists see if the diets made a difference in a risk factor (like blood pressure). Studying diseases like diabetes, cancer, or heart disease typically have to enroll thousands over years and rely on participants to choose their own food.
Pro: If the study finds a difference in a disease or risk factor, you can be assured the diet caused it.
Con: A study where a diet has no effect may be due to people not sticking with the diet or possibly a result of a trial that was too short or too small.
In my upcoming Part 2 column, I’ll review three additional game changers — The Diabetes Prevention Program (DPP), The DRINK Study, and the Pounds Lost study.
Paying attention to evidence-based scientific research allows us to make more informed choices. The result is a healthier lifestyle and an improved quality of life.
I would like to wish everyone a Happy, Healthy New Year.
Thanks to the Center for Science in the Public Interest, for much of the content in this blog.
Mark Mahoney has been a Registered Dietitian/Nutritionist for over 30 years and completed graduate studies in Public Health at Columbia University. He can be reached at email@example.com.