Which of the currently popular and highly marketed diets are best for weight loss and cardiovascular health? Can science answer this question?
I just took a 90 minute online “course” taught by Eric Rimm of the Harvard School of Public Health; I think he does a good job of summarizing the scientific evidence on this topic and presenting it in a way that the average layperson can understand.
You can sign up for free here. If you’re not interested in spending your time watching him, here are my take-home points:
1. When evaluating the efficacy of a diet to control weight, the best evidence comes from observational studies that involve tens of thousands patients over decades and/or (preferably) randomized control trials that last at least two years.
2. The Paleo, Wheat-belly, gluten-free, Atkins, South Beach, and Zone diets do not have good evidence supporting sustained weight loss or health benefits. In general, people who follow these diets will be consuming lots of fresh vegetables, nuts and “healthy” fats and avoiding processed food, which is good, and this likely explains any benefits achieved.
3. Of all the diets, the low-fat diet (Ornish/Pritikin/China Study are the extreme examples of this) is the only one which has strong evidence showing an absence of benefit.
Yes, the diet that was recommended to Americans for 30 years does not help with weight loss in the long run for the vast majority of individuals.
As Rimm says “We need to eliminate the dogma that low fat is needed for weight loss.”
Dr. Rimm spends a good amount of time on this, highlighting findings from a study of 50 thousand women (the Women’s Health Initiative), which lasted for 9 years. In the first year, women on the low-fat diet (counseled to consume <20% of calories in the form of fat), lost more weight than those on the usual diet, however, in subsequent years they gained back the weight and did not differ from the higher fat consuming group.
There was also no difference in the rates of dying or contracting any disease between the two groups.
The problem with the low fat diet was adherence. Although a very small percentage of individuals can remain on a vegan or really low-fat diet and successfully lose weight and be healthy, the majority of us can’t.
By the end of the study the low fat group had increased their fat consumption to 28% which was not much less than what the usual group was consuming (32%)
Over time, the low fat group gradually added fats because they taste better and they are more satiating.
4. The DASH diet has evidence showing improvement in blood pressure, and cholesterol. The heart of the DASH diet is an eating plan rich in fruits and vegetables, low-fat and nonfat dairy, along with nuts, beans, and seeds.
Unfortunately, it was developed in an era when all fats were considered bad and proof of cardiovascular benefit is lacking.
5. Mediterranean diet. Gets a strong pass from Rimm with multiple studies showing benefits in both weight reduction and reduction of cardiovascular mortality.
The Med diet also demonstrates good long term adherence because of its diversity and inclusion of fat (for taste and satiety).
Improved adherence has been shown to be the major determinant of diet success. When you add regular counseling and support to any diet it works better and can be sustained.
This is the Mediterranean diet I recommend:
1. Lots of fresh fruits and vegetables. These contain fiber, phytochemicals, minerals.
2. Two servings of fish per week.
3. Plenty of nuts (and drupes!), legumes, and seeds.
4. Grains are allowed, preferably all whole grains.
5. Moderate alcohol consumption (1 drink/day for women, 2 drinks/day for men).
6. Olive oil.
7. Meat is allowed.
8. Eggs and dairy are allowed.
Dr. Rimm is still clinging to the idea that all saturated fats should be limited and preferably replaced by PUFAs or MUFAs. He presented this graphic (courtesy of Dr. Willet at Harvard), which illustrates the most prevalent concepts about saturated fat replacement.
Risk of heart disease is on the y axis. According to this graph, If you replace saturated fat with trans fat or sugar/refined starch, risk goes up.
If you replaced saturated fat with unsaturated vegetable fats or whole grains, risks go down.
Most nutritional experts now can agree on the importance of the key components of the Med diet and the lack of efficacy of low fat diets.
The disagreement comes in whether moving that arrow down from saturated fat to unsaturated fat is truly beneficial for weight management or cardiovascular health.
Good Fats and Bad Fats?
My own take on the good fat/bad fat controversy is as follows:
There are multiple types of saturated fats and multiple types of unsaturated fats and the scientific evidence is not currently robust enough to make the claim that replacing any saturated fat with any unsaturated fat is a healthy change.
There is no evidence that low-fat or no fat dairy is healthier than full fat dairy (see here and here). Eating no, or low, fat yogurt with the natural fat replaced by sugar and other additives likely moves the arrow up, raising your risk. This kind of processed food gets a pass from mainstream nutritionists for some reason.
Saturated fat from pasture-raised pigs and cows consumed in moderation is not unhealthy or weight gain promoting.
In the end, Dr. Rimm and I agree on about 95% of the science and recommendations in his course.
Take a look and you can tell your friends that you just passed a Harvard course with flying colors!