Preventive Medicine Column
Dr. David L. Katz
I am just back (on the red eye, no less) from the annual Scientific Advisory Committee meeting of the California Walnut Commission held in Napa Valley. If you could live in Napa Valley, why would you live anywhere else? If you’ve been, you know what I mean.
The California Walnut Commission is the trade group representing walnut growers in the U.S., most of whom have their ranches in California. The commission is involved in some marketing, messaging, and PR- but overwhelmingly oversees the research agenda examining the health effects of walnut intake. And that, of course, is where I come in- and why I was at the meeting.
My lab has completed two studies of walnuts, and I was in Napa to present the second, as well as to participate in discussions of future research priorities.
Our first study, published in Diabetes Care in 2010, showed that adding walnuts to the daily diet of adults with type 2 diabetes for two months significantly improved blood vessel health, as measured by something called endothelial function.
Well known to clinicians, especially cardiologists, and to researchers, endothelial function is not generally familiar to the public at large. In brief, ultrasound is used to measure the ability of blood vessels to dilate when they should. Normal endothelial function is a very strong indicator of low heart disease risk; endothelial dysfunction portends the converse.
Walnuts added to the diet improve endothelial function in type 2 diabetes. Our study also showed that even though we were adding walnuts, and thus calories, to the diet- weight gain did not result. Our study subjects did not gain weight, because they made room in their diets for the walnut calories. Like other nuts, walnuts are energy dense (i.e., high in calories)- but especially when eaten as nature provides them, without additions of sugar or salt, they are very satiating, providing a lasting feeling of fullness. Satiety is, in essence, the ROI for calories consumed, and it appears to be very high for nuts in general, and walnuts specifically.
Our more recent study examined a potential role for walnuts in diabetes prevention. Using a similar design, and measures, we examined effects of roughly 14 walnuts/day for 2 months in overweight adults with signs of the metabolic syndrome, a state that anticipates diabetes. Our results are just in, and will be submitted for publication shortly. In brief, they closely mimic the results of study 1: endothelial function improved, as did blood pressure, with no weight gain.
Results from my lab were just a small part of a rich, 2-day conversation about the diverse health effects of walnut intake, and underlying mechanisms. Scientists from throughout the U.S., from Spain, and from Australia contributed.
Studies were presented indicating that walnuts confer health benefits when included in the Mediterranean diet; are effective at increasing satiety and controlling appetite; contain uniquely active antioxidants; may prevent cancer progression; reduce inflammation; influence gene expression; and may help reduce body fat. They may also enhance male fertility.
With regard to mechanisms- and the active ingredients in walnuts- discussion was equally far-ranging. Like most nuts, walnuts are rich in unsaturated oils, minerals (magnesium, calcium, and potassium), fiber, protein, and antioxidants. Unlike other nuts, walnuts are rich in omega-3 fat, in the form of ALA (alpha linolenic acid).
There is, as some of you likely know, on-going debate about the relative health benefits of plant omega-3 in the form of ALA, and marine animal omega-3, which comes as EPA and DHA. As was explored in some detail by the biochemists at our meeting, ALA converts inefficiently into EPA and DHA in the human body. But on the other hand, it may exert complementary health benefits of its own, and clearly influences the composition of our cells, and hormone production.
While much discussion focused on the ‘active’ ingredients in walnuts- an inevitable consequence of our often reductionistic approach to science- the consensus was clear that the active ingredient in walnuts is almost certainly…walnuts. Studies were presented indicating explicitly that many of those parts exert benefits even in isolation, but that the health effect of the whole is greater than a mere summing of the parts we know.
Even as we met, results of a meta-analysis by Harvard researchers, involving some 200,000 study participants, were published on-line in the American Journal of Clinical Nutrition. The study shows that regular intake of processed meat, and to a lesser extent any red meat, increases diabetes risk substantially. Conversely, more intake of nuts (as well as whole grain and low-fat dairy) and less intake of meat can reduce diabetes risk by as much as 35%. So says Dr. Frank Hu, senior author of the meta-analysis, and a participant at the Napa meeting.
I can say one more thing about walnuts: when the overall nutritional quality of nuts is measured comprehensively, they come out on top. On the NuVal scale from 1 to 100, which has itself been validated against health outcomes in 100,000 people, walnuts score 82. Almonds are close behind at 81, but virtually no other nut comes close. Pistachios score 69; pecans, 67; peanuts 29; and cashews, 25.
It is perhaps to be expected that a group of scientists studying walnuts, and attending a walnut commission meeting, would be walnut fans. In other words, you might dismiss our enthusiasm as bias. But there are two reasons why you shouldn’t.
First, the peer-reviewed science supporting health benefits from walnut intake is broad, and deep, and has had to run a gauntlet of filters designed to weed out biased work. Second, the groups’ bias in favor of walnuts is cart, not horse. The evidence generated by robust research has made proponents of us all- we didn’t just start out that way.
Nuts are encouraged in the Dietary Guidelines, and are a potentially valuable addition to most diets (assuming no allergies to them, of course). Walnuts are the pick of the nut crop in my opinion, as well as that of my many colleagues at the Napa meeting. There are likely benefits to weight and health from adding roughly 1 oz of them (7 nuts) to your diet most days.
That, in a nut shell, is the conviction I brought home with me from California. Chew on it as you see fit.
Dr. David L. Katz; www.davidkatzmd.com