Michael Greger MD

Michael Greger MD

Posted February 16, 2012

Published in Health

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Ask the Doctor: Q&A with Michael Greger, M.D. (#19)

Read More: antioxidant, arugula, atherosclerosis, beets, blood pressure, caffeine, coffee, colon cancer, dermatitis, DNA, eczema, endothelial, energy drink, espresso, flaxseed oil, green tea, health, heart disease, hempseed, hempseed oil, inflammation, nutrition, olive oil, oxalates, potato, stroke, supplement, sweet potato, toxins

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This is another sampling of the more than 900 comments and questions I’ve responded to on the site (so far!). Please feel free to leave any follow-up questions here or on any of the hundreds of videos on the more than a thousand topics covered on And remember, there’s a new video posted every weekday, so to make sure you don’t miss any:


Ionestamot asked on Toxins in Cooked Potatoes?[...] what’s the verdict on gold, red & purple potatoes?

The healthiest potatoes are probably sweet potatoes (see my video about sweet potatoes), but if you are going to eat plain potatoes, the varieties with colored flesh (not just skin) do appear healthier. A new study published this month found that the consumption of 6-8 microwaved purple potatoes a day (they’re pretty small) resulted in a boost in antioxidant capacity of the blood (whereas the plain potato starch control acted as a pro-oxidant) and a drop in blood pressure in overweight individuals. “Thus,” they concluded, “purple potatoes are an effective hypotensive agent and lower the risk of heart disease and stroke in hypertensive subjects without weight gain.” Last year a study found that purple potatoes appeared to suppress both early and late stage human colon cancer cells in vitro, but only if they were fresh. After being in storage a few months their anti-cancer properties diminished. And finally, another study published last year (and this one available full-text), found a trend towards lower inflammation in men eating purple potatoes than white, concluding “Pigmented potato consumption reduced inflammation and DNA damage in healthy adult males. This offers consumers an improved nutritional choice in potato consumption.” Purple sweet potatoes may offer the best of both worlds, as suggested in an in vitro study last year on human fat cells that suggested anti-obesity, anti-oxidant, and anti-inflammatory benefits.

Berryman asked on Preventing childhood allergies: I’d be interested in your opinion of this study I found online at This is regarding hempseed oil, not necessarily for children but for adults with eczema. Have you seen other studies looking at hempseed oil and eczema? Is it ok to take so much oil every day for a month?

I was surprised to find so few articles published in the human medical literature on hempseeds, but maybe I shouldn’t be given the lack of much of an industry lobby and the stigma attached to the plant. Only 4 popped up in a pubmed search (excluding articles written by a “Dr. Ian D. Hempseed.”). The latest (available full-text) was a double-blind placebo-controlled comparison of fish, flax and hempseed oil supplementation that lasted 3 months and found no significant effects of any of them on lipid profile, LDL oxidation or measures of inflammation. This result is similar to what was found in my video Is Distilled Fish Oil Toxin-Free? Before that a study comparing daily tablespoons of flaxseed to hempseed oil similarly didn’t find much effect. The third was the study you cited, which found that 2 daily tablespoons of hempseed oil improved atopic dermatitis (an itchy skin rash) better than the same amount of olive oil. The researchers suggest it may be because of the gamma linoleic acid content of hempseeds, an omega 6 fatty acid that paradoxically appears to have an overall anti-inflammatory effect. Instead of downing the oil, as always I’d suggest eating the whole food–hempseeds–directly (same with flaxseeds, see my video Just the Flax Ma’am). And the final study, “Anaphylaxis to ingestion of hempseed” soundly debunks the wikipedia claim that “In fact, there are no known allergies to hemp foods.”

veggie4every1 asked on Priming the proton pumpWhat about the oxalic acid in beets? Is it good for you to take in large amounts?

As noted in my video about the oxalates in turmeric versus cinnamon (Oxalates in Cinnamon), it’s not just the amount that matters but also how well particular oxalates are absorbed, and the bioavailability of oxalates in beets is relatively poor (6 times less so than spinach, for example). Cooking the beets could cut levels about 25% but for the rare person with a condition like idiopathic calcium nephrolithiasis (a type of kidney stones) that needs a low-oxalate diet, a better high-nitrate vegetable choice would be arugula.

Toxins asked on Ask the Doctor (#18)Hello Dr. Greger. Latest in nutrition Volume 8 was a fantastic DVD. It was very informative and applicable to my lifestyle. I especially liked the healthy ice cream idea, I’m going to try that! I don’t want to pester you with an email, so I thought id post it here. In regards to creatine, you mentioned that most creatine supplements are contaminated with heavy metals, yet consuming a creatine supplement as a vegetarian increased cognitive function. Now my question to you is this: Is there any safe creatine supplement and if there was one, would you recommend consuming it?  Also in reference to caffeine, you mentioned it has positive health benefits. This study shows that the caffeine in coffee caused impaired endothelial function: I know Dr. Esselstyn also brings up this study. What are your thoughts?  

I’m so glad you liked it! (note: volume 8 won’t be available publicly until March; I’m still rolling out volume 7). So everyone understands the context, I’ll leave the creatine question until the video airs (sometime in early May), but let me address the caffeine question, since I’ve been such an outspoken advocate of green tea consumption (see, for example, The Healthiest Beverage and Dietary Brain Wave Alteration). As I detailed in my video The Power of NO, endothelial dysfunction is the first step towards atherosclerosis–our #1 killer–and so we need to keep the inner lining of our arteries healthy by any means necessary. Last year, a study entitled “Impact of Acute Caffeine Ingestion on Endothelial Function in Subjects With and Without Coronary Artery Disease” was published in the American Journal of Cardiology. They performed the most rigorous investigation to date, a randomized, double-blind, placebo-controlled cross-over study that found that caffeine significantly improved endothelial function. They concluded “In conclusion, acute caffeine ingestion significantly improved endothelial function assessed by brachial artery FMD in subjects with and without CAD and was associated with lower plasma markers of inflammation.” That was for the amount of caffeine found in about 2 cups of coffee, 4 cups of black tea, or 8 cups of green. Similar benefits were found previously at a higher dose (3, 6, and 12 cups respectively). So why do studies on brewed coffee, espresso, and energy drinks show negative effects? Well, there are a lot of other substances in these beverages besides caffeine, some of which may also be removed in the decaffeination process. Since there appear to be compounds in coffee that both impair and improve endothelial function (whereas in tea, both green and black, it appears to be all improvement), one might turn to epidemiological studies to look at overall risk and benefit of coffee consumption (see, for example, my Update on Coffee). Though filtered coffee may be good, the evidence supporting the benefits of green tea are much stronger and more consistent. So I continue to recommend people drink tea instead of coffee, not because coffee is bad for you, but because green tea appears to be much better.

JTodd asked on Medical school nutrition educationI’m afraid I’m one of those guilty docs who never filled in the nutrition knowledge gap that medical school left me with. has been such an eye-opener, and I am already seeing some quasi-miraculous changes in some of my more motivated patients. Where else would you suggest I go to supplement my training in nutrition?

My favorite source for continuing medical education credit is It’s co-sponsored by the George Washington University School of Medicine and Health Sciences and the Physicians Committee for Responsible Medicine. In fact they currently have my swine flu presentation up as one of the featured offerings!

-Michael Greger, M.D.

PS: If anyone lives in northern California, I'm giving a free open-t0-the-public presentation on the latest in clinical nutrition this weekend, and just added four new speaking dates in March. See my speaking schedule here.