Have you ever wondered if there’s a natural way to lower your high blood pressure, guard against Alzheimer's, lose weight, and feel better? Well as it turns out there is. Michael Greger, M.D. FACLM, founder of NutritionFacts.org, and author of the instant New York Times bestseller “How Not to Die” celebrates evidence-based nutrition to add years to our life and life to our years.

Nutrition Facts Grab Bag 18

Want to know more about forest bathing, the ideal waist size, and how to make matcha ice cream? You’ve come to the right place.

This episode features audio from Why Does Forest Bathing Boost Natural Killer Cell Function?, What’s the Ideal Waist Size?, and Dr. Greger in the Kitchen: My New Favorite Dessert. Visit the video pages for all sources and doctor’s notes related to this podcast.


You may have heard the expression “knowledge is power.” Well – today – we’re going to give you more power to control your diet and lifestyle – by giving you the facts. Welcome to the Nutrition Facts podcast. I’m your host – Dr. Michael Greger.

It’s time for the Nutrition Facts grab bag, where we look at the latest science on a whole variety of topics. In our first story, we ask – can the aroma of wood essential oils replicate the immune-boosting effects of walking in a forest?

Studies on the effects of “forest bathing,” a traditional practice in Japan of visiting a forest and breathing its air, have found it “can induce…significant increase[s] in the number and activity of natural killer cells” that can last for as long as a month. And, because natural killer cells are one of the ways your body fights cancer (by killing off tumor cells), the findings suggest that forest visits “may have a preventive effect on cancer generation and progression.” Okay, but how? “Why did the forest environment increase…natural killer cell activity?” What is it about the forest environment?

One thought is that the boost may be related to a reduction in stress. If you measure the amount of adrenaline flowing through people’s systems, did hanging out in a forest—but not a city—drop adrenaline levels down? Yes, so that checks out, but drip some adrenaline on human blood cells in a petri dish, and there does not appear to be any effect. The stress hormone cortisol, on the other hand, dramatically suppresses natural killer cell activity. So, maybe the forest led to less stress, less cortisol, which released the natural killer cells under its thumb––and you get the big boost?

We know being surrounded by nature can decrease levels of cortisol in our saliva, but what about our bloodstream? A significant drop after a single day trip to the forest. But a week later, the cortisol was normalizing, and the forest effects sometimes appeared to last an entire month. Anything else that could cause a longer-term immune system change?

Maybe we’ve been missing some of our “Old Friends.” If you sample outdoor air, you can pick up an abundance of microorganisms floating around from the soil or water, which are absent in our indoor air (which is dominated by organisms that either live on us or try to attack us). So, maybe on a day-to-day basis, in terms of keeping our immune system on ready alert, it might not be sufficient to encounter only the biased microbes of the modern synthetic indoor environment that lack some of the Old Friends, and probably bear little resemblance to the microbes we evolved to live with over millions of years.

Or maybe it’s the plants themselves. Maybe it’s the aroma of the forest? Trees produce aromatic volatile compounds called phytoncides, like pinene, which you can breathe into your lungs in the forest. But do these compounds actually get into your bloodstream? One hour in the woods, and you get like a six-fold increase in circulating pinene levels circulating throughout your system. Okay, but to fully connect all the dots, the phytoncides like pinene, these tree essential oils would have to then induce human natural killer cell activity. And…guess what? Phytoncides induce human natural killer cell activity. If you stick natural killer cells in a petri dish with some unsuspecting leukemia cells, your killers can wipe out some of the cancer cells, but add a whiff of cypress, white cedar, eucalyptus, or pine, and the cancer cells don’t stand a chance.

A combination of wood aromas improved the recovery of mice put through the wringer. But this is the study I was looking for. If we want to know if the magic ingredient is the fragrance of the forest, then let’s see if we can get that same boost in natural killer cell activity just vaporizing some essential oil from one of the trees into a hotel room overnight. And it worked! A significant boost in natural killer cell activity, though it just boosted their activity, rather than their number, and being in the actual forest can do both. So, maybe it’s a combination of the tree fragrance and the lower cortisol levels working together?

Ironically, these phytoncide compounds are part of the tree’s own immune system, which we may be able to commandeer. The researchers speculate these compounds may be playing some role in the fact that more heavily forested regions in Japan appear to have lower death rates from breast cancer and prostate cancer. Being out in nature has been found to be an “important coping strategy among cancer patients.” It turns out this could potentially be helping more than just with the coping, thanks to the fragrance of trees.

In our next story, we look at the ideal waist size.

There was a book originally published in the 80s, and then repeatedly republished, entitled Dieting Makes You Fat. Since most people who lose weight go on to regain it, the concern is that there may be adverse health consequences of so-called yo-yo dieting. This idea emerged from animal studies that showed, for example, detrimental effects of starving and refeeding obese rats. This captured the media’s attention, leading to a pervasive common belief about the “dangers” of weight cycling, discouraging people from even trying.

Even the animal data is inconclusive, though. For example, weight cycling mice makes them live longer. Most importantly, though, a review of the human data concluded that “evidence for an adverse effect of weight cycling appears sparse, if it exists at all.” Bottom line: “Yo-Yo Dieting is Better Than None.”

Ideally, we’d get down to a BMI of 20 to 22, but body mass index doesn’t take the composition of the weight into account. For example, bodybuilders are heavy for their height, but can be extremely lean. The gold standard measure of obesity is percentage body fat, but an accurate calculation can be complicated and expensive. All you need to measure BMI is height and weight, but it may underestimate the true prevalence of obesity.

The World Health Organization defines obesity as a body fat percentage over 25 percent in men or 35 percent in women. At a BMI of 25, which is considered just barely overweight, body fat percentages in a representative US sample of adults varied between 14 percent and 35 percent in men, and 26 percent and 43 percent in women. So, you could be normal weight but actually obese. Using the BMI cutoff for obesity, only about 1 in 5 Americans were obese back in the 90s. But based on their body fat, the true proportion even back then was closer to 50 percent. Half of America is not just overweight, but obese.

So, just using BMI, doctors may misclassify more than half of obese individuals as being just overweight or even normal weight, and miss an opportunity to intervene. The important thing is not the label, though, but the health consequences. Ironically, BMI appears to be an even better predictor of cardiovascular disease death than percentage body fat. That suggests that excess weight from any source—fat or lean—may not be healthy in the long run. The lifespan of bodybuilders does seem to be cut short. They have about a third higher mortality rate than the general population. The average age of death was around 48 years old––but this may well be due in part to the toxic effects of anabolic steroids on the heart.

Preeminent nutritional physiologist Ancel Keys (after which “K-rations” were named) suggested the mirror method: “If you really want to know whether you are obese, just undress and look at yourself in the mirror. Don’t worry about our fancy laboratory measurements: you’ll know!” All fat is not the same, though. There is the pinchable superficial flab that you may see jiggling about your body, but then there’s the riskier, deeper visceral fat which coils around and infiltrates your internal organs. Measuring BMI is simple, cheap, and effective, but does not take into account the distribution of fat on the body, whereas waist circumference can provide a measure of the deep underlying belly fat.

Both BMI and waist circumference can be used to predict the risk of death due to excess body fat, but even at the same BMI, there appears to be nearly a straight-line increase in mortality risk with widening waistlines. Someone with “normal-weight central obesity,” meaning someone not even overweight according to BMI, but fat around the middle, may have up to twice the risk of dying compared to even someone who’s obese according to their height and weight. This is why the current recommendations recommend measuring both BMI and waist circumference. This may be especially important for older women, who lose approximately 13 pounds of bone and muscle as they age from 25 to 65, while quadrupling their visceral fat stores (men tend to only double). So, even if a woman doesn’t gain any weight based on the bathroom scale, she may be gaining fat.

What’s the waistline cut-off? Increased risk of metabolic complications starts at an abdominal circumference of 31.5 inches in women, and 37 inches in most men, though closer to 35.5 inches for South Asian, Chinese, and Japanese men. The benchmark for substantially increased risk starts at about 34.5 inches for women, and 40 inches for men. Once you get over an abdominal circumference of about 43 inches in men, mortality rates shoot up about 50 percent compared to men with 8-inch-smaller stomachs, and women suffer 80 percent greater mortality risk at 37.5 inches compared to 27.5 inches. The reading of a measuring tape may translate into years off one’s lifespan.

The good news is the riskiest fat is the easiest to lose. Your body appears smart enough to preferentially shed the villainous visceral fat first. Although it may take losing as much as 20 percent of your weight to realize significant improvements in quality of life for most individuals with severe obesity, your disease risk drops almost immediately. At 3 percent weight loss (just 6 pounds for someone weighing 200 pounds), your blood sugar control and triglycerides start to get better. At 5 percent, your blood pressure and cholesterol improve. Just a 5 percent weight loss (about 10 pounds for someone starting at 200) may cut your risk of developing diabetes in half.

Finally today, if you’re feeling like you need a cool sweet and healthy treat –why now whip up some Matcha ice cream? Here’s how.

This is Dr. Greger in the Kitchen, where I take the science and put it into practice. Today, I’m going to share with you my new favorite dessert: matcha ice cream. 

Matcha is powdered green tea leaves. What’s so great about green tea? Its consumption is associated with lower rates of heart attacks, strokes, and premature death, proven in randomized, controlled trials to prevent precancerous colon polyps from developing, and precancerous prostate lesions from turning into full-blown cancer, and so anti-viral there’s an FDA-approved green tea compound ointment used to reverse warts caused by the HPV virus. That’s why in my Daily Dozen checklist, it is one of the healthiest beverages, along with water and hibiscus tea.

Matcha is probably the healthiest way to drink green tea, since you’re drinking it as a whole food—whereas normally, you just make a hot water-green tea extract, and throw away the leaves. That’s like boiling some collard greens, and throwing away the greens, and just drinking the cooking water! Think what proportion of nutrition you’d be throwing away. The same thing, with tea leaves. Matcha allows you to drink tea whole.

That way, you’ll get all the nutrition, but also all of the potential contaminants. You know, China was late on the ban on leaded gasoline, resulting in higher-than-average lead levels in the soil on Chinese tea plantations. It gets taken up by the plant, but does not tend to leach out into the water when you just make tea the regular way. But if you eat the tea, like matcha, or if you throw tea leaves in a smoothie or something, I would recommend making sure your tea is sourced from Japan, rather than China.

Alright, now this stuff I have here is from Japan, so that’s not a problem. The problem, however, with any matcha tea is the taste. Matcha has a strong grassy, earthy, mossy flavor. So, I figured, why not put it into ice cream? It could be the spoonful of sugar that helps the medicine go down. Of course, traditional ice cream has way more than a spoonful of sugar—not to mention the saturated butterfat. And the dairy proteins can actually interfere with some of the beneficial effects of the green tea compounds. So, how about making a healthy matcha ice cream comprised entirely of whole foods—just two whole foods, to be exact.

Take two frozen super-ripe bananas and put them in a food processor or blender, alongside with a half-teaspoon of the matcha tea—now, more if you can stand it, but it’s pretty strong stuff, so why don’t we start there. And blend it up.

Blended frozen bananas take on this perfect, rich, creamy ice cream texture. Now, the more, the riper, the better. You want lots of little brown spots on there. I’m going to add a little, maybe a little mint garnish here.

The matcha actually makes the blended bananas taste better somehow. So, we’re talking just two whole plant foods—one a dark green leafy. Makes a gourmet decadent-tasting dessert. The more you eat, the healthier you are. That’s the way desserts should be.

We would love it if you could share with us your stories about reinventing your health through evidence-based nutrition. Go to NutritionFacts.org/testimonials. We may share it on our social media to help inspire others.

To see any graphs charts, graphics, images, or studies mentioned here, please go to the Nutrition Facts Podcast landing page. There you’ll find all the detailed information you need – plus links to all of the sources we cite for each of these topics.

For a vital, timely text on the pathogens that cause pandemics – you can order the E-book, audio book, or the hard copy of my latest book “How to Survive a Pandemic.”

For recipes, check out my new “How Not to Diet Cookbook.” It’s beautifully designed, with more than 100 recipes for delicious and nutritious meals. And all proceeds I receive from the sales of my books go to charity.

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