The Best Foods to Slow Your Metabolism

The largest component of our daily energy budget is resting metabolic rate. As I discuss in my video Slowing Our Metabolism with Nitrate-Rich Vegetables, the direct effects of physical activity are relatively small compared to how many calories we expend just living and breathing. Now, during something like training for the US Army’s Special Ops or climbing a four-mile-high mountain, we may burn 4,000 calories a day. For most people, however, the calories we burn just lying around existing exceeds normal physical activities. Thus, our resting metabolic rate can have implications for controlling our weight.

Researchers have shown that dietary nitrate found in beets and green leafy vegetables improves the efficiency of the little power plants within our cells, boosting athletic performance by extracting more energy from every breath. So, if we eat a lot of vegetables, might it slow our metabolism since our body can function so much more efficiently with the calories we give it?

Indeed, researchers found that after giving people a dose of nitrate equivalent to a few servings of spinach or beets, their resting metabolic rates slowed on average about 4 percent. That’s nearly a hundred calories a day. If our bodies burned that many fewer calories each yet we didn’t eat any less, couldn’t we could put on a few pounds? Of course, green leafy vegetables may be the healthiest food on the planet, so we shouldn’t decrease our greens intake to try to control our weight. What’s going on? Researchers think perhaps it was a way our body evolved to use vegetables to help preserve energy during lean times in our ancient past. That is, slowing our metabolism may have benefits for our longevity.

What else similarly slows our metabolism? Caloric restriction, such as eating every other day. This may be one reason why caloric restriction is associated with a longer lifespan in many animals. Maybe like a candle, burning with a smaller flame allows us to last longer. It’s hard to walk around starving all the time, but it’s easy to replicate that same metabolic benefit by eating a big salad every day.

This may be why eating leafy green vegetables is among the six most powerful things we can do to live longer, along with not smoking, not drinking heavily, walking at least an hour a day, getting seven hours of sleep a day, and achieving an ideal weight. Doing even just one of these six may cut our risk of premature death by around 20 to 25 percent.


What’s that about boosting athletic performance? See:

Don’t want to carry beets out onto the track with you? Try fennel seeds: Fennel Seeds to Improve Athletic Performance.

What else can greens do? Check out How to Regenerate Coenzyme Q10 Naturally.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

Fennel Seeds for a Nitrate Boost

Dozens of studies now suggest that the nitrates in vegetables, such as beets and green leafy vegetables, may help not only sick people “as a low-cost prevention and treatment intervention for patients suffering from blood flow disorders” like high blood pressure and peripheral vascular disease, but also healthy people as an effective, natural performance-enhancing aid for athletes. Most of the studies were done with beet juice, though, which is why I was so delighted to see a study on whole beets, which showed the same benefit. But what about studies on whole green leafy vegetables? That’s one of the topics I cover in my video Fennel Seeds to Improve Athletic Performance.

There was a study a while ago suggesting that one of the reasons the Okinawans in Japan looked forward to many more years of good health at the same age at which many Americans and Europeans were dying is all the nitrate in their green leafy vegetables, which tends to bring down blood pressures. The reason I didn’t report on this at the time is because I had never heard of the vegetables in the study. I know what chrysanthemum flowers are, but I didn’t think most of my viewers (or I) would be able to find garland chrisantemum, ta cai, chin gin cai, Osaka shirona, nozavana (or nozawana) pickles, or water dropwort at the local store.

What about less exotic greens, like frozen spinach? Researchers wanted to test the immediate effects on our arteries of a single meal containing a cooked box of frozen spinach, for both arterial stiffness and blood pressure. First, they needed a meal to increase artery stiffness and pressure, so they gave people a chicken and cheese sandwich, which lowered the elasticity of their arteries within hours of eating. But, when they added the spinach, the opposite happened. After chicken and cheese, the force the heart had to pump went up within minutes, but the spinach kept things level. So, a meal with lots of “spinach can lower blood pressure and improve measures of arterial stiffness.”

That’s great for day-to-day cardiovascular health, but what if you want a whole food source that can improve your performance when you’re out hiking, for example? Beets and spinach aren’t the most convenient of foods when you’re out and about. Is there anything we can add easily to our trail mix? Well, if you look at a list of high-nitrate vegetables, you see celery, endive, lettuce, Swiss chard, and the like—not much you can just stick in your pocket. But what about fennel? That’s on the list. Could fennel seeds (which actually aren’t seeds at all, but the whole little fruits of the fennel plant) be the convenient, high-nitrate source we’re looking for?

Fennel seeds are “often used as mouth fresheners after a meal in both the Indian sub-continent and around the world.” You’ll typically see a bowl of fennel seeds, sometimes candy-coated, as you walk out of Indian restaurants. When you chew them, you can get a significant bump in nitric oxide production, which has the predictable vasodilatory effect of opening up blood vessels. This makes them a cheap and easy way to carry a lightweight, nonperishable source of nitrates. Researchers singled out mountaineers, thinking chewing fennel seeds could help maintain oxygen levels at high altitudes and help prevent HAPE—high altitude pulmonary edema—which is one of the leading killers of mountain climbers once you get more than a mile and a half or so over sea level. Don’t confuse HAPE with HAFE, though, which is caused by the expansion of gas at high altitudes—a condition known as high altitude flatus expulsion or “Rocky Mountain barking spiders.”

Fennel seeds may help with that, too, as they’ve been used traditionally as a carminative, meaning a remedy for intestinal gas. “Fennel has also shown antihirsutism activity,” combatting excessive hair growth in women, the so-called bearded woman syndrome. Indeed, applying a little fennel seed cream can significantly reduce it.

If fennel seeds have such a strong hormonal effect, should we be worried about chewing them? There have been cases reported of premature breast development among young girls drinking fennel seed tea a couple times a day for several months. Their estrogen levels were elevated, but, after stopping the tea, their chests and hormone levels went back to normal.

Current guidelines recommend against prolonged use in vulnerable groups—children under 12 and pregnant and breastfeeding women—and perhaps your pet rat, as rodents metabolize a compound in fennel called estragole into a carcinogen, but our cells appear able to detoxify it.


If you’re interested in learning more about using nitrates to improve athletic performance, check out:

Curious about non-nitrate athletic performance tweaks? See:

And what about sports drinks? See: Are Sports Drinks Safe and Effective? and Coconut Water for Athletic Performance vs. Sports Drinks

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

What to Eat to Boost Immunity

What we eat—or don’t eat—can affect our immune system. In my video Using the Produce Aisle to Boost Immune Function I profile a study conducted to determine the effect of the consumption of brightly colored vegetables on the immune system. For the first two weeks, the subjects ate basically no fruits and veggies. Then, they drank one and a half cups of tomato juice every day for two weeks, followed by two weeks of carrot juice, and then two weeks of spinach powder. Within just two weeks of a fruit- and veggie-deficient diet, immune function plummeted. However, just one and a half cups of tomato juice a day brought subjects back from the ashes. It didn’t take five servings a day—just one tall glass of tomato juice produced results. The carrot juice alone didn’t seem to help as well, however, nor did the powder equivalent of about one serving of spinach. This tells me two things: how remarkably we can affect our immune function with simple dietary decisions and, not all veggies are alike.

When this study was repeated looking at other immune markers, the tomato versus carrot appeared more evenly matched. There is one family of vegetables, however, that we definitely don’t want to miss out on. Inflammation and leaky gut can occur all because of an absence in our diet of AHR ligands—in other words, cruciferous vegetables, including cabbage, collards, cauliflower, kale, Brussels sprouts, and broccoli.

Do people who eat healthier actually get sick less? Those who eat more fruits and vegetables appear to have a lower risk of getting an upper respiratory tract infection like the common cold, whether they’re otherwise vegetarian or not. Even just one added apple a day may help keep the doctor away. The common cold is usually so innocuous, though, so why not test against something stronger?

Researchers have also looked at more serious respiratory infections like influenza. Studying the relationship between various risk factors and influenza-related hospitalizations in the United States, they found that a 5 percent increase in the prevalence of obesity was associated with a 6 percent increase in hospitalization rate. Physical inactivity had worse outcomes, resulting in a 7 percent increase in hospitalizations. Low fruit and vegetable consumption, however, had the most impact, increasing flu-related hospitalization rates by 8 percent.

The common cold isn’t always innocuous, though. For instance, a cold during the first trimester of pregnancy is associated with a number of birth defects, including anencephaly, one of the worst, which causes a fatal malformation of the brain. More recent data suggest that the cold-related fever is the real culprit, as anti-fever drugs appear able to prevent the possible birth defects caused by the common cold.

It’s best, of course, not to get sick in the first place. One thousand women and their diets were followed before and during pregnancy. It was found that “[w]omen who consume more fruits and vegetables have a moderate reduction in risk of [upper respiratory tract infection] during pregnancy, and this benefit appears to be derived from both fruits and vegetables instead of either alone.” Whole fruits and vegetables provide a natural balance of all sorts of things that may improve our immune function in a “complementary, combined or synergistic manner that could account for the protective effect observed from high consumption of both fruits and vegetables”—or maybe that’s the only way they got enough in their diet. The women who appeared protected in this study were eating nearly nine servings of fruits and vegetables a day, compared with only five servings of fruits or four of veggies. This suggests that the arbitrary five- or six-a-day minimum may be insufficient for effective immune function.

For example, in one famous study, elderly individuals were randomized into groups that ate either five servings of fruit and veggies a day or two servings a day. The five-a-day group showed an 80 percent improved antibody response to their pneumonia vaccination compared to the two-a-day group. Even though only about 30 percent (12 out of 40 people) of the five-a-day group reached their target levels of servings, they still did six times better than the two-a-day group. But maybe eight, nine, or ten servings a day would have worked even better.


Need a reminder about what those protective Ah receptors are? See The Broccoli Receptor: Our First Line of Defense and Counteracting the Effects of Dioxins Through Diet.

What’s the best way to prepare broccoli? Check out these videos:

In late pregnancy, however, women can overdo it. See Caution: Anti-Inflammatory Foods in the Third Trimester.

What else can we do to lower our risk of upper respiratory tract infections? See:

Also be sure to check out my video, Are Happier People Actually Healthier?, which compares people’s resistance to having the common cold virus dripped into their nostrils.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations: