Best Food for the Common Cold

It is well known that deficiencies in certain vitamins and minerals can lead to significant impairment of immune function and an increased susceptibility to infection that can be reversed by supplementation in deficient individuals. “However, it is unclear to what extent supplementation may aid in maintaining an optimal balance within the immune system in adequately nourished individuals.” As I discuss in my video, Kiwifruit for the Common Cold, researchers in New Zealand tried supplementing with whole kiwifruit. In a petri dish, gold kiwifruit puree appeared to boost natural killer cell activity in human blood and also boost the response to the tetanus vaccine. But, does the same thing that happens in a petri dish happen in people? In another study, researchers found that two immune-related sets of genes were upregulated by eating three kiwifruit a day. But, does that translate into actually helping us fight off infection? We didn’t know the answer until recently.

Researchers in New Zealand found that the consumption of gold kiwifruit reduces the severity and duration of certain upper respiratory tract infection symptoms. Why study gold kiwifruits? The study was funded by the company that owns the patent to gold kiwifruits.

In the study, a few dozen elderly individuals were randomized into one or two groups, eating either two bananas a day or four kiwifruit a day for a month, and then switching. The next month, the banana group ate kiwis, and the kiwi group ate bananas. The rationale for providing banana as the “placebo” was to provide an alternative fruit that had relatively similar calories, but lower nutritional value.

The study’s purpose was to determine whether regular consumption of gold kiwifruit reduces the incidence, duration, and severity of symptoms of upper respiratory tract infections, like the common cold, in comparison to bananas. They found no reduction in overall incidence, but those who did get sick during the kiwifruit phase had significantly reduced severity and duration of head congestion, as well as a reduced duration of sore throat. And not just by a little. The sick banana eaters suffered for five days with a sore throat and congestion, compared to the kiwifruit eaters, who felt better after just a day or two. That’s significant.

The reason the researchers studied older individuals is that they tend to be more susceptible to respiratory infections. Small children are another at-risk group, averaging twice as many upper respiratory tract infections as adults, four to six per year. So, in another study, 66 preschoolers were randomized into the same two groups, kiwifruit versus bananas. In the kiwi group, not only were there again significant improvements in the symptoms of those who got sick, fewer got sick in the first place—45% lower odds of a cold or flu-like illness, which means that the children eating kiwis had almost a 50% reduction in the chance of having a cold or flu.

This suggests that kiwifruit consumption may be a “daily prescription for health”; though about 1 in 150 children report being allergic to kiwifruit, which ranks them number three after milk and eggs, and above peanuts, in a survey of 3,500 kids, of the most common food allergies. So, kiwis are not for everybody.


The upside of the financial conflict of interest is that it offers a rare source of funding for fruit and vegetable research. For more on kiwis, see Kiwifruit for Irritable Bowel Syndrome, Kiwifruit and DNA Repair, and Kiwifruit for Insomnia.

What else can we do to lower our risk of pesky respiratory infections? Check out Can Gargling Prevent the Common Cold?, Preventing the Common Cold with Probiotics?, Nutritional Yeast to Prevent the Common Cold, and Using the Produce Aisle to Boost Immune Function.

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:

Why it’s Better to Drink Green Smoothies With a Straw

A group of women were split into two groups and told to eat as much soup as they wanted, but half were given big spoons and told to eat quickly, while the other half were given small spoons and told to eat slowly. The slow group ended up feeling more satiated—despite eating less food. Prolonged meal duration can allow more time for our body’s own “I’ve-had-enough” signals to develop before too many calories have been consumed. It makes sense. After all, we evolved for millions of years before cooking, when undomesticated fruits and vegetables were much tougher and fibrous. Our body is built to expect us to take our time when eating.

What about when drinking? There weren’t any blenders on the African savannah. In smoothie form, you can drink fruits and vegetables at about two cups a minute—ten times faster than it might take to eat fruits and vegetables in solid form. Liquid calories can be consumed so quickly they can undermine our body’s ability to regulate food intake at healthy levels. It’s not the liquid texture per se, but the high rate of consumption at which liquids are normally consumed. Blend all the smoothies you want, but sip them slowly for a half hour or so rather than gulping them down.

Even when sipped slowly, though, an all-fruit smoothie may not be as filling as eating a whole fruit, so the more greens you can add to your smoothie the better. You can also add ground flax seeds. As you can see in my video, The Downside of Green Smoothies, the thicker the smoothie, the less hungry you are one, two, and even four hours later—and flax seeds make for thick, milkshake-type smoothies. Researchers found that one tablespoon of flax seeds significantly suppresses appetite and calorie intake. You can give someone a meal two hours after the tablespoon of flax seeds, and they eat significantly less—all the while dropping their cholesterol in only one week when eating about a tablespoon of flax seeds each day.

The fat naturally found in flax seeds can also help maximize the absorption of fat-soluble phytonutrients. There’s a threshold for optimal absorption that can be reached with just about three walnuts worth of fat. If we’re trying to reduce added fats, a green smoothie with some nuts, seeds, or avocado can enable us to take full advantage of the healthiest foods on the planet—dark green, leafy vegetables.

Smoothies also allow us to eat parts of fruits and vegetables we might not otherwise. If, instead of the lemon juice called for in Mayo Clinic’s basic green smoothie recipe (shown in the video), you used a little wedge of lemon, you might get some seeds and peel, which in vitro at least, appear to suppress both breast cancer and colon cancer cell growth.

Clinical studies on smoothies show what you’d expect to see from eating great foods like greens and berries—enhanced athletic performance and recovery, boosting the antioxidant power of your bloodstream, and potentially improving arterial function in both the short- and long-term. Kiwifruit smoothies protect against DNA damage, and strawberry smoothies protect against inflammation. Of course, presumably, so would just eating greens, kiwis, and berries intact.

There’s been some concern expressed that drinking green smoothies would bypass the nitrate-reducing bacteria in the mouth, but our body’s way too smart for that; it pumps nitrate back into our salivary glands. Even if we deposited greens directly into our stomach with a tube, we’d still produce the nitric oxide so important for artery health.

Concerns have been raised that the oxalic acid in vegetables might increase kidney stone risk, but, as research shows, the opposite might be the case. (See How to Treat Kidney Stones with Diet.) So are there any downsides of smoothie consumption?

Whether with lemon juice or a lemon wedge, smoothies can be sour. Any time you’re eating or drinking something sour, you have to be careful about eroding the enamel on your teeth. Researchers found that if you soak teeth in a smoothie for an hour, significant enamel is eroded away. But who soaks their teeth in a smoothie for an hour?

What if you instead study the effects of smoothies in situ (meaning in position), as opposed to in vitro (meaning in glass)? If you make people wear slabs of enamel in their mouths while they drink a smoothie to replicate a typical tooth exposure, researchers find almost as much erosion as drinking Diet Coke. So, it’s recommended that smoothies be consumed through a straw, similar to the advice given for other acidic beverages like soda or hibiscus tea. Drinking juice through a straw has less of an acidic effect than swishing it around in your mouth, so avoid swishing around mouthfuls of smoothie in your mouth. You also want to wait at least an hour before brushing so as not to brush your enamel in a softened state; rinsing your mouth with water after drinking smoothies can help rinse away some of the acids to protect your teeth.

One final caveat for smoothies: When I advocate green smoothies to boost fruit and vegetable consumption, I’m talking about whole food smoothies, not those made from juice or with added sugars—or human organs. Some women choose to consume their afterbirth. Though described as “replenishing and delicious,” the problem with eating one’s placenta is that one of the functions of the placenta is to filter out toxins, so it may be contaminated with heavy metals, as well as pose a food poisoning risk if consumed raw, like in a smoothie. Green smoothies are great, but I’d be cautious about drinking certain types of red smoothies.


I have several videos on smoothies: Are Green Smoothies Good for You?, Are Green Smoothies Bad for You?, Green Smoothies: What Does the Science Say?, Liquid Calories: Do Smoothies Lead to Weight Gain?, and A Better Breakfast.

Was the concern about dental erosion new to you? See more in Plant-Based Diets: Oral Health, Plant-Based Diets: Dental Health, and Protecting Teeth from Hibiscus Tea.

What’s with that nitrate thing? See Don’t Use Antiseptic Mouthwash. So, What’s the Best Mouthwash? Watch to find out.

For more on fat-soluble nutrient absorption, check out an ancient video of mine, Forego Fat-Free Dressings?. And for more on oxalates and kidney stones, there’s How to Prevent Kidney Stones With Diet and How to Treat Kidney Stones with Diet.

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 Cure High Blood Pressure

High blood pressure ranks as the number-one risk factor for death and disability in the world. In my video, How to Prevent High Blood Pressure with Diet, I showed how a plant-based diet may prevent high blood pressure. But what do we do if we already have it? That’s the topic of How to Treat High Blood Pressure with Diet

The American Heart Association (AHA), the American College of Cardiology (ACC, and the Centers for Disease Control and Prevention (CDC) recommend lifestyle modification as the first-line treatment. If that doesn’t work, patients may be prescribed a thiazide diuretic (commonly known as a water pill) before getting even more meds until their blood pressure is forced down. Commonly, people will end up on three drugs, though researchers are experimenting with four at a time. Some patients even end up on five different meds.

What’s wrong with skipping the lifestyle modification step and jumping straight to the drugs? Because drugs don’t treat the underlying cause of high blood pressure yet can cause side effects. Less than half of patients stick with even the first-line drugs, perhaps due to such adverse effects as erectile dysfunction, fatigue, and muscle cramps.

What are the recommended lifestyle changes? The AHA, ACC, and CDC recommend controlling one’s weight, salt, and alcohol intake, engaging in regular exercise, and adopting a DASH eating plan.

The DASH diet has been described as a lactovegetarian diet, but it’s not. It emphasizes fruits, vegetables, and low-fat dairy, but only a reduction in meat consumption. Why not even more plant-based? We’ve known for decades that animal products are significantly associated with blood pressure. In fact, if we take vegetarians and give them meat (and pay them enough to eat it!), we can watch their blood pressures go right up.

I’ve talked about the benefits to getting blood pressure down as low as 110 over 70. But who can get that low? Populations centering their diets around whole plant foods. Rural Chinese have been recorded with blood pressures averaging around 110 over 70 their whole lives. They eat plant-based day-to-day, with meat only eaten on special occasions.

How do we know it’s the plant-based nature of their diets that was so protective, though?

Because in the Western world, as the American Heart Association has pointed out, the only folks getting down that low on average were those eating strictly plant-based diets, coming in at about 110 over 65.

So were the creators of the DASH diet just not aware of this landmark research done by Harvard’s Frank Sacks? No, they were aware. The Chair of the Design Committee that came up with the DASH diet was Dr. Sacks himself. In fact, the DASH diet was explicitly designed with the number-one goal of capturing the blood pressure-lowering benefits of a vegetarian diet, yet including enough animal products to make it “palatable” to the general public.

You can see what they were thinking. Just like drugs never work—unless you actually take them. Diets never work—unless you actually eat them. So what’s the point of telling people to eat strictly plant-based if few people will do it? So by soft-peddling the truth and coming up with some kind of compromise diet, the on a population scale maybe you’d do more. Ok, but tell that to the thousand U.S. families a day that lose a loved one to high blood pressure. Maybe it’s time to start telling the American public the truth.

Sacks himself found that the more dairy the lactovegetarians ate, the higher their blood pressures. But they had to make the diet acceptable. Research has since shown that it’s the added plant foods—not the changes in oil, sweets, or dairy—that appears to the critical component of the DASH diet. So why not eat a diet composed entirely of plant foods?

A recent meta-analysis showed vegetarian diets are good, but strictly plant-based diets may be better. In general, vegetarian diets provide protection against cardiovascular diseases, some cancers, and even death. But completely plant-based diets seem to offer additional protection against obesity, hypertension, type-2 diabetes, and heart disease mortality. Based on a study of more than 89,000 people, those eating meat-free diets appear to cut their risk of high blood pressure in half. But those eating meat-free, egg-free, and dairy-free may have 75% lower risk.

What if we’re already eating a whole food, plant-based diet, no processed foods, no table salt, yet still not hitting 110 over 70? Here are some foods recently found to offer additional protection: Just a few tablespoons of ground flaxseeds a day was 2 to 3 times more potent than instituting an aerobic endurance exercise program and induced one of the most powerful, antihypertensive effects ever achieved by a diet-related intervention. Watermelon also appears to be extraordinary, but you’d have to eat around 2 pounds a day. Sounds like my kind of medicine, but it’s hard to get year-round (at least in my neck of the woods). Red wine may help, but only if the alcohol has been taken out. Raw vegetables or cooked? The answer is both, though raw may work better. Beans, split peas, chickpeas, and lentils may also help a bit.

Kiwifruits don’t seem to work at all, even though the study was funded by a kiwifruit company. Maybe they should have taken direction from the California Raisin Marketing Board, which came out with a study showing raisins can reduce blood pressure, but only, apparently, compared to fudge cookies, Cheez-Its, and Chips Ahoy.


The DASH diet is one of the best studied, and it consistently ranks as US News & World Report’s #1 diet. It’s one of the few diets that medical students are taught about in medical school. I was so fascinated to learn of its origins as a compromise between practicality and efficacy.

I’ve talked about the patronizing attitude many doctors have that patients can’t handle the truth in:

What would hearing the truth from your physician sound like? See Fully Consensual Heart Disease Treatment and The Actual Benefit of Diet vs. Drugs.

For more on what plants can do for high blood pressure, see:

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: