Why Are Walnuts and Seaweed Good for Your Gut?


Story at-a-glance:

  • Researchers from the University of Illinois suggest the addition of walnuts to your diet increases the amount, types and functional capacity of beneficial bacterial in your gut
  • A team from Stanford University has isolated a compound in seaweed that may be useful for manipulating gut bacteria strains for therapeutic use, providing evidence that dietary choices do affect gut health
  • One of the first steps you can take to improve your gut health is to eliminate sugar from your diet and eat whole foods, especially grass fed meat, organic vegetables and plenty of healthy fats
  • You also can nourish your gut by eating fermented foods and prebiotic foods, as well as by taking probiotics or sporebiotics

By Dr. Mercola

The importance of fiber to your diet is nothing new. The dietary fiber you get from foods such as nuts and seaweed, for example, acts as a food source for your gut microbiota, helping your beneficial bacteria do their jobs. This involves breaking down complex foods, providing your body with nutrients and helping you feel full.

Most people eat nuts because they are a convenient snack. While almonds and cashews are popular choices, you now have a new reason to choose walnuts. A study at the University of Illinois underscores the value of walnuts not only because they are a healthy fat and good source of fiber, but also because of the specific ways they may benefit your gut microbiome.

Similarly, research out of Stanford University involving seaweed takes gut bacteria to the next level in that scientists have identified a means of manipulating bacterial strains for potential therapeutic use. Both bodies of research underscore the need for optimizing your gut, which I consider to be one of the most important factors in your overall health and well-being. I say that because 70 to 80 percent of your immune system resides within your digestive tract.

Why Walnuts Are Good for You

If you routinely eat nuts but overlook walnuts, you may be missing out. According to PreventDisease.com walnuts have some impressive health benefits:1

  • A diet rich in walnuts and other nuts has been shown to play a role in supporting heart health2,3
  • Approximately 90 percent of the phenols — including flavonoids, phenolic acids and tannins — found in walnuts reside in its skin
  • Eating a handful of walnuts daily is said to reduce blood pressure and lower your risk of cardiovascular disease; walnuts have also been shown to slow tumor growth in animals4,5
  • If you are a man eating a Western-style diet, studies suggest consuming higher levels of polyunsaturated fatty acids (PUFAs), which are found in walnuts, may increase your sperm quality6
  • Consuming walnuts is thought to improve your brain function and cognition7

“Walnuts have been called a ‘superfood’ because they are rich in omega-3 fatty acid, alpha-linolenic acid and fiber and they contain one of the highest concentrations of antioxidants,” states registered dietitian Lauri Byerley, Ph.D., research associate professor of physiology at Louisiana State University Health New Orleans School of Medicine. “Now, an additional superfood benefit of walnuts may be their beneficial changes to your gut microbiota.”8

How Walnuts Support Your Gut Bacteria

A study published in The Journal of Nutrition9 suggests walnuts affect your gut microbiota — the trillions of mostly beneficial microorganisms inhabiting your gut — in beneficial ways.10 Researchers at the University of Illinois at Urbana-Champaign, led by Hannah Holscher, Ph.D., assistant professor of food science and human nutrition, fed 18 participants either 42 grams (about one-third cup) of walnuts or no walnuts during two three-week periods that were separated by a one-week “washout” period.

Blood and fecal samples were collected and analyzed at the beginning and end of each study time frame to evaluate the effects of walnuts on fecal microbiota, bile acids and metabolic markers of health. Walnut intake was shown to increase levels of three bacteria: Clostridium, Faecalibacterium and Roseburia, which produce a metabolic byproduct called butyrate that is believed to improve colon health.

It’s important to note, however, Holscher and her team did not measure butyrate levels. She says, “[W]e can’t say that just because these microbes increased that butyrate also increased. We still need to answer that question.”11 Other interesting highlights from the research include:12

  • Faecalibacterium, which has attracted attention as a potential probiotic bacterium, has been shown to reduce inflammation in animals and animals with higher amounts of it also have better insulin sensitivity
  • Participants who consumed walnuts had lower levels of secondary bile acids, which is significant because, according to Holscher, secondary bile acids have been found to be higher in individuals with colon cancer
  • Individuals consuming walnuts had blood levels of low-density lipoprotein (LDL) cholesterol that were 7 percent lower than the group that did not consume the nuts13

The work Holscher and her team completed builds on earlier research14 conducted at Louisiana State University in 2017. Working with lab rats for up to 10 weeks, Byerly and her colleagues discovered the walnut-eating group had different bacteria in its descending colon than rats eating a replacement diet containing comparable amounts of fat, protein and fiber.

In my opinion, the replacement diet of corn oil, protein casein and a cellulose fiber source was horrible and not an effective basis of comparison to walnuts. With the walnut diet, researchers noted the presence of increased amounts of beneficial bacterial. They also observed the numbers and types of bacteria changed, as did the bacteria’s functional capacity. Said Byerly:15

“We found that walnuts in the diet increased the diversity of bacteria in the gut, and other nonrelated studies have associated less bacterial diversity with obesity and diseases like inflammatory bowel disease. Walnuts increased several bacteria, like Lactobacillus, typically associated with probiotics, suggesting walnuts may act as a prebiotic.”

Seaweed Also May Have a Beneficial Impact on Your Gut Health

In a study published in the journal Nature,16,17,18 researchers at Stanford University School of Medicine have shown it’s possible to manipulate animal diets as a means of influencing their gut bacteria. In fact, the team, led by Justin Sonnenburg, Ph.D., associate professor of microbiology and immunology at Stanford, was able to control the amount of bacterium growing in mice intestines simply by adjusting the amount of a specific carbohydrate in their food and water.

Previously, a 2010 study19,20 published in Nature investigated the transfer of carbohydrate-active enzymes from marine bacteria to the human gut, particularly among Japanese people due to seaweed comprising a large portion of their diet. About the current body of work Sonnenburg said:21

“We’re all endowed with a microbial community in our guts that assembled in a chaotic manner during our first few years of life. Although we continue to acquire new strains throughout life, this acquisition is a poorly orchestrated and not-well-understood process. This study suggests it could be possible to reshape our microbiome in a deliberate manner to enhance health and fight disease.”

Despite the increased attention on the importance of gut health and probiotics, regardless of the source, little is known about why certain strains are more successful than others. As such, Sonnenburg and his colleagues speculated that a “dietary boost” would give specific bacterial strains an advantage inside the gut microbiome.

After choosing members of Bacteroides, the most prominent genus in human guts, the researchers sought to determine if the bacteria could use a carbohydrate called porphyrin, found in the sea vegetable nori. As to the choice of nori and that particular carbohydrate, Sonnenburg stated:22

“The genes that allow a bacterium to digest porphyran are exceedingly rare among humans that don’t have seaweed as a common part of their diet. This allowed us to test whether we could circumvent the rules of complex ecosystems by creating a privileged niche that could favor a single microbe by allowing it to exist in the absence of competition from the 30 trillion other microbes in the gut.”

Will Doctors One Day Be Able to Directly Manipulate Your Gut Bacteria to Fight Disease?

After identifying a nori-eating strain of Bacteroides, the Stanford scientists attempted to introduce it to three groups of lab mice:23

  • Two groups of mice had their gut bacteria wiped out and replaced with naturally occurring gut bacteria from two healthy human donors, each of whom donated exclusively to just one group of mice
  • The third group of mice retained a mouse-specific composition of gut microbiota

When all three groups were fed a standard mouse diet, researchers were successful in engrafting the porphyrin-digesting strain in only two groups, and only to varied and limited degrees. However, when the three groups were fed a porphyrin-rich diet, the bacteria engrafted at similar levels in all mice.

Notably, the team discovered they could precisely calibrate the size of the engrafted bacteria population simply by increasing or decreasing the amount of the isolated nori compound. About the results, which stunned researchers, Sonnenburg said:24

“The results of this dilution experiment blew us away. The direct effect of diet on the bacterial population was very clear. We can use these gene modules to develop a vast toolkit to make therapeutic microbial treatments a reality. Porphyran-digesting genes and a diet rich in seaweed is the first pair, but there could potentially be hundreds more. We’d like to expand this simple paradigm into an array of dietary components and microbes.

Our growing ability to manipulate [gut microbes] is going to change how precision health is practiced. A physician whose patient is about to begin immunotherapy for cancer may choose to also administer a bacterial strain known to activate the immune system, for example.

Conversely, a patient with an autoimmune disease may benefit from a different set of microbiota that can dial down an overactive immune response. They are just a very powerful lever to modulate our biology in health and disease.”

Other Ways You Can Nourish Your Gut

Besides eating a few more walnuts and adding sea vegetables to your diet, below are other ways you can intentionally nourish your gut microbiome.

Fermented foods: I often highlight the importance of eating fermented foods to help “heal and seal” your gut. Culturing vegetables is easy and inexpensive and you also can make your own homemade yogurtkefir or kimchi. These foods are not only packed with good bacteria, but also are detoxifying, immune-boosting and nutrient-rich. Fermented foods provide a wider variety of beneficial bacteria than you could ever receive in supplement form.

Prebiotic foods: A great way to support your friendly gut bacteria is by providing them with prebiotics, which are found primarily in fiber-rich foods. Lab research25 involving young rats found that dietary prebiotics have a significant effect on your rapid eye movement (REM) and non-rapid eye movement (NREM) sleep cycles. The following whole foods help add prebiotic fiber to your diet and improve the health of your microbiome, thus improving your overall health:26,27

Apples Asparagus Banana
Beetroot Breast milk Burdock root
Cashews Chicory root Couscous
Fennel bulb Garlic Grapefruit
Green peas Jerusalem artichokes Jicama
Konjac root Leeks Nectarines
Onion Persimmon Pistachios
Pomegranate Savoy cabbage Seaweed
Shallots Snow peas Tamarillo

Probiotics: While I highly recommend you obtain most of your nutrients from real food, probiotic supplements can be helpful if you are unable to eat fermented foods. For probiotics to do their job, you must nourish your microbiome with real food. That’s true because pathogenic, disease-causing bacteria thrive on sugar and processed food.

By eating whole, natural foods — plenty of healthy fats and organic vegetables, as well as moderate amounts of grass fed meat — you’re supporting the growth of your beneficial gut bacteria. Research suggests the benefits of probiotics aren’t limited to your gut, but also affect your immune system, mental health, mood and even your brain function. Probiotics have been shown to help reduce the symptoms of depression.

Sporebiotics: Spore-based probiotics, or sporebiotics, which are part of a group of derivatives of the microbe called bacillus, are a great complement to probiotics. Sporebiotics consist of the cell wall of bacillus spores, and they help boost your immune tolerance, especially when it comes to antibiotics.

Chronic low-dose exposure to antibiotics through your food, particularly with respect to meat sourced from concentrated animal feeding operations (CAFOs), takes a toll on your gut microbiome. This can result in chronic ill health and an increased risk of drug resistance.

Whatever steps you take — eating walnuts and sea vegetables, eliminating sugar and processed foods, adding prebiotic foods, incorporating fermented foods, taking probiotics or sporebiotics, or all of the above — I encourage you to begin optimizing your gut. A healthy gut will boost your immunity, help your body resist disease and positively affect your health and well-being in ways that may surprise you.


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