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Science Finally Knows Why Eating Fiber Is Necessary

Science Finally Knows Why Eating Fiber Is Necessary

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We’ve known for years the largely indigestible stuff was good for us. Now we understand what it does.

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But it hasn’t always been clear as to why a steady source of fiber is essential at all, given that the stuff is, by definition, the indigestible portion of fruits and veggies.

A new study finally puts the role that fiber plays into perspective, and it doesn’t have to do with calorie intake—but with our gut health instead.

It turns out the answer is that it’s food for our gut microbiome. That’s the name for the multitudes of bacteria in our intestinal system: which keep us healthy and aids in digestion.

It turns out those helpful little guys rely on dietary fiber for their food—and we benefit. While the gut might not have all the enzymes needed to completely break down each and every plant we eat, some of the bacteria within the gut actually have the power to break down these otherwise indigestible foods.

The New York Times reports that the discovery was made by Dr. Fredrik Bäckhed, who tested his theory on mice.

The mice who ate a diet poor in fiber had drastically fewer good—or bad—bacteria in their guts. What’s more, the bacteria there ended up closer to the animal’s intestine wall, causing ailments such as chronic inflammation. Within a few weeks, the mice developed higher blood sugar levels and began to put on weight.

The mice with high-fiber diets suffered from none of these problems.

Getty: shilh / Getty: ECummings00 / Getty: 4kodiak / Getty: AlasdairJames

But there are indications that the situation is not too difficult to reverse. After several weeks, Dr. Bäckhed administered a boost of fiber called inulin to a group of rodents who were eating the low fiber diet. Despite the fact that their diet was poor, the gut biomes in the mice improved.

While there’s a clear link between dietary fiber and aiding your healthy gut, Dr. Bäckhed noted that the mice with added inulin weren’t completely resorted to perfect health. It turns out not all fiber is equal—and the many different kinds of microbes need fiber from a wide variety of sources to stay healthy.

“It points to the boring thing that we all know but no one does,” Dr. Bäckhed told the Times. “If you eat more green veggies and less fries and sweets, you’ll probably be better off in the long term.”

How to Eat 37 Grams of Fiber in a Day

When it comes to getting enough fiber in our diets, most of us fall short. But it's easier than you think to eat the recommended daily intake. For adults 50 and younger you need 25 grams for women and 38 grams for men. This sample menu gives you 37 grams of fiber from tasty, familiar foods:

  • Breakfast: One serving of whole-grain bran flake cereal (5 grams of fiber), topped with half a sliced banana (1.5 grams of fiber) and skim milk
  • Morning snack: 24 almonds (3.3 grams of fiber) mixed with a quarter cup of raisins (2 grams of fiber)
  • Lunch: Turkey sandwich made with 2 slices of whole wheat bread, plus lettuce, and tomato (about 5 grams of fiber total), and an orange (3.1 grams of fiber)
  • Afternoon snack: Yogurt topped with half a cup of blueberries (2 grams of fiber)
  • Dinner: Grilled fish served alongside a salad made with romaine lettuce and shredded carrots (2.6 grams of fiber), plus half a cup of cooked spinach (2.1 grams of fiber), and half a cup of lentils (7.5 grams of fiber)
  • After-dinner treat: 3 cups popped popcorn (3.5 grams of fiber)

Some facts about meat

Think of meat as a protein sponge. Raw muscle is about 70% liquid. This liquid is not blood, which is dark red, almost black, thick, and it clots . There is no measurable blood in a properly slaughtered and butchered animal. Most of the liquids in muscle are myowater, water laden with the protein called myoglobin.

Myowater is thin and usually pink, and it doesn’t coagulate like blood. It is what you see on your plate when you cut into a piece of meat fresh from the heat. Remember, Zuzu, everytime someone calls it “blood”, somewhere a teenager becomes a vegetarian.

What we call juiciness is not just a matter of how much water is in the meat we are eating. There are many factors.

Insoluble Fiber

This is found in the seeds and skins of fruit (so always eat your peels) as well as whole-wheat bread and brown rice.

The health benefits include:

Weight loss: Like soluble fiber, insoluble fiber can play a key role in controlling weight by staving off hunger pangs.

Digestive health: Eating lots of insoluble fiber also helps keeps you regular, and if you do get constipated, adding more of it to your diet can get things moving. Insoluble fiber can also improve bowel-related health problems, like constipation, hemorrhoids, and fecal incontinence (problems controlling your bowel movements.)


Oregon State University Linus Pauling Institute: Micronutrient Information Center: "Fiber."

University of Colorado, Colorado Springs: "Soluble Fiber vs. Insoluble Fiber."

Cleveland Clinic: "Improving Your Health with Fiber." "Healthy Diet in Adults."

Harvard School of Public Health, The Nutrition Source: "Fiber."

Academy of Nutrition and Dietetics Oncology Nutrition: "Constipation, Diarrhea and Fiber."

Kaiser Permanente: "Fiber Facts: Why Fiber Is Important."

Montefiore Medical Center: "Don't Forget the Fiber in Your Low-Carb Diet."

Tracie Jackson, RD, LMNT, CDE, nutrition therapist, Nebraska Medicine. "Patient information: High-fiber diet (Beyond the Basics)."

Rebecca Blake, RD, director of clinical nutrition, Mount Sinai Beth Israel.

Oregon State University Extension Service: "Fiber - The Fabric of Heart Health."

University of Massachusetts Medical School: "What you can do to LOWER your TRIGLYCERIDES."


As a service to our readers, Harvard Health Publishing provides access to our library of archived content. Please note the date of last review or update on all articles. No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.


We would like to eat a more plant based diet but find the protein & carb issue confusing. My husband is diabetic so I first choose a protein (usually lean chicken, fish or beef). Then I choose low carb vegies and then some small amount of carb., if he eats too much carb his blood sugar goes up, especially if he hasn’t had protein. If he has eggs at breakfast he’s fine, if he has oatmeal with no protein his blood sugar goes up. If beans & grains are the protein on a plant based diet, they’re also high carbs. How can this work for a diabetic? Also, to me, the word ‘protein’ has always meant some kind of meat, cheese or eggs and the proper portions for a protein I understand. I have a hard time thinking of beans & grains as protein and how they are equivalent to meat, eggs & cheese. To me they are a carb and I worry that he won’t get enough protein. Any suggestions?

To help your husband control his blood glucose he can continue to consume small amounts of fish, chicken and eggs with his meals. Beans and grains are plant proteins, but do contain carbohydrate. Most animal based proteins contain more protein and only a very small amount of carbohydrate. Healthy eating patterns are not a one size fits all. Individuals should choose foods that work best for them and support decreasing their health risks.

Always good to examine various sides of an issue. Towards the beginning of this article it’s mentioned that our body’s cells prefer glucose for energy, which only comes from carbohydrates. The only animal product with any carbs is dairy. At some point in human evolution we must realize that adults have no need for breast milk after infancy,, especially the milk of another species. There are mountains of the most reliable scientific research that supports the superior health benefits of a whole-food, 100% plant-sourced diet. A good deal of this is found at the NutritionFacts website, which is a .org site. Another crucial issue which we cannot afford to ignore, if we want the human race to survive, is the devastating impact of all animal agriculture (including fowl and fish) on the environment and resource usage. Many reliable resources are easy to,find online, among them includes a site with information collected from various sources:

Think that it is not enough to eat healthy foods, but to add herbs and to check that the food that is eaten is as organic as possible, and that there are no unnecessary spraying or raise some of your food in your garden or on your balcony.
Recently I investigated the subject and came to an interesting book The Lost Book of Remedies, how to find them and how to grow on your own.

Dr. T Colin Campbell coined the term “whole food, plant based diet” and he did define it as meaning only plant based foods and no animal products so I think the author incorrectly defines the term “plant based” which is confusing. Plant based does mean vegan.

I have one question to ask you

Tomatoes are very easy to prepare. You don’t have to cook them, you can eat it as a salad , raw…..But easy things have adverse side effects. Does tomatoes cause kidney stones?

There is no research to support that tomatoes cause kidney stones – so enjoy them

Your dietary advise was very valuable. I am only a vegetarian ( I thought vegan and vegetarian meant the same) . I think people eat meat more for its taste than for its nutrition. Unfortunately taste and nutrition seem to be going in opposite directions. I became a diabetic before becoming a vegetarian .I also have a bit of cholesterol. I have been able to reduce same by frequently taking green tea, roasted peanuts and olive oil. I have the following questions to ask you?

1) Your article mentioned barley. Does barley give the same amount of carbohydrates /energy/nutrition as Oates does for breakfast? can it replace Oates at least once a weak? 2) Your article also mentioned feta cheese. Is this the same as cottage cheese for people with cholesterol? 3) Doctors now say coconut milk is okay for cholesterol. I now take tea and even Oates with coconut milk. It tastes relay good and my cholesterol level has not gone up due to it either. Please answer the above.

Barley and oats are both whole grains. The nutritional content is somewhat different. Comparing 2 oz. raw : barley is higher in total carbohydrate (44 g) vs. oats (38 g) higher in fiber (9 g) vs. (6g) , but lower in protein (6 g) vs. (10 g). Both are low in saturated fat, sodium and sugar. Barley can be used as a replacement for oats.
Comparing feta cheese with low fat (1%) cottage cheese: (

1/4 cup) feta cheese is significantly higher in saturated fat (5 g) vs. (1g) so when looking for heart healthy food choices to help reduce LDL cholesterol – low fat cottage cheese has the edge. However – feta cheese does not need to be avoided – it has a wonderful taste and can be used in moderation.
Coconut milk is high in saturated fat and therefore a low fat milk is a better choice to use on a daily basis. However, for special occasions – using coconut milk in recipes that traditionally call for its inclusion is fine.

Harvard’s healthy eating is the most reliable. I sincerely follow the suggestions for a better health. Apart from the Mediterranean diet
Could you please tell us about the Nordic diet as well. Thank you for your admirable service to the community

The Nordic diet is similar to the Mediterranean eating pattern in that it is plant based. The diet includes whole grains, berries and other fruits, vegetables, legumes and fish, especially fatty fish such as herring, salmon and mackerel. Both diets include some eggs and dairy, but limit red meat, processed foods and sweets. See blog below for more details.

Could you please comment, maybe in a future post, a bit about on the epidemiological meta-analyses that show how animal protein can really affect human health even in small amount? Great post. Thanks for sharing your expertise.

Regarding the first paragraph…
I thought beans were legumes. Your posting says….”legumes and beans.” .

Sorry for the confusion. Legumes are a class of vegetables that includes beans, peas and lentils.

Is this like the Zane method whitch I underdatand you approve?

This flexible, plant based approach to diet makes it easy to eat in restaurants and for family members that may not share your diet ideas to dine with you. I especially like the flexitarian diet. Above all, I personally avoid refined sugar and flour, but never explain to others why because either they will feel bad or not understand anyway. Instead, Just joke about it, like, never on Mondays.

I am surprised that you offer so many grilled menus, when grilling has been associated with cancer acceleration. You can eat plant based dishes without charcoal.

As a retired physician I can not determine which grains are whole grains because of labeling of breads etc. Multigrain, whole wheat, but other grains are listed etc. Help me.

In reviewing bread labels – look for breads that have “whole” as the first ingredient. I also suggest that consumers choose breads that have only a few ingredients and are without additives and sugar.

I’d caution readers to view the claims that olive oil is “heart healthy” with a bit of scepticism. Ask the question of “healthy compared to what”. That said, I’m really pleased that Harvard is showing that a plant-based diet leads to healthy long term outcomes. I’d add that if you want to lose weight fast, go vegan, exercise, and avoid sugary snacks. I lost 40 pounds in one year by ditching dairy products. Now 75 years old, I can hike any rugged trail I want. Before losing the excess weight, I found hiking mountains to be very challenging.

This is one of the most important and informative blogs I have received from Harvard. Thank you!

What Is Fiber & How Does It Work? Everything You Need to Know

We’ll venture to guess it’s been a while… or you never have. And there’s no shame in that. But it is important to note that dietary fiber is much more than just a crazy term or fad that can be ignored. Eating a fiber-rich diet is extremely important and absolutely imperative to keep bodily functions running smoothly.

Yet to most of us, fiber still remains a mystery. Here’s exactly what it is, how it works and why you need it.

What is fiber?

Fiber is the part of plant foods that your body can’t digest or absorb. It is also referred to as roughage or bulk.

“By classification, fiber is considered an indigestible carbohydrate,” explains board certified emergency physician and a Fellow of the American College of Emergency Physicians, Dr. Elaine Rancatore.

But fiber is very different from other carbohydrates &mdash and much lower in calories.

The benefits of fiber

Don’t let the term “indigestible” throw you off. Although fiber is not digestible, it is good for the body in many ways, says naturopathic physician Wendy Wells.

“Eating fiber increases the immune system in your gut, feeds the good probiotic bacteria there, keeps the digestive lining healthy and absorbs and pulls out excess hormones, cholesterol, fat and toxins from the body,” Wells says.

And pulling out those toxins is part of what make fiber so important.

“Many of the health problems we face start from a poor or sluggish digestion caused from built-up waste material in our colon,” says nutrition expert Siv Sjöholm. “Fiber helps us digest and pass the foods we eat. By increasing our fiber intake, we decrease the risks of common diseases such as cardiovascular disease, cancer, diabetes and obesity.”

But the benefits of fiber don’t stop there. Nutrition and wellness coach Michelle Pfennighaus outlined these added bonuses:

  • Fiber slows the absorption of sugar into your bloodstream to help maintain stable blood sugar levels and help prevent Type-2 diabetes.
  • Fiber lowers bad LDL cholesterol and promotes heart health.
  • Fiber helps keep you feeling full and satisfied and more in control of your appetite and weight.
  • Fiber keeps your digestive system healthy and regular, helping you to avoid constipation and the risk of diverticulitis.
  • Fiber reduces the risk of certain types of cancer, including colon and breast cancer.

The different types of fiber

There are two types of dietary fiber, and you need both.

Insoluble fiber adds the bulk needed to clean out the colon and regulate bowel movements. This fiber, or roughage, acts like a sponge. As it absorbs water, it swells inside your intestine and produces a feeling of fullness. The insoluble fiber moves through the digestive system to remove waste, toxins and materials your body doesn’t need.

Soluble fiber comes from fruit, some vegetables, brown rice, beans, barley peas, lentils, oats and bran. Soluble fiber mixes with water and digestive enzymes made by the liver to create a gel. This gel works chemically to prevent and reduce the body’s absorption of substances that may be harmful. It is soluble fiber that helps control blood sugar and reduces cholesterol.

How to eat more

To increase your fiber intake, try adding these foods to your diet:

A version of this article was originally published in April 2012.

The Health Benefits of Carbohydrates

So how important are carbs and why do we need carbohydrates in our diet? The National Library of Medicine (NLM) explains that carbohydrates play a role in glucose and insulin metabolism, as well as cholesterol and triglyceride metabolism and fermentation.

When carbohydrates are digested, they are broken down into glucose to be either used as energy or stored in the liver and muscles for future use.

Here are all the reasons why carbs are necessary.

1. More Energy

Part of the importance of carbs is that they are your body's preferred energy source. The Mayo Clinic explains that once the sugars and starches in carbohydrates are broken down and absorbed, they enter your bloodstream, which is then called blood glucose. This glucose in the blood stimulates the pancreas to secrete insulin. Insulin tells the body to either absorb the glucose to use as energy or to store it.

This process, as explained in the November 2014 issue of ​Advances in Nutrition​, is significant because this glucose is used as the main energy source for the brain, red blood cells and the central nervous system. Your body needs glucose to have the energy to do everything from breathing to weight training.

In addition, your brain needs glucose to function properly. If you don't take in enough carbohydrates, you can become weak, lethargic and unable to focus on even simple tasks.

2. Weight Control

Carbs are often blamed for weight gain, but the truth is that they are crucial for healthy weight control. You should eat 14 grams of fiber for every 1,000 calories that you consume each day, per the Academy of Nutrition and Dietetics. The only sources of fiber are foods rich in carbohydrates, so it's nearly impossible to get enough dietary fiber on a low-carb diet.

Fiber-rich foods add bulk to your diet, making you feel full more quickly and satisfying your appetite for longer. High-fiber foods are generally low in calories as well, so getting enough fiber can help you lose weight.

3. Heart Health

Dietary fiber prevents cholesterol from accumulating in your arteries and creating dangerous blockages that can lead to a heart attack or stroke, per the Academy of Nutrition and Dietetics. Eating whole-grain foods, such as fresh fruit, vegetables, whole wheat, oats, bran and quinoa, gives you valuable fiber that can protect your heart and keep you feeling your best.

Avoid simple carbohydrates, such as cakes, cookies, products made with white flour and processed foods, which are generally low in fiber and often high in fat and added sugar.

4. Improved Digestion

Getting enough fiber-rich carbohydrates can help prevent digestive problems, such as constipation and indigestion, per the Academy of Nutrition and Dietetics.

Insoluble fiber, the type of fiber that doesn't break down during digestion, is also known as roughage. It pushes other food along your digestive tract, speeding up the digestive process. It also adds bulk to your stool, making it easier to pass bowel movements. Without a sufficient intake of carbohydrates, you may not get enough fiber to keep your digestive system regular.

Five Ways Eating Starch Finally Helped Me Lose the Weight.

When I turned 30, something switched in my metabolism and I started gaining weight.

Before I realized it, I had gained 20 pounds in a matter of three years. I’m only 5’2″, and this growth spurt was not affecting my height, let me tell you.

Honestly, I had tried everything—from eating low carbs and high protein, to cutting out all alcoholic beverages, to eating gluten-free, to joining an accountability group online. I even joined a “Biggest Loser” contest at work.

Two weeks into the contest, I thought I was doing so well as I hadn’t eaten gluten or indulged in any wine in two weeks. Additionally, I’d been working out consistently. When I hopped on the scale, to my horror, not only had I not lost any weight, I’d gained a pound and one percent body fat.

About a month and a half ago, I began the vegan lifestyle for the third time in my life, but this time armed with the book, The Starch Solution by Dr. Douglas McDougall, M.D. My friend and health guru, Cheeks, sent it to me in the mail. My aha moment was when I realized it was time to cut out the excess oil and fats from my diet and start eating more starch. I was ecstatic—I love potatoes and rice.

At first, starch consumption seemed so contrary to popular belief and the science I had been reading—especially with regard to weight loss.

It seems that most people I knew who gave up consuming starch and carbs and ate a Paleo diet were losing so much weight.

Despite my doubts, I decided to give The Starch Solution a try. Hesitantly, I ate potatoes and bread: the arch enemies of the modern dieter. What I’m starting to realize now is that it’s not the starch that was adding the pounds, but the dairy, meat, and excess fats (with which starches are often paired).

Surprisingly, after only two weeks of adopting this new lifestyle and way of eating, I was weighed for the aforementioned contest at work, and found that I had lost seven pounds and three percent body fat.

Now, I’m starting to have more energy, and I’m noticing that the bloat that seemed to be perpetually present is starting to disappear. About a month and a half in, I’m down 12 pounds.

Dairy is designed to help baby cows become large adult cows.

According to Dr. Michael Klaper, dairy was never meant for human consumption. Antiquated science said the main source of calcium was dairy. However, this theory has since been proven false. Dairy may help bone density, but it actually increases the risk of fractures. The dairy industry is the one paying for these scientific studies. They sure have a lot of money vested in keeping their product on the market.

Quality, absorbable calcium is found in cruciferous vegetables, including broccoli, spinach, kale, and arugula, and, ironically, calcium supplements can actually do more harm than good.

Check out this video from the documentary, “Cowspiracy” with Dr. Michael Klaper, explaining the dangers of dairy and reasons to avoid it:

Butter and oil are not doing us any favors.

Whole food fats in the form of avocados, almonds, flax seeds, sunflower seeds, and the like are much better for our hearts and overall health at a cellular level. The Mediterranean diet has been touted as a heart-healthy way to eat. This is true in many cases because of the starches that these populations eat, like whole grains, pasta, fruits, and veggies.

Olive oil is also highly present in this culture’s diet, but these people appear to be healthy despite the oil they consume. They’re mainly consuming a plant based, meat-free diet.

“All large populations of trim, healthy people, throughout verifiable history have obtained the bulk of their calories from starch.”

The American Diet consists of 20 percent starch, 70 percent meat and dairy, 5 percent fruits, and 5 percent vegetables, while The Starch Solution consists of 70 percent starch, 0 percent meat and dairy, 10 percent fruits, and 20 percent vegetables. For more rapid weight loss, incorporate 45 percent starch, 0 percent meat and dairy, 10 percent fruits, and 4 percent vegetables.

The populations that have the least cases of obesity, heart disease, and cancers are in Asia and rural Africa where the bulk of their diets are based around starch, vegetables, and fruit. Because we have been so conditioned in the United States to look at the food pyramids—which have been funded by these animal agriculture groups—it may seem contrary to our everyday perception of what is healthy.

“When we switch from the Standard American Diet (S.A.D.) to a starch based diet we increase the fossil fuel availability by 40 fold.”

It is estimated that roughly 16 percent of global warming is is caused by animal agriculture.

This is huge, as transportation exhaust is responsible for only 13 percent of all greenhouse gas emissions. Unfortunately, a leading cause of human-related greenhouse gas emissions is caused by livestock production. Additionally, animal agriculture is also responsible for soil degradation and global deforestation.

Starches and vegetables contain loads of protein and essential amino acids. It is actually a myth that we need animal protein, which is perpetuated by the meat industry. The main source of protein for grazing animals is vegetation. Why can’t humans also go straight to the source to obtain our protein and amino acid needs?

There are better ways to get these necessary building blocks of nutrition in our diets, including eating organic, leafy green vegetables that are rich in absorbable plant proteins, such as amino acids, legumes, nuts, and organic soy. There are also starches that are rich in protein, including but not limited to, yams, brown rice, and sweet potatoes.

As a yogi, one of the yamas or ethical codes is to practice ahimsa or nonviolence to all living beings.

This makes it easier to stay away from meat and dairy products and start eating a starch based diet, especially when we have the right tools needed to succeed. Great cookbooks are vital for a vegan kitchen.

For breakfast, try this oil-free vegan lox and cashew cream cheese recipe that is quickly becoming a staple in my household. For dinner, experiment with this oil-free pesto. And for dessert, consider this vegan, oil-free chocolate cheesecake, which is so good, you’ll wonder if it was actually made using dairy products. Just substitute applesauce for the oil.

There are countless healthy ways to eat in this world. Each of us must find the path that most perfectly works for us.

Personally, I am most happy being a starchivore! Please pass the potatoes.


The vast majority of publications on food acceptability and behavior have considered middle- or high-income populations. However, there is research focused on low-income populations which deserves attention considering that many millions worldwide suffer undernutrition and/or food insecurity. The objective of this review is to highlight what the authors considered to be the most relevant research in the area to thus bring attention to this sensitive area which requires further research. Although there is a certain overlap, the review is classified in the following areas: fruits and vegetables, obesity, food choice, indigenous populations, development of specific food products and, finally, what we consider to be the most promising or necessary research in the field of food choice in low-income populations.

Reader Interactions


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