Complex Carbohydrates

Complex Carbohydrates

In one of our articles on this website (“Plants in Nutrition”), we tried to explain in simpler terms why plants are so fascinating and beneficial for human health.

Although the article was quite lengthy, we barely scratched the surface.

So, we have continued to do so in this article on complex carbohydrates, because they seem very important for our health.


Because there is paramount scientific evidence of the efficiency of plant-based food rich in complex carbohydrates on various digestive disorders, such as constipation, hemorrhoids, colitis, bowel polyps, and colorectal cancer.

Complex carbs don’t mean refined carbs

The term carbohydrates, people usually associate with highly refined products, such as white flour, white sugar, pastries, sweets, white bread, etc. Such refined carbs are completely stripped off many essential nutrients, making it totally unhealthy for regular consumption.

Simple(r) carbs are, in fact, simple sugars. Soon after eating simple sugars, what most people do on a daily basis, glucose levels in the bloodstream rise, followed by insulin spike, which helps glucose enter our cells (to be used as energy).

Overtime, indulging in such food may lead to insulin resistance and cause type 2 diabetes.

However, the term complex carbohydrates are used to describe hundreds of other highly beneficial substances that are an integral part of “natural”, plant foods, rich in nutrients essential for our health.

These chemicals, such as starch, and cellulose (fiber) as well as vitamins, minerals, proteins, fats, and other highly beneficial bioactive compounds are present exclusively in whole foods – fruits and vegetables.

A diet that contains this type of carbs, which are derived from fresh fruits and vegetables as well as whole grains, is the healthiest possible diet to eat and is perfectly in tune with what our body really needs to function properly.

Complex carbs: digestible & indigestible

Complex carbohydrates, such as whole grains, vegetables, legumes, beans, and fruits usually contain a combination of different types of fiber, in different proportions.

Most plant foods contain both indigestible (insoluble) and digestible (soluble) fiber.

However, we cannot completely digest any dietary fiber (from complex carbs), because our body does not produce the enzyme cellulase. Nonetheless, fibers are of the utmost importance for our health.

There are several reasons for that.

As we said earlier, there are numerous researches on fiber intake and digestive cancers that showed a lower risk for colorectal cancer for individuals who regularly consume foods rich in fiber.

And remember, no fiber in animal foods.

In terms of reducing obesity, the density of fiber foods, due to its water-binding capacity, increases the sense of fullness and satiety, thus suppressing the appetite.

The increased volume of intestinal contents stimulates the bowel movements on its way down through the intestine pushing the waste out. That is the reason why foods that contain an insoluble type of dietary fiber can be an excellent natural remedy for constipation; it prevents it and relives it.

Food rich in insoluble fibers, that is, cellulose, lignin, and wheat bran include cereal products, whole grains, brown rice, spelt, rye, and root vegetables – potatoes, carrots, lentils, beans.

Gut microbes and dietary fiber – what science says about the potential healing effect

So, what we have learned so far is that complex carbohydrates are naturally present in plants in the form of starch and fiber. And in plants only. No fibers and starches in animal food.

Accordingly to digestibility, there are 3 types of starches: slowly digestible starch, rapidly digestible starch and resistant starch.

By exposing uncooked, slowly digestible starch grains (wheat, rice, corn, etc.) to thermal treatment, they gelatinize and convert to rapidly digestible starch, that is, become available for the enzymes to degrade them.

Dietary fibers are fractions of the total starch that are not digestible or only partially digestible in the colon. This type of starch is called resistant starch.

As humans are unable to digest most complex carbs (e.g., cellulose), simply because they do not have enzymes, they need a ‘helping hand’ of billions of different microbial communities within the gut to complete the job.

During that process, a range of beneficial metabolites is produced, both for host and for bacteria. In other words, dietary fiber ferments to short-chain-fatty-acids (SCFAs), butyrate, propionate, ferulic acid, and others, providing the energy for use in the cells.

Dietary digestibility of starches is of great importance for our health.  Unlike slowly digestible, rapidly digestible starch in plants has the ability to quickly spike glucose levels in the blood and, consequently, the insulin response. Furthermore, a diet rich in rapidly digestible starch and low in fiber is linked to type 2 diabetes.

To preserve the amount of slowly digestible starch in plants, and prevent their conversion into rapidly digestible starches, it is necessary to control the cooking temperature.

That’s why it is important to follow suggestions of total cooking temperature for pasta, cereals, rice, and so on.

Again, there is a clear link between slowly digestible starch and their beneficial potential for our health.

Prolonged digestion is important due to allowing for a slower and more steady release of glucose in the blood; it contributes to the feeling of satiety and fulness.

Moreover, obesity is linked with lower bacteria species necessary for fiber digestion. Increased intake of food rich in fiber may have a beneficial effect on the expansion and diversity of fiber-metabolizing microbes, and therefore, the production of “good” metabolites.

Gut microbiota species and our immune response

The human gut contains billions of diverse microbes that play an important role in our systemic health, performing not only digestive functions but also supporting our immune response.

Simply put, our body is home to trillions of bacteria, helping us to digest food and maintaining a balance between them in order to preserve host health. And a diet rich in fiber, that is, whole plant-based foods leads to increased microbial diversity. In fact, the composition of our gut bacteria reflects our long-term dietary habits, besides other important factors such as age, and genetic predispositions.

Again, a diet rich in fiber stimulates the formation of diverse bacterial communities that release key metabolites during this process, necessary for digestion, immunity, and proper cognitive functioning. As we said earlier, the short-chain-fatty acids are one of the most important organic metabolites that affect not only digestion but also cell signaling and proliferation.

A completely different story tells us diet rich in animal proteins and fats that lead to the increase in gut microbial species and production of metabolites that can be detrimental in the long run, such as N-nitroso compounds, amines, ammonia, phenolic, and other carcinogenic and pro-inflammatory chemicals.

But, dietary fibers and starches can even do more – they can neutralize these harmful consequences of metabolism of animal-based foods. (If you failed as a vegetarian, at least don’t forget to add vegetables to your burgers).

Furthermore, a plant-based diet rich in fiber, together with the fiber-metabolizing microbiota play a key role in the production and secretion of the stomach mucus, which is an impervious layer to many harmful pathogens and prevents infections and inflammatory diseases.

Fiber and gallbladder disease

As we know, bile acids are necessary for lipid and carbohydrate metabolism, intestinal transport and absorption and its mucus layer, and many more.

Foods rich in fiber play an important role in regulatory functions of bile acids and prevention of their accumulation in the liver, that occurs in certain gallbladder conditions such as cholelithiasis.

Some primary bile acids are highly toxic and some bacterial enzymes (the same that bind and metabolize dietary fiber) in the intestine are necessary for their conversion into secondary bile acids, which are beneficial for our health. That further indicates that dietary fiber may serve as a mediator that regulates the level of bile acids and prevents their accumulation in the colon. Deregulation of bile acids in the colon is associated with polyps and colorectal cancer.

However, more research is necessary to understand these complex interactions, to be able to make general recommendations regarding the amount of dietary fiber intake (considering an individual response and overall health status of the person).

The human gut contains billions of diverse microbes that play an important role in our systemic health, performing not only digestive functions but also supporting our immune response.


Maintaining balance within the gut microbial communities means a stable internal environment of the organism, so-called ‘homeostasis.’

Digestible fiber or soluble fiber can be found in skins of many fruits and vegetables, oatmeal, flaxseed, nuts, beans, blueberries, apples, and more.

This type of fiber dissolves in water (within food), and it is partially digested by the enzymes in the larger intestine forming a gel-like mass and making the stool softer and bulkier. Soluble fiber has clinical relevance for a variety of conditions such as diabetes and hypercholesterolemia (high blood cholesterol), to name a few.

Additionally, soluble fibers bind bile acids, a simple sugar, and cholesterol in the intestines, and slow down their absorption into the blood. The dietary fiber also suppresses the activity of some digestive enzymes, which results in more food being pushed out via stool, which may contribute to weight reduction.

Furthermore, the foods with this type of fiber can dilute and binds some carcinogenic chemicals, and act as significant health promoters.


Cereal products are made from annual crops, whole grains are used in nutrition: maize, rice, wheat, corn, oat, rye, bran, barley, often in combination with dried fruit.

This type of food is advertised as an excellent source of energy, vitamins, and minerals, as well as dietary fiber.

Sometimes true, but not always true.

To compensate for the loss of vitamins and other important nutrients during food processing, producers often “fortify” the food with too much fat (to make it crispy and to control texture), chocolate, a simple sugar, and countless other additives.

It is clear that such foods certainly can’t benefit our health.

It delivers energy rapidly due to the high glycemic index,  and it is not different from other highly processed foods.

The worst breakfast cereals in terms of nutritional value (low fiber, high in fat and sugar), include chocolate-filled corn flakes, muesli with added sugar, sugar-frosted cornflakes, granola with chocolate, and many more.

Good, wholegrain cereals, no sugar added

Unlike these unhealthy varieties that mostly come in attractive shapes and texture, there are also much healthier breakfast cereals.

We just need to carefully read the nutritional label in terms of serving size, total calorie and look for cereals without chocolate, sugars, and fats.

Even better if we can make our own combination using fresh fruit and oatmeal, for example.

This type of cereals may seem not so appealing at first glance. But, as we all know, the first impression could be misleading.

To sum it up by saying that plant-based food is what we need on our menu every single day, not just “sometimes” is not an exaggeration.

Wholegrain cereals, with NO added sugars, fats, and additives, that is exactly what we should be eating if we care for a healthy gut.


By Vera Bonderovic, MSc Medical Biochemistry edited:


    • Holscher HD. Dietary fiber and prebiotics and the gastrointestinal microbiota. Gut Microbes. 2017;8(2):172-184. doi:10.1080/19490976.2017.1290756
    • O’Keefe SJ, Chung D, Mahmoud N, et al. Why do African Americans get more colon cancer than Native Africans?. J Nutr. 2007;137(1 Suppl):175S-182S. doi:10.1093/jn/137.1.175S
    • Naumann, S.; Schweiggert-Weisz, U.; Eglmeier, J.; Haller, D.; Eisner, P. In Vitro Interactions of Dietary Fibre Enriched Food Ingredients with Primary and Secondary Bile Acids. Nutrients201911, 1424
    • Hannah D. Holscher (2017) Dietary fiber and prebiotics and the gastrointestinal microbiota, Gut Microbes, 8:2, 172-184, DOI: 1080/19490976.2017.1290756
    • Kassem Makki, Edward C. Deehan, Jens Walter, Fredrik Bäckhed, The Impact of Dietary Fiber on Gut Microbiota in Host Health and Disease, Cell Host & Microbe, Volume 23, Issue 6, 2018, Pages 705-715, ISSN 1931-3128.


Add your comment or reply. Your email address will not be published. Required fields are marked *