In recent times, there has been a public outcry for kidney support. In some studies, Tannor et al.(2019) explained that limiting the use of herbal preparations may mitigate chronic kidney in developing countries. 

However, I am yet to read one empirical study that revealed that the recent incidence of kidney diseases is triggered by herbal medicines. There is a need to research our local diets and their possible impacts on our health.

This is because, in recent times, emerging studies are proving that a diet rich in resistant starch could help improve our kidneys and neglect them affecting our kidneys. Recently, Rhonda Witwer, the founder of resistantstarch.com educated me more on resistant starch diets and their impact on our health due to one of the articles she read on resistant starch diets in our local foods.

Hence, in this article, I explore more studies on kidney health and resistant starch diets.

The Kidneys

The truth is that most people are not focused on their kidneys.  From a consumer wellness perspective, kidney health isn’t a “thing” at all.  It probably should be, given the rapid growth of kidney disease in developing countries.

The CDC 2017 Fact Sheet contains many helpful American statistics.

The kidneys serve as a filter for the bloodstream.  If the kidneys are not working well, harmful metabolites begin to accumulate in circulation in the blood. Dialysis can be used to filter and remove many of these compounds. Some compounds are particularly problematic because they cannot be removed by dialysis as they are bound to albumin and are very large.

Gut Health and its Impact on the Kidneys

Two protein metabolites in particular are of particular concern in kidney health: indoxyl sulfate (IS) and p-cresyl sulfate. Both of these compounds are produced in the gut and are broken down from amino acids in proteins (indoxyl sulfate is a tryptophan derivative and p-cresyl sulfate is a derivate of tyrosine and phenylalanine).

One study by Lau & Vaziri (2017) established that the intestinal microbiota is the nucleus of this systemic inflammation and oxidative stress characteristic of chronic kidney disease.

However, the gut microbiome differs in those with CKD compared to healthy adults. It is not clear, however, whether the kidneys or the intestinal tract causes or contributes to the inflammation and oxidative stress.

However, what we do know according to a study by Lau(2018), urea infiltrates the gut, increasing the production of ammonia and ammonium hydroxide, which causes damage to the intestinal gut barrier and promotes intestinal wall inflammation.

 Others explained that the protein fermentation in the gut could start the process instead. In any case, this ‘gut-kidney axis’ is an area of active research.

Resistant starch and the kidneys

Resistant starch could impact the kidneys in three major ways thus helping to maintain their health:

(1) Resistant starch reduces toxic nitrogen-containing compounds in the blood plasma produced in the gut, notably indoxyl sulfate.

(2) Resistant starch reduces inflammation and oxidative stress.

(3) Resistant starch impacts Vitamin D metabolism.

Explanation

• Resistant starch reduces plasma toxins by increasing fecal bulk.

An old study by Cummings BJN (1996) found that every 1 gram of resistant starch consumed, provides about 1.6-2.6 grams of extra fecal mass created, depending on the source and structure of the specific resistant starch. However, we know from studies, a diet rich in green bananas and plantain can deliver as high as over 80% of resistant starch diet.  This means that such diets are good to maintain our kidney health.

Because resistant starch diets resist digestion and go straight to the colon and ferment to generate more good bacteria;

Another study by  Birkett et al.(1996) explained this bacterial growth consumes nitrogen and is a “healthy” way to dispose of protein.  The study further established that resistant starch reduced intestinal ammonia, phenol, cresol, and total phenols.

Another animal study by Le Leu (2007) found that resistant starch reduces the protein fermentation by-products in the gut. This means the fecal bulk removes protein breakdown and nitrogen-based compounds from the body. Thus, the nitrogen-containing compounds are not absorbed into the body and do not have to be disposed of through the kidneys.

One of these nitrogen-containing compounds, Indoxyl sulfate, is a uremic toxin that induces inflammation and leukocyte activation.  It is associated with increased mortality in CKD patients. Two clinical trials confirmed that resistant starch reduces indoxyl sulfate and other compounds in the blood plasma of people with chronic kidney disease.

The first study was conducted by Dr. Timothy Meyer and his colleagues at Stanford University showed that 18 grams of resistant starch/day reduced indoxyl sulfate by 27% in 40 patients on hemodialysis in California, USA.  This study also showed a trend of a 24% reduction of p-cresol sulfate, reduced plasma urea, nitrogen, and plasma album, but these were not statistically significant(Sirich 2014).

The second study was conducted by Dr. Denise Mafra and her colleagues at the Universidade Federal Fluminense in Rio de Janeiro, Brazil found that 16 grams of resistant starch/day reduced indoxyl sulfate levels in 31 chronic kidney disease patients undergoing hemodialysis in Brazil (Esgalhado 2018). This study also found reduced plasma levels of thiobarbituric acid reactive substances (TBARS) and IL-6 and a trend to reduced protein carbonyl levels.  There was no difference in plasma levels of p-cresyl sulfate. Animal studies have also shown similar benefits (Chen BBB 2016, Kieffer AJPRP 2016).

• Resistant starch reduces inflammation.  

Dr. Denise Mafra’s clinical trial in Brazil established that reduced inflammation biomarkers, interleukin-6 (IL-6) and high-sensitive C-reactive protein (hs-CRP), and oxidative stress markers thiobarbituric acid reactive substances (TBARS). In addition, there was a trend to reduced protein carbonyl (p-0.06)(Esgalhado 2018)

Another clinical study also found reduced inflammation and oxidative stress biomarkers in hemodialysis patients.  Dr. Vaziri of UC Medical School in Irvine, California, and his collaborators at the Tabriz University of Medical Sciences in Iran fed 46 hemodialysis patients 25 grams of resistant starch/day for 8 weeks and showed reduced TNF-a, IL-6, and malondialdehyde.  Serum urea and creatinine concentrations also significantly declined (Tayebi Khosroshahi 2018). There was no significant difference in hs-CRP, serum interleukin-1b, and total antioxidant activity.  These participants also reported improvements in constipation.

A new animal study by Martin Kriegel and his colleagues at Yale University demonstrated that resistant starch improved the gut barrier and reduced the translocation of bacteria in mouse models of lupus. (Zegarra-Ruiz CHM 2018).  While the focus of this study is not kidney-related, it demonstrated that resistant starch restored the gut barrier. Loss of integrity of the gut barrier is a major contributor to systemic inflammation.  This mechanism could also be contributing to reducing inflammation and oxidative stress seen in clinical trials.

Dr. John Arthur and Dr. Boris Zybailov at the University of Arkansas are also working on the impact of resistant starch on kidney health. They also found that resistant starch produces massive changes in proteins as well as the entire gut microbiome in animal models. They concluded that resistant starch shifted the microbiome from mucin degraders to butyrate producers (Zybaylov 2018). They found that the increased butyrate led to enhancements of oxidative stress and inflammation as well as other biological processes. This team has also initiated an NIH-funded clinical trial with resistant starch in children and adults with stage 3 chronic kidney disease (Clinical Trials.gov Identifier NCT03356990).

Dr. Vaziri has published additional animal studies investigating various mechanisms related to resistant starch and kidney function(Kieffer 2016, Vaziri 2014)

• Resistant starch attenuated the urinary loss of Vitamin D metabolites in animal models (Koh 2016, Smazal 2013, Koh 2014). 

The kidneys activate 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol.  Though this research is still early, it adds to the body of evidence demonstrating positive kidney health benefits following the dietary consumption of resistant starch.

There is now sufficient clinical evidence to conclude that dietary consumption of resistant starch helps maintain healthy kidneys and healthy kidney function.

Additional information is available in some excellent review articles: Lau & Vaziri Clinical Science 2018,  Moraes & Mafra European Journal of Nutrition 2016

Take Home

Before processed foods, our forefathers daily ate 30-50 grams of resistant starch every day - in intact whole grains, beans, under-ripe bananas and plantain, leftover fufu, and raw starchy foods such as cassava related, cocoyam, maize diets, yams et al.  This means they were eating good starchy diets then. In the modern era, we turn away from these good resistant starch diets thinking they were bad for our health rather.

In the modern era, people eating a "modern" diet get only 2 to 5 grams of resistant starch a day. Loss of this critical nutrient is contributing to the chronic health problems suffered by billions of people around the world according to studies on resistant starch.

What you have to know is that any food that contains resistant starch simply means: starch that resists digestion in the small intestine. It reaches the large intestine and feeds the good bacteria that live in our guts. It turns out that this gut fermentation is really important to our metabolism. Scientific studies are showing that we need to return to the historically consumed slow-to-digest resistant starch and eat far less easy-to-digest, soft, highly refined carbohydrates. Studies have demonstrated positive kidney health benefits following the dietary consumption of resistant starch.

Though unripe bananas and plantain are rich sources of resistant starch diets, people with kidney disease should limit them due to their high potassium content. Doctors recommend less than 2,000 gm of potassium per day.

NB: Prof. Nyarkotey has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical associations to justify his write-ups. My articles are for educational purposes and do not serve as Medical advice for Treatment. I aim to educate the public about evidence-based scientific Naturopathic Therapies.

The author is a Professor of Naturopathic Healthcare and President of Nyarkotey College of Holistic Medicine & Technology (NUCHMT)/African Naturopathic Foundation. E-mail: collegeofholisticmedicine@gmail.com.