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Why I Avoid Foods Rich in Resistant Starch: The Evolutionary Rationale for Less Nutritional Resistance

http://darwinian-medicine.com/why-i-avoid-foods-rich-in-resistant-starch-the-evolutionary-rationale-for-less-nutritional-resistance/

If you’re someone who’s been involved in the evolutionary nutrition community for some time, then you’re undoubtedly familiar with Resistant Starch (RS), which is a special type of starch that we humans are incapable of digesting without the help of the bacteria that dwell in our bowels. RS rose to fame some years back, spurred on by bloggers and health/fitness writers who argued that it was equivalent to a hidden jewel that the nutritional community, and in particular evolutionary eaters, had failed to detect, unearth, and exploit. Some put this exciting jewel up on a pedestal, and claimed that it could bring a whole new dimension to nutrition.

Like many others, I was drawn to the allure of the jewel and the remarkable properties it was said to possess. However, over time, after I’d experimented with RS for some time and organically entered onto a line of reasoning that was independent of the one I’d seen presented by RS-proponents, I came to realise that the recently unearthed stone was no real jewel, but rather a fairly unspecial aggregate of minerals presented as a gem…

My history with resistant starch
Back in the day when I was experimenting with RS, I used to eat slightly green/unripe bananas on a frequent basis. For a period of time, I also consumed cold potatoes, which had been cooked and then refrigerated for many hours, as well as some cold rice and raw potato starch, which, like incompletely ripened bananas, are rich in RS.

The reason why this experiment was short-lived, as opposed to something that turned into a habitual practise of mine, is simple: It didn’t make me feel well. Initially, I felt that the addition of RS to my diet gave me a boost, health and energy wise, and I also felt that it benefitted my gut health; however, this situation not only quickly subsided, but actually mutated into something hostile. It got to a point where whenever I took in a lot of one or more of the aforementioned foods, I became moody/sad and tired.

The effects, which were quite pronounced, were certainly not due to “incomplete accommodation” to the new diet, as I consumed RS for quite some time and didn’t really experience any gut issues. Furthermore, they were clearly related to RS, as they never presented themselves when I ate ripe bananas or warm, newly cooked rice or potatoes, and kicked in roughly following the hours it took for the starch molecules to reach my lower gut, where they were subjected to microbial degradation.

In the time that has passed since then, I’ve occasionally tried eating some RS rich foods again, just to check and see if I still react the same way. I do. I’ve come to realise that I to some extent got caught up in the hype surrounding RS back in the day. I didn’t jump in with both feet or start loudly promoting the consumption of RS; however, I was certainly intrigued by what it was said to offer. It took some unpleasant physiological reactions, which spurred some rational, skeptical, and independent thinking, to get me out of the haze.

Given what I now know about the evolution of the human diet and biology, I don’t find it surprising that I reacted so negatively to the inclusion of a lot of RS in my diet…

Is it really true that we humans have a long history of eating foods rich in resistant starch behind us, and that we’re therefore well adapted to consuming such foods?
One of the statements I repeatedly came across online back in the day when the RS mania was at its most intense centers on the role that RS played in ancestral human diets. Many of the people who encouraged the consumption of RS-rich foods argued that RS had been an important part of the human diet for many millions of years, and that our intake of this nutritional substance has recently plummeted. Implicit in these notions is the belief that we’re not only biologically suited, as a result of evolutionary, adaptive processes, to take in quite a bit of RS, but that we need to do so in order to fulfill our nutritional/microbial requirements.

I never saw any real, hard-hitting evidence presented in support of this argument. Mostly, people were just speculating and jumping to conclusions based on theories they’d formed regarding what our ancestors ate. At the time, I didn’t immediately perceive those ideas as being invalid; however, over time, as I started thinking more about the issue at hand, I came to realise that they were unsubstantiated and flawed.

It’s really somewhat surprising that the idea that RS made up a large part of ancestral human diets has been allowed to perpetuate, considering that one can quickly deduce, from putting together what we know about the technological equipment that ancient humans utilized and what they ate, as well as how we are designed, that our primal forebears generally wouldn’t have taken in a lot of RS…

4 reasons why I’m skeptical of the idea that we’re biologically suited to take in a lot of resistant starch
1. The refrigerator only came into play very recently
Our hunter-gatherer forebears mostly lived in warm or temperate areas of the world and ate mostly fresh food. They obviously didn’t eat starchy foods that had been cooked and then refrigerated for many hours, seeing as they didn’t have access to any form of refrigeration unit. At night-time, temperatures tend to drop, irrespective of where one lives; however, such natural temperature fluctuations obviously have little in common with the way we tend to set and regulate the temperature of modern refrigerators.

Generally, African hunter-gatherers such as the Hadza, which are particularly relevant to discussions about the human diet, as they live in a part of the world where humans are believed to have originated, eat primarily fresh food, often roasting the tubers they’ve been known to consume over a fire before eating them. In cold parts of the world, it’s more feasible to store food for a longer period of time; however, in such areas, animal source foods, as opposed to plant foods, tend to be the primary component of hunter-gatherer diets.

Basically, given what we know about the nutritional practices of our ancestors, it seems highly implausible that RS3 – the type of resistant starch that’s found in starch-containing foods that have been cooked and cooled – made up a large part of the natural diets of our ancient ancestors.

2. Cereal grains – the primary source of resistant starch in the modern human diet – only became a significant part of our diets following the Agricultural Revolution
The typical modern man derives most of the resistant starch he takes in from grains – a source of RS1. Seeing as it wasn’t until the Agricultural Revolution started sweeping the globe some 12.000 years ago that grains, as well as various types of legumes (e.g., beans), became a major part of humans’ dietary repertoire, it goes without saying that our primal forebears didn’t take in significant quantities of RS in the form of those foods.

This is obviously relevant to discussions about human health and nutrition, as a compelling body of evidence indicates that post-agricultural diet-induced selection pressures haven’t been intense enough to reconfigure the human biology so that it matches well with the nutritional properties of a grain-heavy diet.

3. Foods rich in resistant starch, such as green bananas and plantains, are unpalatable and difficult to chew
One can learn a lot about what types of foods that we’re designed to crave and eat, as well as the foodstuffs we would do best to avoid, simply from paying attention to one’s dietary experiences. There’s a reason why we find ripe, sweet, and big Granny Smith apples attractive and tasty and the fast food industry has made good business selling foods high in sugar and fat. That reason is rooted in our evolutionary past, in the sense that our ancestors would have benefitted, in a Darwinian sense, from seeking out and eating ripe fruit, honey, marrow, and other calorie-dense foods. The problem we have today is that we’re now surrounded by sugary and/or fatty foods, some of which are many times more potent than the ones our primal forebears had available to them.

Conversely, we tend to find things we’ve historically not eaten or benefitted from staying away from off-putting in some way. Such items may for example have a weird smell, taste bitter, or be hard to chew. This is relevant to the discussion about RS, as foods that contain unusually high levels of RS, such as green bananas and cold potatoes, don’t appeal to most people’s tastes. Not only is the edible portion of the former difficult to access and chew, but it has a dry, unstimulating taste to it. Personally, I find that it doesn’t feel right to eat it. As for cold potatoes, I don’t think anyone would say that they taste fantastic. They’re arguably insipid.

I’m certainly not the only one who feels this way, as evidenced by the fact that people have been trying to device clever ways of getting such foods into their bodies in such a manner that their experience of the taste and feel of the food is minimized. For example, online, you’ll find recipes for plantain-smoothies largely manufactured for the purpose of tapping into the purported health benefits of RS.

Given that our nutritional proclivities were shaped over evolutionary time in accordance with the principles of Darwinian selection, the fact that foods that are very high in RS have several characteristics that make them unpalatable and difficult to eat should cause us to think twice before we include such foods in our dietary repertoire, as it strongly suggests that they don’t agree with our bodies.

4. The notion that starchy foods made up a large part of preagricultural human diets is highly speculative
The general belief within the evolutionary health community is that starch didn’t make up a large part of preagricultural human diets. However, recently, some researchers have questioned that belief (1), and the role of carbohydrate in human evolution has sporadically received attention by the media, largely as a result of recent scattered findings of starchy human-associated remains (2, 3). It’s definitely good that research is being done and that there’s a discussion in this area. Also, one shouldn’t forget that many contemporary African hunter-gatherers take in appreciable amounts of starchy foods.

With that being said, it’s important to recognize that the “radical” notion that Paleolithic humans took in a lot of starchy foods is highly speculative, and goes against what the majority of evolutionary nutritionists and researchers hold to be true based on the available scientific data. Personally, I question the veracity of the claim, for a number of reasons, one of which has to do with the oral health effects of starch consumption. Furthermore, it’s important to recognize that before our ancestors understood how to make and control fire; they were not able to break down starchy roots and tubers through the application of heat.

Finally, and most importantly, even if starchy plant foods made up an appreciable part of our primal forebears’ diets, that doesn’t necessarily mean that it was common to take in a lot of resistant starch, seeing as heating/cooking, which is required to break down starchy foods, reduces the RS content (4, 5). Cooked potatoes, for examples, are quite low in RS.

I haven’t been able to find any good data on the RS intake of modern African hunter-gatherers who eat appreciable amounts of starchy foods. I find it unlikely that it’s particularly high, in part because RS has some abrasive characteristics (The Hadza typically spit out the rough, insoluble fibers of the tubers they eat, but not before chewing on it for some time (6)), but I’m certainly open to being challenged on this point.

The bottom line: The ancestral human diets that contributed to shaping the human physiology over millions of years of evolution appear to have been quite low in resistant starch
There’s no doubt that the diets of our naturally living ancestors were tough and fibrous; however, based on the aforementioned discussion, it appears highly implausible that they were rich in RS. This is not to say that our primal forebears didn’t take in any RS at all. They undoubtedly did, primarily through the consumption of certain types of seeds, legumes, and underground storage organs. However, they didn’t consume foods that are supremely rich in RS.

This is something you’ll quickly come to recognize if you go on Google and look up lists of foods high in this special type of starch. One of the things you’re bound to notice if you view the lists under evolutionary light is that they are dominated by foods and food products that didn’t become a part of our species’ dietary repertoire until very recently in our evolutionary history. Processed products rich in potato starch, banana starch, and the like are obviously newcomers, but so are most of the starchy whole foods that you’ll find on such lists, albeit to a lesser extent.

healthy-foods-bowl
Contemporary diets created on the basis of Paleolithic nutrition principles come out as being high in fiber, albeit fairly low in RS, unless a lot of unripe bananas are consumed that is. Is this lack of RS really a bad thing…?
In contrast, the dominant foods of hunter-gatherer diets (meat, seafood, eggs, nuts, ripe fruit, and vegetables) are generally low in RS, at least when compared to unripe bananas, cooked and cooled rice, etc. Given that this is the case, it’s not surprising that research has shown that people who consume a diet based on Paleolithic nutrition principles take in less resistant starch than people who adhere to a diet created on the basis of government-supported dietary guidelines (7).

Perhaps needless to say, the Paleolithic diets that are prescribed in modern experimental trials don’t necessarily correspond 100% to the original human diet in terms of their nutritional composition. It’s reasonable to assume that they’re generally fairly well matched though.

Why does this matter?
It matters because our nutritional requirements and tolerance were shaped over evolutionary time via natural selection. Foods that have only recently been introduced into our dietary repertoire and that have a nutrient configuration that differs markedly from that of the foods we’re evolutionarily accustomed to eating are liable to provoke problems. Modern foods and food products that are extremely rich in RS certainly fit this bill. One would expect them to be problematic, capable of producing gut microbial communities that don’t agree with our human selves.

My main concern is that an intake of resistant starch at a level that’s many times higher than the evolutionarily normal intake level for our species may lead to “excessive” growth of a particular set of colonic bacteria and immoderate generation of certain microbe-produced compounds, in particular butyrate and other Short-Chain Fatty Acids (SCFAs), which may enter and have widespread effects on the host.

scfas-metabolic-regulation
SCFAs produced via colonic fermentation have far-reaching physiological effects, as illustrated by this figure derived from a paper by Mayu Kasubuchi et al., which depicts the effects of SCFAs on host metabolic functions. SCFAs are important; however, excessive production, as may result from a high intake of resistant starch, may be detrimental.
SCFAs produced through colonic fermentation are generally considered to be beneficial to host health. There’s no doubt that compounds such as propionate and butyrate are important in the context of human physiological functioning and health; however, as always, more isn’t necessarily better. Just like it’s not a good thing to take in “unrestrained” amounts of vitamin C or fermentable inulin (two compounds that are generally considered to be beneficial), it’s not necessarily advantageous to bring so much resistant starch into one’s body that one’s colonic bacteria get to have a big acid-generating feast.

Seeing as the scientific community has devoted little to no attention to the potential dangers of resistant starch, it’s impossible to a mount a compelling body of data derived from experimental studies in support of these statements; however, they certainly have a theoretical and evolutionary rationale behind them. Furthermore, several studies support the notion that more isn’t necessarily better when it comes to SCFAs (8, 9, 10).

What about the studies that seem to show that resistant starch is good for us?
I know perfectly well that a number of studies that have come out over the most recent decades give the impression that RS is good for us. At least that’s what they appear to show upon first inspection. What’s important to recognize though is that most of the studies in this area by no means conclusively show that a high intake of RS favorably affects health. Rather, the available data largely pertain to the effects of RS on gut microbiota composition. Of note, much of the findings come from animal studies and in vitro experiments.

I very much question the practise of drawing inferences regarding the prudency of a nutritional practise based on an assessment of how that practise affects gut microbiota composition, for the simple reason that human-microbe interactions are convoluted and intricate. Not only are our gut microbiotas highly complex, but there’s great inter-individual compositional variation. One can certainly speculate that a particular diet-induced compositional shift is beneficial, but if no additional analyses pertaining to the health outcomes of the shift are carried out, I would be very resistant to make health inferences based on the findings. This is particularly true if the research in question was carried out in animal models.

With that being said, the findings of recent randomized trials suggest that certain RS preparations may benefit some aspects of health, at least in a subset of people (e.g., 11, 12, 13, 14). I have no intention of trying to pick apart, discredit, or invalidate each and every one of these studies; however, I do have some general thoughts pertaining to them. First of all, I ask myself whether similar results would have been achieved in populations of people eating a healthy, fiber rich diet. Secondly, I ask myself how much of an impact (if any) the companies that produce the RS products that have been employed in some trials have had over the design and outcome of the studies. Thirdly, I find the general lack of focus on the potential downsides of RS consumption concerning. Virtually all of the focus is on the potential benefits of consuming RS. Hence, one shouldn’t necessarily expect the available literature to reflect the true realities of the whole RS thing.

In the end, you’ll have to decide for yourself what to take from the studies and make up your own mind about what to believe. Personally, I tend to let evolutionary/biological insights guide me, but if that’s not your cup of tea, then you may reach different conclusions than I do.

A brief summary, as well as some concluding remarks
Resistant starch gained widespread attention from evolutionary eaters some years back, propelled into the spotlight by people who were highly enthusiastic about the great things that they believed this compound had to offer humanity. Like many others, I was sufficiently intrigued to start dabbling with the whole RS thing. Over time, however, I not only realised that RS didn’t live up to the hype, but I became increasingly skeptical of this nutritional substrate and eventually made it a policy of mine to avoid unripe bananas, potatoes that had been cooked and then cooled, isolated potato starch, and other foods that are unusually high in RS, in large part because I noticed that such foods adversely affected my mood, mental health, and energy levels, and also because I realised that the idea that it’s good to take in a lot of RS lacks evolutionary support.

In the end, each person will have to test and see what works best for them and make up their own mind about what to believe. I’d urge caution though, as RS has arguably been profoundly overpraised. Almost all of the information that’s available online would lead one to believe that RS is our friend. Hopefully this article can help bring a bit more balance to the score by raising awareness of the potential dangers of diets rich in RS, thereby potentially helping people to make better and more informed dietary decisions.