Personalized nutrition is a fancy term that’s being thrown around a lot lately. If you go on Google and search around for new diet trends, then chances are you’ll come across articles on this dietary concept. Some of these articles will likely tell you that personalized nutrition will soon be a well-known and dominant concept in nutrition and that, in a not so distant future, it will become common practice to personalize nutrition based on genotype and microbiome composition. In other words, if you go and see a dietitian in the future, you will be asked to carry out a range of tests, including blood glucose analyses and genome sequencing, and the dietitian will, based on the results of those tests, design a personalized diet plan for you.
To most people, I think this idea sounds fascinating and revolutionary. Who wouldn’t like to know exactly what diet that is the best fit for their body and genetic make-up? The personalized nutrition concept certainly sounds revolutionary, which is probably why it has become so popular. People are always looking for something new and better. They quickly get tired of generic diets. Particularly those who’ve tried a wide variety of different diets in an attempt to lose weight or improve their health, without experiencing the results they were hoping for, are probably highly enthusiastic about the personalized nutrition concept, thinking that they can finally get a plan that’s made just for them; a plan they will be able to follow and that will give them the results they want.
Not everyone is so enthusiastic about personalized nutrition though. Personally, I’m somewhat skeptical of its merits. I don’t like to be the bringer of bad news, but the fact is that the premise that underlies this emerging nutritional concept has several holes in it. These holes aren’t small and barely visible; they are quite large…
What is personalized nutrition?
Let’s imagine that we go out on the streets and ask a bunch of random people if they are willing to participate in a nutritional experiment. After some time, we manage to collect a group of a dozen or so volunteers, who we bring with us to a medical facility that’s stocked with all sorts of different lab equipment that can be used to analyze everything from cholesterol levels to microbiota composition to body fat levels.
The first thing we do upon initiation of the project is to collect information about the participants’ weight, height, and physical appearance. We find that the participants are a very heterogeneous group with regards to their body composition and physical structure. Some are thin, others are large; some are tall, some are short; and the range of BMIs is quite broad.
The next thing we do is to inspect how their bodies look and work on the inside. We analyze their gut microbiota via DNA sequencing of fecal material, carry out genetic tests, and check how their blood glucose levels respond to the intake of various foods.
These tests further reveal that the participants differ in several respects. There’s great interindividual variability with regards to all of the findings. Some of the participants harbor a microbiota dominated by the group of bacteria called Firmicutes, others by the group Bacteroidetes, and some have genetic mutations that aren’t found in the rest of the participants. Moreover, the blood glucose levels of the participants vary widely following the consumption of food: some get a rapid spike in glucose after they eat a carbohydrate-rich meal whereas the blood glucose levels of others increase in a more gradual fashion.
These types of results are not uncommon in nutritional research. It’s well known that each and every one of us carries a unique microbiota and that we are not 100% identical with regards to our genetic make-up. Moreover, it has been shown that there’s a lot of interindividual variability with regards to diet-induced changes in blood glucose levels (1). Some people are capable of producing more insulin than others and/or are more sensitive to the effects this hormone has on cells throughout the body.
These are the facts upon which the personalized nutrition concept is built. Since studies have revealed that no two people are identical with regards to their metabolic and digestive capabilities; it’s assumed that each and every one of us has unique nutritional needs, and hence, should be prescribed a personalized diet plan. It’s a very logical assumption to make. It’s not necessarily a correct assumption though.
I’m not going to disagree that it’s 100% necessary to take factors such as physical activity levels, health status, and body composition into account when designing a diet plan for an individual. That said, I question the idea that people differ widely with regards to their basic nutritional needs.
How different are we, really?
The weight of the evidence indicates that all modern members of Homo sapiens are descendants of a group of humans who lived in Africa some hundred thousand years ago (2). Starting about 70.000 years ago, our ancestors began migrating out of Africa and took up residence in various parts of the world. Ever since then, the genetic diversity of the human population has kept on increasing as a result of geographical dispersion, mutations, and selection.
That said, Homo sapiens is still a fairly homogenous group with regards to its members’ genetic composition. The genome of your neighbor is not that different from yours. In the context of nutrition, it’s well known that there has been positive selection for lactase persistence and increased AMY1 copy number within some populations; however, these genetic changes fade in comparison to all of the similar traits that we all share.
The fact is that we humans are all very similar with regards to our general physical structure (2). Our digestive and metabolic systems are very much alike; and our nutritional needs don’t differ that much. Some of us descend from ancestors who were early adopters of the starch-heavy diet that became popular following the Agricultural Revolution, and hence, may carry genes that confer an increased ability to digest carbohydrates. Others descend from a line of ancestors who lived in a cold habitat and ate a lot of animal source food. These people often carry unique adaptations that allow them to better tolerate a diet high in fat and/or protein. For example, recent research has revealed that the Inuit are equipped with genetic adaptations to a high-fat diet (3).
With all of that being said, the differences that exist between people with regards to metabolic and digestive capabilities are, in general, not huge.
So, why then, have studies shown that one specific diet or meal can induce very different hormonal and metabolic responses in different individuals? While some people can eat a breakfast of bread and juice without experiencing any troubles with their blood glucose; others can get dangerously high levels of sugar in their blood from the same meal.
Can this phenomenon solely be explained by inter-individual genetic variability? No… Evolution may not have prepared us for the modern sugar-laden and starch-heavy-diet; however, that doesn’t mean that we’re not adapted to digest carbohydrates. If the consumption of carbohydrates makes your blood glucose levels rise to alarming levels, then there’s something wrong with your body. The solution isn’t to go on a life-long, super restricted diet or pump yourself full of insulin, unless of course you have a genetic defect that compromises your ability to digest carbohydrates or your beta-cells are irreparably damaged. Rather, the solution must be to address the underlying problems that cause your blood sugar to spiral out of control following the consumption of food.
Inter-individual differences in microbiota composition may largely account for inter-individual differences in responses to dietary interventions
The gut microbiota is a central player in the metabolic theatre of the human body. Over the past several years, it has become increasingly clear to me that the microbiota plays a critical role in regulating host insulin sensitivity and production. I know believe, based on everything I’ve read, that gut dysbiosis is the fundamental cause of type-1 diabetes. The microbiota has also been shown to play an important role in the pathogenesis of insulin resistance and type-2 diabetes (4). Fat mass, by itself, is also a major contributor to insulin resistance, due in part to the fact that it releases a range of inflammatory mediators.
This leads us over to the main problem I see with the personalized nutrition concept. Instead of addressing the underlying problems that cause decreased or impaired tolerance to various nutrients (e.g., carbohydrates), the approach of personalized nutrition is to design a diet based on the symptomatology of the individual in question. If the individual shows signs of glucose intolerance, a very low-carbohydrate diet is prescribed, but nothing is done to remedy the problems that caused the intolerance in the first place.
Perhaps needless to say, I don’t like this approach. I much prefer to get at the root of the problem. There’s a reason why people respond differently to different diets and meals. The differences don’t just arise out of thin air. It’s my belief that interindividual variability in microbiota composition can explain much of the differences in outcome and tolerability that are observed in clinical nutrition research.
The microbes that dwell in your gut are involved in pretty much every part of the digestive processes that go on inside your body. They break down many of the nutrients you take in; they regulate your immune system, thereby affecting how your insulin-producing beta cells function and how your muscles, liver, and other tissues respond to the action of insulin and other hormones; and they regulate the permeability of your intestine, thereby controlling what substances that are allowed to pass from the gut and into systemic circulation. It has also become clear to me that the gut microbiota is involved in regulating hepatic gluconeogenesis. I strongly suspect that differences in fasting blood glucose levels between people can largely be explained by interindividual variation in gut microbiota composition.
A lot of people in the world today harbor a degraded, species-inappropriate microbiota and carry a lot of excess adipose tissue. Hence, it shouldn’t come as a surprise that many contemporary humans are metabolically deranged and have blood glucose levels that fall in the prediabetic or diabetic range. When compared to hunter-gatherers, most westerners have poor glucose tolerance, poor insulin sensitivity, and poor leptin sensitivity, among other things (5, 6).
A better approach
The approach of personalized nutrition is to tailor each person’s diet according to the person’s gut microbiota composition and glucose tolerance, among other things. I’m skeptical of this approach. If a person is overweight, has poor glucose tolerance, and harbors a degraded and imbalanced microbiota, I much rather prefer to fix his microbiota and get him to lose weight, and thereby improve his glucose tolerance and insulin production and sensitivity, as opposed to keeping him on a very special diet.
This is not to say that I don’t like low-carb diets. I do. All I’m trying to say is that I think it’s much wiser and better to try to restore the body of the person in question to a fully functional state, as opposed to trying to design a diet that allows him to function fairly good despite his current compromised state.
The human microbiota is a highly dynamic community of organisms. It changes rapidly following changes in diet. Rather than sculpting the diet according to the composition of the microbiota, it’s probably much better to sculpt the microbiota via diet changes, and if necessary, other interventions. In other words, we shouldn’t assume that the microbiota is a static community and that each person should eat a diet that is specifically tailored for his microbiota. There are some exceptions to this rule (e.g., some people with gastrointestinal disease have a severely inflamed gut and may need to be on a special diet); however, in general, the principle above holds true.
The main problem I see with the personalized nutrition approach is that microbiome and genome analyses don’t really tell us that much. You can’t simply look at a person’s genetic code or microbiota structure and know what type of diet he should be on. Instead of trying to piece together information derived from these types of analyses, I much prefer to use the evolutionary template, in combination with modern scientific research, as the basis for designing diet plans. Microbiome and genome analyses, as well as other tests that are an essential part of the personalized nutrition concept, can also be useful, but they should not be the sole or primary tools we rely on.
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