Thursday, January 9, 2014

Meat-based diet and inflammation? Makes sense from an evolutionary perspective

A new study in the journal Nature set the Paleo blogosphere ablaze last month.  This study looked at the effect of diet on changes in gut bacteria and was discussed in my Health Research Recap found here.  I went over how I agree that a diet that focuses on meat is likely not the best choice if your goal is to be healthy.  In this blog I am going to expand on this topic using the evolutionary template to theorize why this happens.

One of the more interesting aspects of the study in question, found here, is that gut bacteria switched rapidly when going to the extremes of diet.  In my opinion, this is a testament to how important a diverse array of bacteria in the gut is to maintaining metabolic flexibility.  Humans can thrive on a wide array of diets and the ability of bacteria in the gut to respond to quick changes in the availability of food from the environment is what makes us metabolically flexible.  From an evolutionary perspective this makes sense because once our ancestors left Africa, the most likely to survive would have been the ancestors who could adapt to changes in the types of food that were available due to seasonal changes in the climate.

As Summer leads to Fall and Fall leads to Winter, the availability of fruit and other foods that were preferred by our ancestors who remained in Africa would not have been available in more seasonal climates during a time when you couldn't just order takeout.  To adapt, they likely needed to consume fallback foods that were available in a wintery climate during that time of year.  This would include meats and root vegetables whose availability would change less with the seasons.  With this change from a diet high in fruits and vegetables to one with a proportionally smaller amount of these foods and a higher proportion of animal flesh, you are changing the substrate that the bacteria in your gut can ferment from mostly fermentable carbohydrates to amino acids.  This, in turn, should change the proportion of bacteria in the gut, favoring the types of gut bacteria that ferment amino acids and scaling back the type that ferment carbohydrates.  Could there be some evolutionary benefit for this switch?  I think so.

As the diversity of available food reduces during the winter time, there would be a benefit to extracting more energy out of that food.  Most people believe that carbohydrates and protein contain the same amount of calories per gram at 4 calories, but this is not true.  In a bomb calorimeter, carbohydrates average 4.2 calories of energy per gram while proteins average 5.65 calories per gram.  Some of this increased energy content is due to the nitrogen content of protein, which humans don't use for energy.  Even if half of the extra energy is from nitrogen, that still leaves an extra 17% more energy in protein per gram than carbohydrates.  This is huge when food becomes scarce.  One of the first things one notices as they implement a low carbohydrate diet is constipation.  Maybe this delay in food transit through the digestive tract allows us to absorb more of that energy or it gives bacteria in the colon more time to ferment amino acids that make it there, providing more energy than we can extract on our own.

On top of the potential for extra extractable energy contained in protein, there is the inflammation associated with the bacteria identified in the study.  While our current food environment makes this a bad thing, in the face of food scarcity, this inflammation may be beneficial.  Inflammation is associated with insulin resistance and obesity, two conditions associated with the theoretical thrifty genotype that is more likely to experience these conditions.  Perhaps the inflammation associated with this change in gut bacteria is expressing these thrifty genes to a greater extent and allowing the host to get by with less food.  In other words, the inflammation these types of bacteria create functions as a signaling molecule to the host, letting their body know that it's time to be thrifty and more efficient with cellular energy consumption and storage as the types of available food declines.  This inflammation coupled with moderate to high carbohydrate consumption could improve fat stores during the end of fall as the wintery season approaches and begins to take hold.

We can compare this to what we see today in Type 2 diabetes.  In Type 2 diabetes, people become insulin resistant which causes their blood glucose to rise.  What we see in people with Type 2 diabetes is an eventual spilling of fatty acids in to the blood stream during this process.  This is due to the extremely high levels of blood glucose we are able to attain today with the types of foods that are available in addition to the fact that we are able to exceed our ability to store fat.  Who knows how this process would play out as it did back when our ancestors were moving out of Africa searching for food and being on the low end of their fat storage capabilities.  If it plays out in a similar fashion to the beginning stages of Type 2 diabetes, it is likely that they experienced an enhanced ability to accumulate body fat, a beneficial effect when going in to a season where carbohydrates won't be available.  What we are now seeing with Type 2 diabetes could potentially be the consequences of our past environment selecting traits that were advantageous at the time that are now no longer advantageous.  A recent study in the journal Nature indicates that this may be so.

The study looked at people of Latin American descent and identified a gene associated with Type 2 diabetes that is very prevalent in that population(approximately 50%) called SLC16A11.  This genetic variant alters fat metabolism and causes an increase in cellular fat levels.  People with one copy of this variant are 25% more likely to develop Type 2 diabetes while people with two copies are 50% more likely to develop it when compared to people without this variant.  All of this science mumbo jumbo is nice, but the science behind this pales in comparison to where this gene likely came from; Neanderthals.

This is extremely interesting because Neanderthals left Africa much earlier than we did and those that were more adapted to efficiently storing fat were far more likely to be successful in environments where food sources were seasonal.  Much of the Neanderthal anatomy was suited to cold weather because they had experienced it for millenia before we made our way out of Africa, it only makes sense that their physiology would be optimized for that environment as well.  If this is one of the thrifty genes, perhaps as their diet changed moving in to winter, so did their gut bacteria,.  This, in turn, may have increased expression of this gene, as well as others, which would make them more "thrifty" and, thus, better adapted to their environment via improved fat storage.

Another piece of potential "evidence" comes from those undertaking low carbohydrate diets.  It is common knowledge that low carbohydrate diets aren't typically effective for fat loss when protein intake is high.  The conventional thinking is that this is due to protein causing insulin secretion as well as the ability of certain amino acids to convert to glucose and raise blood glucose levels.  However, it is interesting that high protein intake in the face of low fiber intake causes such a dramatic shift in the composition of gut bacteria given that the vast majority of  low carbohydrate diets tend to be high in protein and low in fiber(Atkins diet).  While a much more gradual shift in the availability of foods is more likely, the ability to adapt quickly would be highly beneficial.

Overall, I think there is a lot of evidence that supports the notion that a diet that is primarily meat, potentially even grass-fed beef, may cause inflammation due to increased protein consumption changing the landscape of the host's gut bacteria.  There appears to be a large benefit to the host through this process.  You have to be careful with what you can generalize this to, however.  I don't believe this indicates that moderate meat consumption is bad, and the ratio of meat to fiber is probably a much bigger determinant of this effect as is the ability of the host to digest protein.  Protein consumption, per se, is not the problem.  The ability of undigested protein to make it's way to the colon where the types of gut bacteria that can ferment amino acids can act on it is the chief concern.  In the absence of high carbohydrate consumption, this inflammation may even be benign.

Preliminary results from the Human Gut Project indicate that people who consume a Paleo diet tend to have higher levels of bacteria that are associated with inflammation, but most of these people have fewer complaints related to inflammatory conditions.  I do think this points to a more moderate approach to meat consumption than most people undertaking a Paleo diet implement for the simple fact that most overconsume protein from meat and nuts.  However, this doesn't vindicate the consumption of protein from vegetable sources.  In fact, given that a lot of these proteins are difficult for humans to digest, especially grains as well as soy and other legumes, they could potentially amplify this effect, especially when fiber intake is low.  Since vegans often rely on protein powders rather than whole food sources to fulfill their protein needs, they are essentially consuming high doses of protein that is more likely to become problematic, even at lower total protein intakes  Therefore, consuming concentrated sources of harder to digest proteins is not a great idea if you are looking to control inflammation, especially if you are also consuming large amounts of carbohydrates as is typical of the vegan diet.