The Human Ecosystem — How Our Health Depends on Which Tissue Wins the Cellular Competition for Nutrients
An article published in the Frontiers of Physiology in 2018 proposed a theory using the ecosystem analogy to explain metabolic health, obesity, and metabolic diseases plaguing western society. This paper states that our bodies are like complex ecosystems. Our cells compete with each other for nutrients just like in large scale ecosystems with hundreds of thousands of living organisms. This is a compelling analogy because it explains the obesity epidemic, but it also explains why your diet isn’t the only thing important for your health. In fact, if you aren’t overeating (calories in ~ calories out), and you are getting all your essential macronutrients and micronutrients, how you process the foods you eat matter quite a lot more than whether or not you are keto, vegan, vegetarian, carnivore, or paleo. By extension, health behaviors like exercise matter more than what kind of diet you are on because exercise provides the stimulus to tell your body what to do with the nutrients you give it.
The ecosystem analogy is a very interesting comparison, and most useful in understanding how cells acquire nutrients from the foods we eat. Just like predators in the African grasslands compete for the same food sources, so do our cells. If hyenas outcompete lions for food, the lions will starve to death. If some cells in our bodies are better at acquiring nutrients than others, the “others” will starve.
Every cell in our bodies require some combination of fat, carbohydrate, and protein to maintain metabolism and cellular processes. The degree to which cells can take up these nutrients and utilize them for metabolism and build cellular machinery is dependent on their ability to compete with their neighboring cells for a given amount of nutrients. So, whether or not “Cell A” gets a sugar molecule depends on how well it can compete for that sugar molecule compared to its neighbor “Cell B”. It’s easy to imagine a scenario where one cell type steals all the nutrients of another cell type. This can be a problem under certain conditions.
Full disclosure: This analogy is an oversimplification and false dichotomy of physiological systems in regard to our health, but for the purposes of this ecosystem analogy, and for the sake of brevity, we are going to use a fat versus skeletal muscle comparison.
Two of the most important cell types for our metabolic and physical health are fat cells and muscle cells. These two tissues compete for nutrients in the bloodstream. Skeletal muscle tissue and adipose tissue (fat tissue) are our two major storage sites for fat and sugars that we consume. Many of the chronic diseases we face as a society today are a result of an imbalance in the acquisition of nutrients between fat tissue and skeletal muscle.
Increases in fat cells number and size contributes to insulin resistance, diabetes, cardiovascular diseases, and certain cancers. These diseases, driven by fat accumulation, are a consequence of the ability of fat cells to outcompete skeletal muscle and other tissues for nutrients. How does it do that?
Adipose tissue is physiologically unique. Fat cells have the ability to hypertrophy, which means each individual cell can grow in size, BUT they also have the ability to grow in number, this is called hyperplasia. Unlike other tissues of the body, fat is the only tissue that does not appear to have a limit for growth in cell size or cell number. It will continue to grow if nutrients are continually provided.
Other cell types in our bodies like neurons, kidney cells, and liver cells all have a limit on how big they can get (hypertrophy). So, despite excess nutrient availability they will not grow past a certain point. Skeletal muscle cells are the only other tissue in the body that have comparable hypertrophic growth abilities to fat cells. Skeletal muscle continues to grow if you provide the right set of stimuli (nutrients and exercise). Interestingly skeletal muscle does not normally grow new cells, meaning hyperplasia does not occur in skeletal muscle. (*hyperplasia can occur in skeletal muscle under certain conditions like severe muscle damage).
The ability of fat cells to undergo hypertrophy and hyperplasia gives adipose tissue a competitive advantage to acquire nutrients better than other tissues. This is because the more massive a tissue is relative to other tissues in the body the better it will be at acquiring nutrients. As the fat cells get larger and replicate, they get better at stealing nutrients from other cells and tissues. It’s important to note that it is not some inherent property of fat cells that make them better at acquiring nutrients, rather, it is merely the fact that the cell surface area increases which increases the cells ability to interact with and take up fats and carbohydrates into the tissue relative to other tissues that have less surface area. This is why it’s so hard to lose adipose tissue once it has been gained because whatever you eat still gets preferentially targeted to the fat cells over the other cells, even if you are in a caloric deficit.
One of the more interesting things this dynamic creates is an “apparent caloric deficit” in cells that govern appetite in your brain. The fat cells become so good at taking nutrients out of the blood stream that the cells in your brain responsible for sensing nutrients in the blood stream “think” that you are starving. So, these cells send a signal that increases appetite! Even though the individual has plenty of nutrients stored in the fat cells, their brains think they are starving, which is another reason why once becoming overweight or obese from increased adiposity it is extremely difficult to reverse. The individual is constantly having to combat their own hunger cues.
Here is another way to think about it: Sedentary behavior and chronic overfeeding over long periods of time provides a “training stimulus” for fat cells. These “trained” fat cells become increasingly good at taking nutrients from the blood stream and storing them in fat cells thus preventing other cells from using those nutrients.
So how do we solve this problem? Part of the answer is exercise. Exercise provides the right conditions for the skeletal muscle and other lean tissues of the body to have a competitive advantage over the fat cells. It is the most powerful stimulus we know of that preferentially target nutrients to lean tissue (skeletal muscle), but it also is a stimulus that triggers fat cells to release fat into the blood stream to “feed” the exercising skeletal muscle. So, in this regard it is a double-edged sword. Exercise increases the sensitivity of skeletal muscle to a meal, and also improves the extraction of nutrients from fat cells. Furthermore, the lean tissue remains sensitive to nutrients and meals long after the exercise bout is finished.
All of these concepts discussed above explain why people who train 5–6 days a week can eat pretty much anything without worrying about gaining weight. The weight that they do gain from overfeeding is preferentially targeted to lean tissue, which is a healthier deposition than if those same nutrients went into adipose tissue. This actually increases the window of calorie balance for an athlete or someone who is very active. For example, this means that if an athlete needs to consume 2800 calories a day, their range could be between 2400 and 3200 and still be healthy, while a sedentary person with the same 2800 calorie requirement has a much smaller healthy window (2600–3000 calories instead). This concept is illustrated in figure 3 of the paper (here).
Exercise is also a great appetite regulator. People who exercise regularly tend to have more consistent and regular hunger cues which allows them to control food intake better than someone who is sedentary. Not only that but some evidence suggests that people who exercise regularly stop eating when the feel full more often than people who are sedentary. Taken together this means that active people have better appetite control and are less likely to overeat than sedentary individuals thus allowing for more precise weight management.
Altogether, exercise is the only stimulus that increases calorie consumption, as well as preferentially target the nutrients you eat to the lean tissues of your body instead of the fat tissue. To quote the authors of the manuscript:
“It is not what is eaten (i.e. diet) that engenders health or disease, but what one’s body does with what is eaten (i.e. nutrient metabolism).”
At this point it should be clear that diet is merely necessary but not sufficient for optimal health. However, many people are obsessive about what they put in their body. This is more and more apparent to me as I see the rise of diet tribes (keto, vegan, vegetarian, carnivore, paleo, low carb etc….). The thing that these zealots don’t seem to realize is that as long as you are getting quality food, are not nutrient deficient, and not overeating, it doesn’t really matter what kind of diet you’re on. The most important thing about your diet is how your body handles what you feed it. A healthy diet is one component of your overall health and wellbeing, but how you process the food is much more important. Therefore, our diet is only healthy relative to the other health habits we pursue — it is entirely possible for a diet to be healthy in one circumstance (high physical activity and good sleep habits), but unhealthy in another context (sedentary behavior and poor sleep habits).
Each of our cells are competing with all the others for nutrients and resources, some of them are better at competing than others. We have to provide the right “healthy” stimulus to give the competitive advantage for the lean tissue to preferentially acquire nutrients. So maybe ditch the fad diet and hit the weight room, your ecosystem will thank you for it.
