Last week in Part I, I reviewed the energy balance for the human body and explained how it controls whether or not we lose weight while dieting. This week, I’ll share with you some my observations on how this energy balance applies to my own weight loss efforts.

The weight loss industry in the United States brings in over $65 billion dollars a year in revenue and includes a wide array of diet systems. Each of these systems tends to contain their own biological explanation on why their recommended approach will help you melt away the pounds. In my experience, there are two key factors which are usually left out of these discussions. The first is the outsized impact of the hundreds of trillions of bacteria which live in your intestinal tract. These little friends of ours, collectively known as the gut microbiome, play a pivotal role in the digestion and absorption of food calories. The bacteria consume calories to sustain their own metabolisms, and they also control how efficient we are at absorbing the energy from different types of foods. The second missing factor is that we “waste” a portion of the energy content of the food we eat by letting it pass through our bodies undigested. The percentage of calories which are “wasted” is also largely determined by the actions of our microbiome.

The bacteria in our intestines are so numerous that the majority of the DNA within our bodies is not our own, but is from the bacteria. Further, the range of species of intestinal bacteria varies greatly from person to person. Only about one third of the species are common among all people, while the remainder arise from personal experience. Differences in the diversity and number of bacteria among people helps to explain such things as why water can make tourists in some locations sick but does not affect the residents, why some people are more efficient in digesting  foods like beans or cabbage than others, and (as I reviewed in Bacteria and Obesity, A Surprising Link) why some people have a very difficult time losing weight. Not only does the makeup of the bacteria population vary from person to person, but it changes over time within a person as a result of age, illness, antibiotic use, environmental factors, international travel, and diet.

To further investigate the role of the gut microbiome on dieting, I have been using myself as a test subject. I am 48 years old, 5’9”, have a body fat content of 18.5%, and currently weigh 159 lbs. For the past two decades, except for one brief sojourn up to 177 lbs, my weight has varied between 155 and 170 lbs. Generally speaking, when my weight reaches 170 lbs, I put myself on a diet and increase my amount of exercise. Below is a chart of my weight from various diets from 2003 through 2014 plotted by month into the diet.  The average is displayed with the dashed line.

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The shapes of these curves are remarkably similar.  Generally speaking, I drop from 170 to 160 lbs in two months and then my weight drifts down to between 155 and 160 lbs and holds steady for awhile. The similarity between the shapes of the curves is far more consistent than my behavior from diet to diet. Since I am the test subject, I know that in some of these diets I exercised like crazy, swimming in the morning, running at lunch, and lifting weights. In others, I mostly just adjusted my food intake. But no matter what, the results were the same. In addition, even though I have around 18% body fat when I weigh 155-158 lbs, a normal but not particularly low level for a man my age, I stop losing weight at this point even though I carry on with my diet and exercise routines. I have noted the similarity of these curves in the past but never gave it much thought. But as I have learned more about the functioning of intestinal bacteria, I began to suspect that their behavior was the key to understanding the details of this weight loss data. Let’s explore this idea further by analyzing daily weight measurements from my current diet, which started in mid-August:

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For the first 7-10 days (shown by the circled data), even though I increased my exercise, curtailed my calorie intake, and made better food choices, my weight did not change at all. This is consistent with my experience from previous diets. After this the lag time, I began to lose weight at the same rate as all of my previous diets. As time proceeds, I expect that my weight will level out in the 155-160 range, stay there for a period of time, and then eventually drift back up to 170.

The theory I have developed from analyzing these data is that my ability to affect my body weight is largely indirect. By this, I mean that I think the direct result of my dieting behavior is to influence the composition of my gut microbiome. Consequently, this composition is what determines whether or not I lose weight. Let me walk through my reasoning with you.

Consider the conditions just before I started a diet. There would have been a period of time during which I was providing my intestinal bacteria with an excess of calories and plenty of sugars and fats. During this time, the bacteria which enjoy these conditions would have thrived and established a population distribution which is very effective in ensuring that my body absorbs energy from fats and sugars. Then, when I see that I weigh 170 lbs, I throw these bacteria a curve ball by limiting the calories available to them and by depriving them of sugars and fats.

When I limit foods with sugars and fats for a while, the population of the bacteria species which facilitate efficient absorption of fats and sugars is reduced. If I maintain my diet for long enough, I deplete the population of these types of bacteria to such an extent that I cannot efficiently absorb calories from sugars and fats from the occasional splurge. Effectively, when I am in the lower end of my weight range I have manipulated the conditions within my microbiome such that I “waste” more food calories. The observation that I seem to be able to go back to eating cheeseburgers and fries without gaining weight eventually leads to complacency on my part. My complacent behavior then allows the fat-loving, sugar-craving, excess-calorie desiring species of bacteria to repopulate my intestines, and the percentage of calories that I am “wasting” goes down. Absent renewed efforts on my part, it’s back to 170 lbs we go.

If what I call my indirect weight loss theory is correct, then the future of the business and science of weight loss is in finding ways to manipulate intestinal bacteria populations such that they are compatible with lower weights. You can already see this happening in the growth in popularity of foods which claim to be “probiotic.” The intent of these foods is to provide the correct balance of nutrients to the gut microbiome to shift the population balance to a “healthier” equilibrium or one more consistent with weight loss. While I have not looked at the data behind any of these claims, in principle this mechanism seems viable.

The next step in the evolution of this approach, which is already being pursued by multiple biotech companies, is the direct introduction of specific species of bacteria into people’s intestines to promote weight loss. The first company to successfully receive an FDA license for such a product is going to make quite a bit of money.

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