Dieting is big business in the United States. If you include the $21 billion spent on diet sodas, the weight loss industry takes in over $65 billion dollars a year. A commonly cited statistic is that at any particular moment, one half of all adults in the U.S. are on a diet. In the interest of both testing the science of dieting for this column and dropping a few pounds myself, I am currently one of them.

The diet industry is filled with books and theories. There are protein diets, cabbage diets, grapefruit diets, and diets where you are supposed to chemically test your urine on a daily basis. Most diet systems include claims that they will either rev up your metabolism or “burn” fat or, both. I think that deep down we know that most of these claims are pure hokum, but the desire to drop a few pounds often helps to expand our willing suspension of disbelief. And so we buy the books and follow the instructions.

From an engineering perspective, dieting is just a heat and material balance. In equation form, this is:

(Mass in)  – (Mass out) = (Mass Accumulated)

(Energy in) – (Energy used) – (Energy out) = (Energy Accumulated)

The material balance is straightforward. We consume and excrete both solids and liquids. If we consume more than we excrete, then we gain weight. What makes dieting more scientifically interesting, and allows room for the diet industry to make a number of pseudo-scientific claims, is that the simple material balance is controlled by a much more complicated energy balance. I’ll discuss the details of this below, but the key linkage arises from the fact that when your body finds that it has extra energy available, it produces and stores fat to use as an energy source at a later date.

The energy in foods is contained within the chemical bonds of the molecules that you eat. (Please note that in order to avoid saying food and beverages over and over I am using the term food to include both.) In order for your body to use the energy in food, these chemical bonds need to be broken apart during the digestion process in the stomach. The process of digesting requires energy in differing amounts depending on the type of food. For example, digesting proteins requires approximately 20-30% of the incoming calorie content, while digesting fats requires only 1-3%. This characteristic goes a long way to explaining the enduring popularity of protein-heavy diets such as Atkins. For our energy balance, I will define the net energy (Enet) as the amount of calories in the food consumed minus the calories required to digest them.

With the definition for Enet in hand, let’s write an energy balance for the body:

Enet = Emet   +  Eexer  +  Ewaste + Ebact  +  Estored

Where:

  • Emet = the energy of your metabolism on a normal day
  • Eexer = the energy you expend in exercising
  • Ewaste = the energy content of your urine and feces
  • Ebact = the energy that the bacteria living in your intestines consume for their own metabolisms
  • Estored = the energy either added to or removed from the cells in your body in the form of fats

Let’s discuss these parameters using me as an example. I am 48 years old and 5’9” tall.  I exercise regularly and, as I type this column, weigh 159 lbs.

An average man of my age and weight uses approximately 1800 calories a day by simply existing, and another 500 or so from walking around and other miscellaneous movement. Therefore my Emet is approximately 2300 calories a day.

Vigorous exercise, such as running or swimming, for a man of my size consumes about 700 calories an hour. I exercise about four hours a week, using 2,800 calories. Therefore, my Eexer averages out to about 400 calories per day.

The digestion process is not 100% efficient, and, thus allows approximately 5% of the calories consumed to pass right through. Therefore, on a day when I eat 3,000 calories (a typical value for me), my Ewaste is 150 calories.

Ebact is the energy that the millions and millions of bacteria which live in your intestines consume to stay alive. We live in an intricate symbiosis with them and, as I explained in Bacteria and Obesity, A Surprising Link, the diversity and population of intestinal bacteria can vary significantly from person to person to great effect. In fact, our designation of some individuals as having a “high metabolism” is almost certainly a result of their having a favorable population of bacteria in their gastrointestinal tract. We’ll estimate what my Ebact is a bit later.

Estored is the key parameter for your dieting results. Let’s reorganize our energy balance to make that clear:

Estored = Enet – Emet – Eexer – Ewaste – Ebact

If Estored is greater than zero, your body will store energy, primarily in the form of fat. If Estored is less than zero, you will lose weight.

Here is an estimated average daily energy balance for me with the negative numbers in parentheses:

Enet                        3,000

Emet                       (2,300)

Excer                       (400)

Ewaste                    (150)

Doing the math, we now have Estored = 300 – Ebact. Therefore, if my intestinal bacteria are consuming 300 or more calories a day, Estored will be negative and I will lose weight. If they consume less than 300 calories, I will gain weight.

If everything were as simple as I have laid out in this equation, all the mysteries of dieting would fall away. I could calculate exactly the weight loss I wanted to achieve, dial in my food intake and exercise, and arrive at a pre-selected weight in a pre-selected time (within reasonable limits). But, as I will discuss next week in Part II, my own dieting data suggests that there is something going on that is more subtle than my equation can currently describe. So it still needs an upgrade.

As I suspect you will have guessed by now, the wild card, just as in the column on obesity, is the bacteria. So come back next week for a discussion of the mysteries of your microbiome and to check in on my diet.

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