Page 67 - GTM-2-1

Global Translational Medicine Mass balance

As an example, the oxidation of one generic triglyceride The total stored nutritional energy is 1,600 kcal + 1,600
molecule is as follows: kcal + 1,800 kcal = 5,000 kcal.
C H O + 78O → 55CO + 52H O + HEAT 8,084 kcal This example illustrates, therefore, that the property of
55 104 2 2 2
food that is related to mass gain is its mass, not energy. The
Reactants Products first subject, in effect, has accumulated substantially more
C H O 860 g 55CO 2 2,420 g nutritional energy than the second subject, yet both have
104
55
+78O 2,496 g + 52H O 936 g experienced the same degree of weight gain.
2 2
3,356 g 3,356 g
4.2. Weight loss is the result of mass elimination, not
It is important to note that there is mass only in reactants energy expenditure
and products but not in energy (calories). Consider the oxidation of 100 g of glucose:
4. Mass-energy equivalence principle C H O + 6O → 6H O + 6CO + HEAT (720 kcal/mol
2
2
2
6
6
12
of C H O )
The mass-energy equivalence principle implies that 6 12 6
when energy is lost in chemical reactions, the system This requires the uptake of 107 g of oxygen (O ) as 100 g
2
will also lose a corresponding amount of mass. However, glucose (C H O ) × (192 g O /180 g C H O ) ≈ 107 g O .
6
2
6
12
12
6
2
6
as far as the regulation of body mass is concerned, this The Law of Conservation of Mass implies that the mass
equivalence principle has been misunderstood. This global of the products = mass of the reactants. The amount of
misconception requires detailed clarification. water (H O) and carbon dioxide (CO ) formed is 207 g as
2
2
the mass of the products = mass of the reactants = 100 g
How is energy intake and expenditure not the governing C H O + 107 g O = 207 g.
factors that determine if the body stores the food we eat 6 12 6 2
as fat or not? How could one change that? How can the Now, assume that all the produced water and carbon
mass of food change that? If the eventual weight loss is dioxide are used in the following way:
from water, urea, or something else, it is still determined (i) Water becomes intracellular water in newborn cells;
by whether or not the body replaces it or even stores more (ii) Hydrolysis reaction (i.e., the cleavage of a chemical
than was used. Where is the gap where energy expenditure bond by adding a water molecule that becomes part
is not representative of the substrate (i.e., mass) being used? of the reaction products); for example, the release of
To understand why nutrient mass, not nutritional thyroid hormones thyroxine (T4) and triiodothyronine
energy, is the entity that determines body mass fluctuations, (T3) requires hydrolysis;
one must think in terms of arithmetic and analytical (iii) Carboxylation reaction (i.e., the addition of carbon
chemistry as shown in the next subsections. The caloric dioxide to a molecule); for example, carboxylation of
values of macronutrients are rounded. acetyl-CoA during fatty acid synthesis.
Notice that in the aforementioned situation, 400 kcal
4.1. Weight gain is the result of mass accumulation, have been expended by oxidizing 100 g of glucose, yet
not energy accumulation body mass will not decrease when heat is dissipated
Consider two individuals that gained 1 kg of non-water but rather when the 207 g of reaction products are
body mass, as they accumulate 1,000 g of absorbed eliminated, which in the described case did not since, as
macronutrients within their body cells. The macronutrient illustrated, oxidation products become part of the body
distribution of the first subject is shown below. mass.
(i) 200 g of protein = 200 g × 4 kcal/g = 800 kcal The important message in this section is that that
(ii) 300 g of carbohydrate = 300 g × 4 kcal/g = 1,200 kcal energy balance and mass balance are separate balances in
(iii) 500 g of fat = 500 g × 9 kcal/g = 4,500 kcal the human body [1,3] . This fact should be kept in mind when
reading the sections below.
The total stored nutritional energy is 800 kcal +
1,200 kcal + 4,500 kcal = 6,500 kcal. 5. Energy balance cannot occur at body
The macronutrient distribution of the second subject is mass stability
shown below.
According to the Law of Conservation of Mass, body mass
(i) 400 g of protein = 400 g × 4 kcal/g = 1,600 kcal stability (i.e., mass balance) can occur only when the mean
(ii) 400 g of carbohydrate = 400 g × 4 kcal/g = 1,600 kcal absorbed mass of each macronutrient is the same as its
(iii) 200 g of fat = 200g × 9 kcal/g = 1,800 kcal respective mean oxidized mass. Otherwise, body mass will

Volume 2 Issue 1 (2023) 3 https://doi.org/10.36922/gtm.222