2 The major nutrients and body energy consumption

  •  You should understand how much energy we can extract from nutrients.
  • You should understand the relation between chemical equilibrium and free energy
  • Energy requirements of a human being
  • Energy generation through oxidation
  • Equilibrium and free energy
Fig. 1 Organic molecules contain a large amount of Energy
The human body is in constant turnover. Many proteins and nucleic acids are constantly degraded and resynthesised. These processes protect the body against harmful damage and provide a mechanism to regulate body functions. Body functions such as movement, brain activity and metabolic activity require energy, which is derived from fuels in our food. The main fuels are carbohydrates, fats and protein, jointly called macronutrients. In addition, our nutrition contains micronutrients, which are only required in smaller quantities. These are ions and minerals, vitamins and other compounds, which are not essential but often beneficial, such as antioxidants or choline. A balanced nutrition provides all these compounds in sufficient quantities. An excess of carbohydrates and fats, in particular, leads to obesity, while lack of any of these components will lead to malnutrition and deficiencies. Our body is well adapted to irregular meals and extended periods of starvation. By contrast, the large increase of adult-onset diabetes (type 2 diabetes) shows that our body is not well adapted to the continuous provision of food throughout the day. Thus, it is of fundamental importance to understand the feeding/fasting cycle and the regulatory mechanisms underlying it. Moreover, we need to understand our metabolism as a balance sheet with inputs and outputs that should match each other (see video in the introduction to the course). The basic metabolic rate (BMR) of a human being can be estimated by simple formulas. 
BMR (kJ) = 100 x weight in kg
This is the energy required when we sleep and do not consume additional energy due to physical activity. To adjust for physical activity the following adjustments should be made:  
· 30% of the BMR for a sedentary person (a student learning)
· 60-70% of the BMR for a person who engages in about 2 hours of moderate exercise per day. 
· 100% or more of the BMR for a person who does several hours of heavy exercise per day.
Thus a female with a body weight of 50 kg and little other activity will use about 5000 + 1500 = 6500 kJ per day. It is instructive to compare this to household energy consumption. The energy of 1kJ converts to 0.28 Watt hours. Hence, 6500 kJ equate to 1820 Watt hours, this would allow a 75 W bulb to glow for 24 h. Thus, a human uses as much energy as 60 W – 100 W light bulb. Remarkably, a professional cyclist can generate about 400 W over a long period of time – and needs to eat accordingly.
How much do we need to eat per day to provide our body with enough energy? 
An overview of the energy content of our major nutrients is given in the Table 1 below. 
Nutrient Energy (kJ/g)
Carbohydrates
17
Fat
37
Protein
16
Alcohol
29

Fat has the highest energy density. For this reason, frying of food in oil adds substantial amounts of energy to a meal. While a 100 g boiled potato has an energy content of about 290kJ, 100g fried chips already have an energy content of 1210 kJ, because they contain 15 g of fat. The energy content of a normal dinner provides more than a third of the daily required energy. For example, a Pizza has an energy content of 2000-2500 kJ and a bottle of beer about 600 kJ. Add 400 kJ for a regular scoop of ice cream and the meal would provide about 50% of the energy for a day. The human body stores all three major nutrients. Carbohydrates are stored as the glucose polymer glycogen in muscle and liver; fat is stored as fat mainly in adipose tissue and protein is stored as muscle mass. Off these stores, glycogen serves as the short-term energy reservoir between meals. Fat and protein are used both as short-term and long-term energy supply. Glycogen stores are limited (100 g in liver, 400 g in muscles), protein stores can be increased by training and are very large (20-30 kg in a 70 kg adult), but not fully dispensable. Fat storage appears to be almost unlimited and largely dispensable. Even in a non-obese adult (70 kg) 15 kg fat is found.

How do we know the energy content of different food groups? Wilbur Atwater and colleagues developed a room-sized human calorimeter for human nutrition studies in 1893 (see Image below). The exhaled air was passed through bottles of sulfuric acid to trap water and through bottle containing NaOH to trap carbon dioxide. Oxygen was provided from a cylinder. This allowed measuring the exchanges of water, carbon dioxide and oxygen by weighing the corresponding containers. In 1900 they published the caloric content of the major nutrients (in kcal/g) as 4, 4 and 9 for carbohydrates, protein and fat, respectively, which are still correct today.

https://sportsci.org/news/history/atwater/atwater.html

In the previous module we looked at the building blocks of a cell. Not surprisingly, the major building blocks of a cell are also our main nutrients, namely carbohydrates, protein and fat (Fig. 2). DNA and RNA are not significant nutrients because nucleobases cannot be used for energy generation. Only the ribose subunit can be used as a carbohydrate.

Fig. 2 Overview of nutrient metabolism in mammalian cells
In this book we will follow the metabolism of the three macronutrients and discover how their metabolism converges to generate ATP. Fig 2 may look confusing now, but will become more and more familiar as we discover the metabolism of our major nutrients.
Table 1 demonstrates that fat has a significantly higher energy content per g weight than carbohydrate and protein. The reason for this will become clear when we discuss the metabolism of these compounds. However, just by inspecting the structures of the major food components in Fig. 3 reveals that protein and carbohydrates are more oxidised than fat (i.e. contain more oxygen per molecule).
Fig. 3 Structural comparison of major nutrients

Because oxygen has a strong tendency to acquire electrons, full oxidation of a nutrient is associated with a large negative free energy, while hydrolysis leaves most of the molecule intact generating only a small amount of energy  (Table 2).  

Reaction Standard free Energy Type of Reaction
Glucose + 6O2 –> 6CO2+6H2O
-2840kJ/mol
Oxidation
Palmitate+23O2 –> 16CO2+16H2O
-9770kJ/mol
Oxidation
Maltose + H2O –> 2 glucose
-15.5kJ/mol
Hydrolysis

The amount of energy associated with oxidation is illustrated by the burning of organic material. Wood fire logs are largely made up of cellulose, a carbohydrate. These can provide heat energy for long periods of time. Similarly wax and fat can be used to make candles, the burning of which provides energy. Burning is fast oxidation of organic material resulting in the generation of CO2 and H2O. The heat released in this process comes from the rearrangement of atomic bonds during the oxidation process. The more oxidised a molecule is, the less energy can be derived from it. This is nicely illustrated in Fig 4, which shows the energy obtained through full combustion of methane, methanol, formaldehyde, formic acid and carbon dioxide. This explains why fat has a higher energy content per g than carbohydrates and protein.

The general principle of organismic metabolism is the conservation and storage of the energy available through oxidation of nutrients.
To understand why some reactions can produce large amounts of energy, we need to understand the principle of chemical equilibrium. Let us assume a chemical reaction:
A+B <–> C+D
When starting with, for instance, 1Mol each of A, B, C and D the reaction will run until it reaches equilibrium. If the arrangement of electrons, charges and bonds is more favourable (has a lower energy) in the products, the reaction will run towards the products C and D. Vice versa, if the substrates are more stable, the products C and D will be converted to substrates. Thus, every reaction is characterised by the extent and direction it runs to reach equilibrium. As a result, the standard free energy of a reaction delta G0 is defined as:
Keq defines the concentrations of substrates and products at equilibrium. R is the gas constant and T the absolute temperature. Any reaction that runs spontaneously towards the products (runs forward towards C and D) will thus have a negative free energy (numerator > denominator), while any reactions that will proceed towards the substrates (runs backward to accumulate A and B) will have a positive free energy.   
The standard free energy G0 is not entirely useful for biological processes, because it assumes a starting concentration of 1 M. For protons this would mean the reaction would be carried out at pH = 0 (equals 1 M protons), far outside biological compatibility. The concentration of water in biological systems, by contrast, is much higher, namely 55.5 M. As a result, its concentration remains constant even though a reaction may use up water (hydrolysis) or produce it (elimination). Thus, a transformed standard state is used in biochemical tables and calculations, namely ΔG0′. This transformed standard free energy sets the activity of water as 1, H+ at 10-7 M (pH = 7) and Mg2+at 1 mM. Please note that this convention is only relevant for reactions in which water, protons or magnesium ions take part. Thus, the above shown equation changes to:
The most important factor that determines the free energy of a reaction is therefore how far the starting concentrations of all reactants in a biological system are away from the equilibrium. The relation between G0′ and the distance between starting concentrations and the equilibrium concentrations becomes obvious when the K’eq is tabulated against the free energy (Table 3)
Distance of K' eq from starting condition (1M) Free Energy
1000
-17.1 kJ
100
-11.4 kJ
10
-5.7 kJ
1
0 kJ
0.1
5.7 kJ
0.01
11.4 kJ
0.001
17.1 kJ

The impact of the concentration has an important consequence for biological reactions. All reactions involving water as a reactant (hydrolysis) will be driven by the high concentration of water in biological systems (55.5 M). Thus, hydrolysing a nutrient polymer such as starch or protein into its building blocks (sugar and amino acids) will always be favored and have a negative free energy (Table 2). Moreover, breaking a polymer down into individual building blocks also increases the entropy of the products.

To calculate the actual free energy using in vivo concentrations of reactants and products the following formula is used:

In this equation [A][B][C][D] are the prevailing in vivo concentrations, while the energy under standard conditions is incorporated into  ΔG0′.

Full combustion of organic material is associated with a much larger negative free energy than hydrolysis, because we are starting with almost 100% reactants (for example cellulose) and end up with almost 100% products (CO2 and H2O). Please remember that the law of mass conservation applies. In the reaction C6H12O6 + 6O2 → 6CO2 + 6H2O the two substrates have the same weight as the two products. The products are much more stable than the substrates, because of a favourable electron configuration and because we are generating a large amount of gas molecules (H2O vapour and CO2), which have a far more random arrangement in space (higher entropy) than a solid block of wood.

Italian physiologist Santorio Santorio (1561-1636) was the first to quantify nutrient intake and waste production by spending a considerable part of his adult life on a large balance of his own construction. He consistently found that the weight of his food and drinks considerably exceeded the total weight of his faeces and urine. He correctly inferred that the body gave off some invisible substance, which we now know to be H2O vapour and CO2.

Fig. 5 Santorio Santorio consuming a meal while sitting on a balance.

In mathematical terms the change of electron configuration and the change of order can be summarised as

 △G=△H−T△S

The change of free energy is determined by the change of the electron configuration △H, which can be measured as heat and the change of entropy △S (which is temperature dependent). The entropy of a system can be judged by the number of arrangements the molecules in the system can have. Accordingly, gas molecules have a much more random localisation than molecules in a solid material.  To determine H the enthalpy of the substrates is subtracted from the enthalpy of the products. In a favourable reaction the substrates have a higher energy than the products, thus the difference will be negative. In a favourable reaction the entropy will increase, thus a negative sign is used before the entropy term.  

Watch you own metabolism!

Table 2 shows that burning of nutrients generates H2O and CO2 . You are burning nutrients right now. Exhale onto the screen of your mobile phone and it will fog up. This is water vapour generated by the combustion process. CO2 can be detected by forming carbonic acid in water. Watch the video below to see the demonstration.

Design an experiment

Design an experiment to quantify the amount of carbon dioxide in breath.

Answer

There are many ways to achieve this. The video shows that CO2 acidifies a slightly buffered solution. You could measure the pH change and using the Henderson-Hasselbalch equation calculate the amount of CO2 dissolved in the water. In a more sophisticated way CO2 can be measured directly by infra-red spectroscopy or Mass-spectrometry.

Oxygen is very electro-negative and a strong acceptor of electrons. During oxidation carbon becomes carbon dioxide and hydrogen becomes water. The electron configuration of these products is much more stable (low energy) than that of the reactants (organic nutrients).

  • Our major nutrients are carbohydrates, fat and protein. We need to consume sufficient amounts to cover our energy demands and unavoidable losses. The amounts required can be readily estimated using simple formulas.
  • Full oxidation of nutrients provides large amounts of energy.
  • The general principle of human metabolism is the conservation and storage of the energy available through oxidation of nutrients.
  •  Only reactions that are far away from equilibrium can provide free energy for work.
  1. 80 trading 24 [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
  2. Fig. 2 By the author using ChemDraw
  3. Fig. 3 By the author using ChemDraw
  4. Fig. 4 By the author using ChemDraw
  5. Fig. 5 CC BY 4.0 Wikimedia