Week 13: Aerobic Metabolism of Mice, Men, and Elephants. Relative VO2, Thermodynamics, and Lifespan

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1. Fast Metabolism vs Slow Metabolism Part 1: Comparing Mice and Men

Alaskan huskies, mice, and many other smaller animals use relatively way more oxygen than humans do. To fairly compare animals and humans and see who has a faster or slower metabolism – we divide absolute VO2 per minute by an animal’s body weight.

  • Relative VO2 = VO2 per minute/body weight

Assume you weigh 65kg. (143lb)  Assume your absolute VO2 at rest (1 met) is 1/3 liter per minute. (0.3 L =333ml)

  • Relative VO2 = 333ml/min over 65kg.
  • = 5.12 ml/min per kg.

Compared to:

A mouse weighs 25 g  or .00025 kg. Absolute VO2 at rest = 68ml/min.

  • Rel. VO2 = 68ml/min over .00025kg
  • = 27.32 ml/min per kg. This is approximately 5x the amount a human uses at rest.

A fast metabolism is when an animal or human at rest has a higher VO2 adjusted for body weight. (a higher relative VO2)

Elephant weighs 3800kg. Absolute VO2 at rest = 4.5 Liters/min or 4500 ml/min

  • Rel. VO2 = 4500ml/min over 3800kg
  • =   1.18 ml/min per kg.  This is 23% of a human’s O2 consumption when adjusted for body weight.



Huge animals like elephants with slower metabolisms generally have a lower VO2 adjusted for body weight. And vice versa: Animals with faster metabolisms use more oxygen relatively.

2. Fast vs Slow Metabolism Part 2: The Biological Reality of Producing Heat and O2 Consumption. Thermodynamics and Weight Loss Simplified.

1-fireless-cookersPeople with slow metabolisms are like ‘fireless cookers’.

In other words, they radiate less heat compared to people with faster metabolisms. Fireless cookers can eat less calories and still gain weight compared to people who can eat more calories and not gain weight.

The difference is like comparing an LED light bulb to an old incandescent light bulb. The same energy can go in each bulb, but one wastes more heat.

The ‘fireless cooker’ is a person whose calories don’t convert to as much heat compared to ‘thinner people’ – so they save it as mass.

Consider a bird – like a mouse – who must eat enough food to avoid freezing to death overnight.


Most of the calories eaten by a bird do not get converted to weight, but are instead wasted as heat to keep it warm.

A bird that dies from the cold actually may have died of starvation as the main cause.

Birds, mice, and other small mammals use more O2 (adjusted for body weight) and produce more heat in order to survive in cold weather – or just radiate more heat at any temperature compared to larger mammals.

In short, faster metabolisms radiate more heat by using oxygen faster and more explosively  – which is the cause of radiating more heat at any temperature compared to animals with slower metabolisms.

  • The more heat you waste, the less of your calories will be saved as mass.
  • If all the calories you eat were converted to heat – you’d cease to store weight.

Example of using O2 faster and more explosively in a ‘cell space’.

Firing up a charcoal BBQ with liquid oxygen:


3. Crazy Calculations for Fun

How fast would a human heart have to beat to match a mouse’s rate of oxygen consumed at rest?

The answer is 427 beat per minute! I calculated this two ways by hand. (click to enlarge)


4. Lifespan and Metabolism Part 1: Heart Rate, Blood Circulation Speed, Relative VO2

Heart rate determines speed of blood circulation in animals.

Mouse at rest: 600 bpm

Human at rest: 70 bpm

Elephant at rest: 35 bpm

At 600 beats per minute (bpm) it takes about 8 seconds for blood leaving the mouses heart to return to the heart.

This means blood is traveling tremendously faster through a mouse’s capillaries – rushing past the cells where O2 is consumed. A crude drawing of comparing speeds at the cell level looks like this:





We already know the rate of passage of O2 into a mouse’s cell is almost 5x greater than a human’s – based on knowing relative VO2 from the previous section:

Human: 5.12 ml/min per kg

Mouse: 27.32 ml/min per kg


The fast circulation rate and high O2 consumption in small animals is directly related to lifespan.

The types of fat in the cells of animals determines the speed at which O2 is consumed.

The faster O2 is consumed, the shorter the lifespan, explained next section.



5. Lifespan and Metabolism Part 2: The Price of Living Fast is to Die Young

The Price is of Living Fast is to Die Young

Small mammals die at younger ages compared to larger mammals. Why?

  • The answer is because the cells anatomical ‘design’ allow oxygen to pass in faster  – and use more oxygen compared to large animals.


What unique ‘design’ structure of a cell determines the rate of O2 passage?

First, imagine an 8 lane tollway has 8 tollbooths for gates for a heavy stream of cars to pass through. Obviously traffic would pass through slower if there was only one gate available.

The mouses cells basically have more ‘cell gates’ for oxygen to pass through compared to humans, so oxygen passes through in much greater amounts – in addition to the blood speeding by faster.

The human brain is (for practical purposes) comparable to a mouse’s cells because human brain cells consumes more oxygen than any other cell at rest.


What Makes a Cell Gate? (See Grand Schematic)

The Answer is Unsaturated Fat – Specifically Polyunsaturated Fatty Acids (PUFA)

  • The amount of PUFA’s in your body is related to inflammation and ‘free radical chain reactions’  or ‘oxidative stress’ which damage your cells and lead to tumor growth.
  • Omega 3 fatty acids (fish oil: EPA, DHA) and Omega 6 fatty acids are PUFA’s.

The table below shows the direct reduction in lifespan in smaller mammals as a function of the amount of PUFAs in their cells. The term peroxidizability index refers to the amount of PUFA’s in the cells. The more PUFA the shorter the lifespan.


When you eat PUFA’s – from any source, mainly vegetable oils, canola, flax-seed, and fish oils – your cell membranes get made from these fatty acids.

The human brain is the most susceptible tissue in the human body to oxidative stress and free radical damage because of its high PUFA count. (See Circle Schematic)

In short, oxygen damages fatty acids that are not saturated. The more unsaturated, the more unstable the fat is, and all the more easy it is for oxygen to ‘peroxidize’ the PUFAs in cells – or in food.

Saturated fats – especially the short and medium chained fatty acids from dairy fat and coconut oil are preferred fuel for cells, stable, and basically used instantly by the body when you eat them.

  • Gates = PUFA.  PUFA like toll booths on a tollway with 8 lanes. More gates/toll booths allow faster passage.
  • PUFA = Omega 6 and Omega 3 Long Chain Fats (Damages small animals and damages our brain)