Fitness and Fatness

Fitness, Fatness and Health

If you asked the average person in the street, is diet or exercise better at reducing body fat – I’d be 99% sure people would answer “diet”.

For years I’ve been hearing things like ‘80% diet, 20% exercise’ some nod to Pareto’s law. Or, ‘you can’t out train a fork’ – my personal fave.

And, whilst I don’t disagree that diet has a massive impact on weight, I would caution under-rating exercise and its effect on body composition.

I would like to hear – ‘it depends’ a lot more often because, well it does.

What diet does / doesn’t do

Changing your diet can reduce the energy you put into your body and depending on your choices, can change your insulin levels, protein turnover (muscle), and blood lipid profile.

All this is extremely positive news.  With those changes in the mix, you can lose body fat.

Diet also allows you to make dramatic changes and get dramatic results quickly when compared to exercise.

Reducing your energy intake by 1000 calories a day is impactful, but it isn’t practical in the long term.

The biological adaptations to dieting (namely metabolic compensation) means you’ll need to reduce and reduce energy intake to maintain an effect while your body learns to reduce energy use by moving less, lowering immunity, reducing fat free mass (FFM) and fighting you everyday to get you to eat more. I’m not even mentioning the psychological responses to rapid calorie restriction – which are deep seated in the human brain and primal in nature.

Dieting is extremely difficult to maintain in today’s food environment without working to modify and manage your personal environment (exposures, habits and coping strategies) long term. Without behaviour management techniques, diet is at best temporarily effective.  Dieting, by itself, doesn’t work in the long-term.

What exercise does / doesn’t do

Exercise does burn some energy and depending on the type, tends to increase fat free mass (FFM) which is the more significant energy using part of your body composition.  This means at rest, exercise tends to increase your energy use.

Exercise improves mental health, physical function, bolsters the immune system and reduces inflamation.  In fact, the list of exercise benefits is so long – I won’t even attempt to write it here.  Exercise affects every cell in the body, improving your mitochondrial function(the batteries in almost all cells). Without cellular energy, you are toast. Dysfunction, disease and eventually death occurs.

In general, research shows that fit people live longer, have more health years, have a more positive outlook and are more able to cope with stress.

You can progress exercise incrementally so that more and more energy is burnt and more and more function is gained.  Unlike diet, the metabolic adaptations to exercise progression are inherently positive – you are designed to move and your body appreciates any movement you can give it.

Exercise spills over – it has a ‘life permeability’. Fit people tend to move more often when not willfully exercising.  They tend to engage in more activity at any opportunity because they can cope with it and don’t see it as a chore.

Because exercise places a demand on the body, it cannot be given in big doses, quickly.  If you overreach (too much effort in one bout of exercise) or over-train (too frequent / not recovering from exercise) your body lets you know about it by being sore and you tend to slow down.  So, with exercise, you generally can’t change your body composition rapidly (in weeks) but you can in months and years.  Long-term it’s a winner.

Why does the favouritism for diet over exercise exist?

The top reasons I can fathom for why we favour diet over exercise are;

• We think short term – so we want results now, the sooner – the better.  That means diet fits our ‘rapid results’ mentality and exercise doesn’t. Exercise is a steady long-term play.
• We like simple explanations – the caloric equation is simple and food is labelled with calorie values, so diet fits.  I don’t think the calorie equation is valuable for a behavioural challenge which is what obesity is. But, as humans, we hate the complex – so give me a number on a package and tell me what to eat – diets are simple.
• We are acting from pain – it’s hard to admit, but for most of us, we are not happy with our bodies – and some would even say ‘desperate’ when we are making our choices.  Running from the pain of not being happy leads to bad decisions, that unfortunately repeat.
• We are information exhausted – there is so much diet information out there, and it’s a marketers playground.  They know with a few key images and words they can get us to part with our hard-earned cash for things that won’t work.  It’s exhausting and impractical but profitable for someone.  I see it in the exercise area too, which grinds me, but it’s less effective than diet marketing. Plus, I live in the exercise domain so I can fight the good fight and call out the fads for what they are. I feel a little less frazzled when I see the latest gizmo as a result.
• We believe humans are special – humans have solved so many amazing challenges, that we think we are super smart.  So, it’s just a matter of time before someone solves obesity – with a particular pill or potion or diet. Because of this, when a new diet headline hits, we are more likely to ‘buy-in’.
• We like concrete things not abstract things – so a diet, being a list of meals / snacks / or rules is very concrete – ‘eat this, not that’.  Exercise, goal setting, planning, maintenance and support are all abstract things so diet is more favourable as a construct than exercise.

These are just my brief musings – I’m sure you’ll have yours on reflection too. Feel free to share them in the comments section.

Why exercise may be more important than you think

In this section I’d like to take you on a thought experiment using two examples, in this case both men but you can use women in your mind if that helps – the rationale should hold.

In this thought experiment:

Person One:

• Walks / jogs 10-15 kilometres (2 hours at 7.5 km/h average) around midday (often in the heat) – 365 days a year [ref 6]. That’s circa 20,000 steps a day for those pedometer buffs out there.
• They also complete; 40 minutes a day in a full squat position (4 x 10 min periods), 60 lunges (getting up and down, and traversing terrain), 20 push ups / dips, 20 bodyweight pulls, and 1 kilometre of carrying – every other day.
• They eat as much as they can get (ad libitum diet – at liberty/as desired), as often as they can get it (gather it, kill it), but all from nature’s foods (meat, seafood, grain, root vegetables, fruit, nuts and honey) and minimally processed (that is, heated or mashed / mixed / crushed).

Person Two:

• Sits most of the day – 9 hours [ref 1].
• Takes about 5100 [ref 3] steps a day (3-5 hours of moving from place A to B usually in a building or between rooms).
• Spends the remainder of the time lying down (10 hours).
• They eat 2500 calories a day [ref 4] using the healthy plate guide [ref 5].

What would they look like?

What would the body composition of each person be?

Who would have the healthier immune system, cardiovascular system, skeletal system, and mind?

It’s an interesting thought experiment. Person one is an example from our hunting and gathering ancestors. They look lean, mean, happy machines.

Person two is your very inactive modern day human (office worker, Netflix junkie) but a dedicated dieter as well.  They may look weak, flabby, slow and sad.

Who would fatten up nicely?

Person one could take on calories from almost any source and remain in pretty good shape (and likely health). I would contend they could eat 50% of their calories from any source (think pastries and pizza if you like) and still, with the remainder of their nutrition being roughly okay, they’d be in pretty good nick.

I would even suggest they could eat 50% more calories (3750) and likely be in better health. The reason being, at 2500 calories they would need to adapt by constraining energy use [ref 7-8] therein suppressing reproductive potential and immunity (and lean tissue development and bone density too probably).

Person one is your average 44 year old male hunter gatherer, 170cm tall, very active, and requires 3,391 calories a day to meet energy needs [ref 16].

Person two would only need to look at a pie and a milkshake and it may kill them. It would certainly blow their health and body composition to pieces pretty quickly – not because they don’t burn calories – but because they can’t burn enough. Person two simply doesn’t have enough potential for energy utilisation (at rest and during exercise) to shift the needle.

Person two is a 44 year old male office worker, 170cm tall, very inactive, and requires 2,064 calories a day to meet energy needs [ref 16].

Are there examples in real life?

Inherently we know that these scenarios would hold true in practical terms. Particularly if you’ve been very active at some stage (remember biking to school, playing at morning tea and lunch, doing a sport practise after school, biking home and then resting). Or, you’ve been around someone training hard for a sustained period (2-3 hours, 6 days a week).

As an example, I know of athletes who fit the profile of person one and who don’t eat perfectly at all – in fact I’ve been involved in research where we were stuffing fat shakes down athletes’ gullets to see what would happen to performance (these were endurance cyclists – about a third responded very well to fat shakes by the way).

Charles Barkley (my favourite NBA player of all time) used to eat Maccas breakfasts on the warm up bike at practice (the first practise of the day – with a game every 2-3 days, and conditioning almost every day).

I know the challenge of getting enough energy into a highly fit athlete who trains twice a day, so they don’t fall apart at the seams.  4000 calories and not an ounce of excess body fat on them. The naturally lean ones get sick of eating entirely and struggle to gain and maintain weight.  Body composition is 40-70% genetic [ref 9] after all, and just as it can be hard for some people to lose body fat, it can be difficult for some to put muscle on.

I also know of people who fit profile two. They are very inactive but eat extraordinarily carefully to keep the weight off. Unfortunately they (like you and I) are in a very polluted food environment trying to do this.  It’s akin to trying not to get wet when it’s raining in Auckland.  Or not get blown around when in Wellington.  It’s exhausting.

What are we evolved to do?

So, what I’m saying is; we evolved in an environment where movement was necessary to survive (eat). Abundant and varied activity were the norm.

Hunters gatherers (homo erectus) 2 million years ago became specialist long distance trekkers, carriers and throwers [ref 20].  In tropical settings they dug for 2-3 hours a day with sticks to find tubers.

It is probable that our ancestors developed long distance running ability to complete persistence hunting. Persistence hunting involved run/walking after a galloping animal, for an average of 31kms, until it overheats and passes out – at which time it was clubbed with a rock. Think Kalahari, and about 70% of a marathon distance with 6 of your mates. Followed by a feed of meat and eyeballs.

Interestingly humans have about 2 hours of stored energy (glycogen) and weeks of fat stored on their body. As such, we can exercise at 70-75% of our maximum for about 2 hours before we tire.  That’s the glycogen (storage form of glucose) running out – and it’s where the ‘wall’ occurs in the marathon.

Following the evolutionary rationale, you would expect that these persistence hunters were likely traveling at an average speed of about 10-12km/h for a 2-3 hour hunt (waking at 6.5km/h and running at 13.5 km/h). They had an estimated VO2max of 57.2 [ref 17].  This is higher than the Tsimane (51.1) who live in Bolivia (jungle terrain) but would have been common in 15-40 year old African hunter-gatherers.

I’m not saying we should aim for that level of fitness – what I’m saying is that’s what our biology evolved toward and we need to be conscious of that when we consider what to do about our health. I would posit that it is the extreme loss in fitness that is contributing to many of the illnesses and diseases (including obesity) that we are facing.

Diet contributes, but where extreme inactivity is the norm, diet becomes titanic in it’s impact.

What should be our first step in health?

The question that has been in my head for many years is ‘would we be better to get fit first, then consider if we wanted or even needed to modify our diet’?

For someone who is very unfit, diet is paramount as there’s no chance of using excess energy taken in.  If you’re unfit, your cells are very poor at using fat as fuel, because they are not good at getting oxygen in (more on that in a minute). So, you get fat, and stay fat quite easily.

For someone who is very fit, diet is less important save for the need for enough energy, and enough fibre and nutrients – which given the total food intake for the very active is easier to achieve.

Put simply, whether diet or exercise is more important possibly depends on your current fitness level – or metabolic health (in another tongue).

If I had my choice – unlimited fitness or limited food?

So, if you asked me which world I’d like to see – one where every person could walk/jog 15 kms in 2 hours flat, and could squat, carry, do push ups and lunges etc

Or,

A world where everyone could stick to a 1200-1500 calorie diet to reduce body fat…

I would definitely choose the former as I think it would make everyone happier, healthier for longer, and more engaged in living.

What are we motivated to do?

I don’t believe we ate food to move.  I believe we moved to eat.  We were hunters and gatherers, and later farmers (also an incredibly active lifestyle before machines).

As such, our motivation for movement is significantly less in the modern world, and we are significantly more likely to eat – as that is our primary driver.

With this motivational back drop, we shouldn’t be trying to diet. We should focus on building ample activity and fitness into our lives. Aiming to be as fit as possible which makes food a necessary fuel not an evil to avoid.

We should also be aggressively managing our food environment as we are not well equipped to say ‘no’ to high incentive foods. To put it plainly, the big food companies have us by the donuts.

By the way, the only other activity our ancestors appear to have engaged in voluntarily, was dance or play.

Researchers suggest this was to communicate, to story tell, to prepare, and to encourage human connection and expression. In other words, we move to eat, to communicate and to copulate.

We also probably moved a bit to fight – but I’m including that in communication – just a very physical form!  All said and done, humans without the threat of scarcity or resource loss, are not violent or war-like. We are inherently good when operating within the community fabric in which we evolved.

How unfit are we?

I think one of our issues is – we’ve forgotten what ‘fit’ really is. There’s this notion of ‘fit’ in the modern world (can climb some stairs) and then there’s what we are designed to do – ‘old fit’ I’ll call it.

A measure of aerobic fitness is VO2Max. Below is a comparison of hunter gatherer aerobic fitness (‘old fit’ – in this case the Tsimane tribes in Bolivia) vs the average Canadians.

You can see that from the age of 20 onward (when school is over, and sports participation declines, and many of us enter tertiary study or sedentary work) there is an explosion of de-conditioning. By their mid 20s Canadian women are around 25% less able to produce energy in working cells. Canadian men are around 15% less able to do so when compared to their hunter gatherer counterparts.

To quantify this fitness difference another way, based on the Cooper 12 min run test (a field test used to measure aerobic fitness – where you simply go as far as you can in 12 mins [ref 14]) – a 25 year old female hunter gatherer can travel 2790 metres in 12 minutes, whereas the average Canadian female can cover only 2220 metres in the same time.

The hunter gatherer would be one and a half laps of a standard 400m track ahead – after just 12 minutes. Think about the energy the hunter gatherer can metabolise and use with that superior work output.  And, the amount of fat oxidation that the hunter gatherer does at rest where, as a fitter individual (with a higher proportion of fat free mass), they burn significantly more fat as fuel than their Canadian equivalent.  Can you see why being less fit, means you’re highly likely to be more fat?

I should mention, that between 10-14 almost all first world populations I looked at are slightly fitter than the hunter gatherers – likely due to our school lives, and our sporting / recreation pursuits.  Unfortunately, this is also changing.

The significance is that first world populations go from slightly fitter, to 11-14% less fit than hunter gatherers, in 4 years. Then the decline simply continues. On average, we never regain the fitness we lose and our ability to use fat as fuel, and burn energy is depressed decade upon decade. It doesn’t have to be this way as fitness level is reversible.

For comparision, hunter gatherers at age 65 have aerobic fitness similar to their 11-14 year old selves. Canadian males – not so much.  Canadian males have lost 50% of their aerobic capacity (54.8 to 27.5).

How much fat free mass do we have?

One measure of body composition is FFM (fat free mass).  This is all tissue in the body (bone, connective, muscle, brain) that isn’t fat mass.

FFM is significantly correlated to BMR (basal metabolic rate – how much energy you burn at rest [ref 15]). Below is a comparison of hunter gatherer FFM% (percentage of total mass) vs the average American (I moved countries here as US data was easier to find).

The hunter gatherer female has 20% more FFM (muscle, bone, organ tissue) and 60% less fat mass.  They are fit, and relatively lean.

My proposition is, if I gave more calories to the Tsimane female, given their activity levels, they may gain lean mass and improve immunity / reproductive potential. If I gave more calories to the American female, they’d store it as fat, as their FFM demands are disproportionately low because they are unfit and their body composition suggests the same.

So what actually happens when you are fit?

Here is a very short list of some basic things that happen when you gain fitness:

• Burn more energy overall
• Burn more energy at rest (increased basal metabolic rate)
• More sensitive to food intake (satiety, dramatically improved insulin sensitivity)
• All body system more likely to work as they should (think liver, kidney, bowel, heart, mind, skin – everything)
• Live longer, at any weight
• Use more fat as fuel at all work rates (rest and any level of activity)
• Better bone density, muscle strength, flexibility, posture, ligament and tendon health, and reduced joint inflammation
• Significantly improved mental health (this should be a big hint that activity is a significant part of our jam)

What makes us fit?

There are many aspects of fitness; flexibility, strength, power, endurance – etc.  Here, I’m just going to focus on our ability to produce energy to keep cells working as they should, during rest or exercise – which is mainly endurance type activity.

An ancient friend

Did you know, you have 37.2 trillion cells in your body. Within each they have little batteries that produce most of the energy our cells need.  These batteries are called mitochondria.  Mitochondria are fascinating for these reasons (and more):

 

1. They make up 10% of your body weight – there are lots of them, about 1000 per cell and even more depending on tissue type.

2. They move around your cell and produce energy where it’s needed (like a portable generator) and can make up 25% of the cells mass.

3. They can link up with each other or divide depending on what the conditions are in the cell.

4. They have their own DNA (mtDNA) which comes from your mum (dad’s is thrown away – too risky).

5. They originated when one type of cell (Eukaryote) combined with a bacteria as oxygen became prevalent on earth (really old cells!).

6. They pass mutations from generation to generation within your body – so if mtDNA gets damaged it has a lineage for your future (thought to be one aspect of aging and cancer).

7. They have very impermeable walls so they can use electrons inside their inner membrane to make ATP (all cells energy currency) and not damage the other parts of the cell (think of oxygen and small fires).

8. They use glucose or fats for fuel, so the more of them you have and the better functioning they are – the more fat you can burn.

9. The partial pressure (p50mito [ref 13]) required to move oxygen across the mitochondrial membrane predicts 66% of BMR (basal metabolic rate). Simply put, the better the mitochondria are at pulling oxygen in, the higher your resting metabolic rate (more energy you burn at rest) in all tissues.

10. You can have more mitochondria (density) and/or better mitochondria (function). For best results, you want lots of them, all high functioning (translated – you want a high aerobic fitness and lots of FFM)

11. The more oxygen you get into mitochondria the more life you have in those tissues – so de-oxygenation is dysfunction, disease and eventually death. This is why hyperbaric oxygen treatment is becoming popular for recovery from stroke amongst other ailments.

12. Under stress (exercise) mitochondria that are not functioning well (don’t make the grade) self destruct (called mitophagy) and new mitochondria are built from their parts – this turnover refreshes the energy capacity of the cell improving function and longevity.

13. Different tissues have different volumes of mitochondria with the brain and liver having more (up to 2500 per cell [ref 18]).

I’ll talk more about mitochondria in future articles but the above 13 items (a very limited number when it comes to mitochondria) explain why fitness correlates well with health and longevity.  It appears mitochondria are ground zero for health and the many other benefits of fitness.

Do we lose fitness before we gain fatness?

Okay, so being fit = good, and not being fit = bad.

Motivation to move = low, and motivation to eat = high.

So, if I stopped moving as much as I did before (as we do post teenage years) and gained body fat (because we generally eat the same due to our behavioural patterns), simultaneously losing muscle and slowly losing FFM (bone density, heart size, lung size etc).

And then, eventually, I ate more because I wasn’t as sensitive to food intake (satiety decreased and insulin sensitivity dropped) and I wasn’t getting the dopamine (happy hormone) my brain needed from movement like I was before (you also get a huge cocktail of drugs from exercise – not just dopamine). Plus, food marketing surrounded me with rubbish choices, brightly packaged and constantly promoted.

Then, the natural solution would be to DIET??!!

There is faulty logic here.  You had less mitochondrial (density) and those mitochondria didn’t work as well as before (function). You weren’t as able to burn fat.

At the same time you took in high sugar / high fat foods (for dopamine and the promise of love) and your fat cells multiplied (hyperplasia) and grew (hypertrophy).

Hand in hand your muscles shrunk (atrophy) and you used less energy at rest (FFM dropped, p50 mito dropped, so BMR lowered).

And, we still believe diet is 80% of the solution and exercise is 20%?

I think, at the very least, it depends

Summary:

• Being fit dramatically increases your ability to burn fat as a fuel because your mitochondrial function and density improves.
• Being fit prolongs your health years (the good ones without nagging disease) and total years.
• Improved fitness aids insulin sensitivity therein decreasing hyperglycemic (high blood sugar) events and reducing fat storage (warding off diabetes as well).
• Fitter people use fat more readily at every workrate (including rest) so have a much higher potential to lose body fat.
• Fitness is progressive/regressive, you use it or lose it as your biology constantly strives for optimisation within the environment it experiences – so it’s not too late to start, and it’s always too early to stop.
• Exercise increases bone mass, muscle mass, organ function, blood vessel health, mental health – in fact exercise is the one thing we do that affects every cell in the body – we are designed to move.
• We are not motivated to move but we are motivated to eat – so we must plan as much movement into our lives as possible, that we enjoy, and we must plan to avoid environments and adjust eating habits that do not support our health and activity.

Metabolic rate – post script (PS)

A final remark (rant ;)) …

• If I decreased your energy utilisation in the 37.2 trillion cells in your body by 20% for two decades (compared to your cells when you were a teenager), what do you think would happen?
• Then, that decrease predominantly affected the cells ability to use fat at rest (and you were at rest most of the time), what do you think would happen?
• If I subsequently increased fat mass by 60% (compared to what your fat mass would have been if you were active), what do you think would happen?
• What if the mitochondria that run that big brain of yours (which by itself needs at least 20% of all energy you produce [ref 19]) weren’t functioning as they once were, is there any chance you’d be a little low, forgetful, foggy, and de-motivated?

You can see from the figure above that any decrease in FFM (muscle, skin, bone, gut, liver function, heart function, kidney function, brain function) will disproportionately decrease BMR when compared to fat (adipose tissue).

The math should be rather convincing.

• Organs (6% body weight) use 58% of your energy (they have a very high metabolic rate).  If you don’t move, you don’t push as much blood through your organs. Therein BMR is dramatically lower.
• Muscle (40% body weight) and skin, bone, gut (33% body weight) combined (73%) use 38% of your energy (they have a moderate metabolic rate).  If you don’t move, you lose muscle and bone density, gut motility reduces, and skin renews more slowly.  Therein BMR is lower.
• The 21% of body fat (noting nowadays most people carry 28-40% fat mass) contributes a poultry <5% energy use.

Unless you are very fit already, it is highly likely that the key to your health and body composition is to implement a robust, enjoyable, supported, and regular exercise plan targeting fitness (aerobic and muscle tissue development / retention) above all else.

It may even be 80% of the solution!

References

1. Sitting and steps in Australian working population – ref 1 Miller, R., Brown, W. Steps and sitting in a working population. Int. J. Behav. Med. 11, 219–224 (2004). https://doi.org/10.1207/s15327558ijbm1104_5
2. 19,200 steps a day for 1.89M person – ref 2 https://exrx.net/Calculators/StepDistance
3. 5111 steps per day – ref 3 BASSETT, DAVID R. JR.1; WYATT, HOLLY R.2; THOMPSON, HELEN2; PETERS, JOHN C.3; HILL, JAMES O.2 Pedometer-Measured Physical Activity and Health Behaviors in U.S. Adults, Medicine & Science in Sports & Exercise: October 2010 – Volume 42 – Issue 10 – p 1819-1825
   doi: 10.1249/MSS.0b013e3181dc2e54
4. Calorie intake recommended for men – ref 4 https://www.nhs.uk/common-health-questions/food-and-diet/what-should-my-daily-intake-of-calories-be/
5. Healthy plate guide UK – ref 5 https://www.gov.uk/government/publications/the-eatwell-guide
6. Hunter gatherer energy use – ref 6 https://www.researchgate.net/publication/274317202_Energy_expenditure_and_activity_among_Hadza_hunter-gatherers
7. Constrained TEE model – ref 7 https://journals.physiology.org/doi/full/10.1152/physiol.00027.2018
8. Constrained TEE model – ref 8 Fernández-Verdejo, R., Alcantara, J.M.A., Galgani, J.E. et al. Deciphering the constrained total energy expenditure model in humans by associating accelerometer-measured physical activity from wrist and hip. Sci Rep 11, 12302 (2021). https://doi.org/10.1038/s41598-021-91750-x
9. Body composition heritability – ref 9 Li X, Qi L. Gene-Environment Interactions on Body Fat Distribution. Int J Mol Sci. 2019 Jul 27;20(15):3690. doi: 10.3390/ijms20153690. PMID: 31357654; PMCID: PMC6696304.
10. Body composition heritability children vs adolescents – ref 10 Brener, A., Waksman, Y., Rosenfeld, T. et al. The heritability of body composition. BMC Pediatr 21, 225 (2021). https://doi.org/10.1186/s12887-021-02695-z
11. Average American body composition – ref 11 St-Onge MP. Are normal-weight Americans over-fat? Obesity (Silver Spring). 2010 Nov;18(11):2067-8. doi: 10.1038/oby.2010.103. PMID: 20978478; PMCID: PMC3837418.
12. Hunter Gatherer fitness and body composition – ref 12 Pisor AC, Gurven M, Blackwell AD, Kaplan H, Yetish G. Patterns of senescence in human cardiovascular fitness: VO2 max in subsistence and industrialized populations. Am J Hum Biol. 2013 Nov-Dec;25(6):756-69. doi: 10.1002/ajhb.22445. Epub 2013 Sep 10. PMID: 24022886; PMCID: PMC4142762.
13. Mitochondria and basal metabolic rate – ref 13 Larsen, F. J., Schiffer, T. A., Sahlin, K., Ekblom, B., Weitzberg, E., & Lundberg, J. O. (2011). Mitochondrial oxygen affinity predicts basal metabolic rate in humans. The FASEB Journal, 25(8), 2843-2852.
14. Cooper 12 minute run test calculator – ref 14 https://exrx.net/Calculators/MinuteRun
15. Fat free mass (FFM) relationship to basal metabolic rate (BMR) – ref 15 Luke A, Schoeller DA. Basal metabolic rate, fat-free mass, and body cell mass during energy restriction. Metabolism. 1992 Apr;41(4):450-6. doi: 10.1016/0026-0495(92)90083-m. PMID: 1556954.
16. Body weight planner calculation of male hunter gatherer energy requirements – ref 16 https://www.niddk.nih.gov/bwp
17. Hunter gatherer estimated aerobic fitness – ref 17 Boyd Eaton & Stanley Eaton, “An evolutionary perspective on human physical activity,” Comparative Biochemistry and Physiology, 2003.
18. Mitochondria significance and function – ref 18  Pizzorno J. Mitochondria-Fundamental to Life and Health. Integr Med (Encinitas). 2014 Apr;13(2):8-15. PMID: 26770084; PMCID: PMC4684129.
19. Brain metabolism – ref 19 Article Source: Scaling of Brain Metabolism with a Fixed Energy Budget per Neuron: Implications for Neuronal Activity, Plasticity and Evolution. Herculano-Houzel S (2011) Scaling of Brain Metabolism with a Fixed Energy Budget per Neuron: Implications for Neuronal Activity, Plasticity and Evolution. PLOS ONE 6(3): e17514. https://doi.org/10.1371/journal.pone.0017514
20. Hunter gatherer evolution and energy use – ref 20 Lieberman DE. Is Exercise Really Medicine? An Evolutionary Perspective. Curr Sports Med Rep. 2015 Jul-Aug;14(4):313-9. doi: 10.1249/JSR.0000000000000168. PMID: 26166056.