Burn, baby, burn

Metabolism Matters - Eating, Exercise, and Evolution Impact Our Energy Burn

Photo by Anna Sullivan on Unsplash

Metabolism - loosely, the rate at which we burn energy - increases during childhood and decreases with age, but not as steeply as previously thought; the rate you burn energy responds dynamically to what and how much you eat, the demands of maintaining the integrity of every cell and organ in your body, as well as how active you are (but exercise burns much less fuel than you think!)

Key Takeaways

• There has been a revolution in thinking about our metabolism over the past decade or so, and most physicians and popular health gurus are unaware of it

• Eat less and exercise is more-or-less useless advice for almost everyone.

• Our bodies are designed to limit the calories that can be burned through exercise.

• Physical activity is good for us: of that there is no doubt, and you should take regular exercise.

• Humans evolved a dual strategy of hunting and gathering together, increasing the importance of intelligence, and likely leading to an increase in brain size

• Hunting and gathering together - a cooperative approach to obtaining food - changed much about our biology, including our metabolism

• Measurements are now available of the total daily energy expenditure of people ranging from 8 days old to 90+ years old (~6,400 participants); energy use increases during childhood, peaks in the 20s, and gradually decreases with age

• Decreases in metabolism with age are not as steep as previously thought, and may be due in part to decreases in physical activity and muscle mass as people get older - in other words, these changes depend also on your level of activity - your metabolism is plastic!

Our modern foods were never available in the past

There is a staggering range of ultra-processed, calorie-rich, highly-palatable, and easily-digested foods available now, compared to even 20 or 30 years ago, and especially compared to a hundred or more years ago.

Our supermarkets have thousands upon thousands of product lines, with a diversity of food unavailable even a generation ago. Effectively, the problem of food sourcing has been solved - we have lots of food available in our environment, at moderate cost, but of variable and varying nutritional quality.

Cheap calories are everywhere - all we need do is go to our local supermarket, or, using an app, order up a home delivery. We also we have a modern epidemic of obesity (see also here), suggesting people are consuming many more calories than they expend through bodily maintenance and activity. The chart below shows calorie availability (and weight) has gone up more-or-less everywhere across the world.

Share of men (% of adult population), who are overweight or obese based on body mass index (BMI) - change over time

Now, just think about the difference between the porridge oats eaten by someone in the 1850’s, and something like an easily-available modern doughnut.

Oats were usually prepared and eaten without oils or sugar - just cooked in milk and/or water, and, if you were lucky, a spoonful of honey or jam (or maybe cream or fruit - vintage recipes here!). Full-fat, sugared, deep-fried doughnuts would be an unimaginable treat.

Porridge oats are more difficult to digest and burn, by some considerable margin, than doughnuts, and the energy released by digesting porridge is provided over a longer time-scale for the body to use.

High-fibre foods are good for you for lots of other reasons – promoting gut health, lowering cholesterol levels, assisting the development of a healthy gut microbiome, among other things.

But the key point is that the calories in these high-fibre foods are relatively inaccessible, and take a long time for your body to extract - i.e., you are burning energy in order to get more energy to burn!

Metabolism and Health

Metabolism is the name for the process our bodies use to transform the energy we consume from food into energy used by our cells. Understanding metabolism is essential for understanding how our bodies function and how we can maintain optimal health.

We often think of metabolism in terms of weight gain or loss, but it is a much more complex process, central to our health and well-being. New research by Hermann Pontzer and his colleagues in western and non-western societies has shed light on some of the mysteries of metabolism, including how it changes as we age (Open access version here; the graphic here is superb also).

Pontzer and his colleagues measured total daily energy expenditure of c. 6,400 people (8 days old to 90+ years old), finding that metabolism increases during childhood, peaks in the 20s, and gradually decreases thereafter.

For years, it was believed metabolism naturally decreases as we get older, leading to weight gain and other health issues. Age-related declines in metabolism are not as steep as previously thought, and may be due in part to decreases in physical activity and muscle mass as people get older.

Change activity levels and your food menu in later life, and you will change your metabolism. Your metabolism is not somehow fixed at a particular set-point – it responds (slowly) to what you do, how you behave, and what foods you consume (and to ambient temperatures in the environment – if you’re in a warm environment, you need less calorie-dense foods, compared to a cold environment).

NB: Herman Pontzer has published a wonderful book on metabolism: Burn: The Misunderstood Science of Metabolism. It is science-writing at its very finest, and I can’t recommend it highly enough. I’ve drawn on the thinking and the findings in Burn for this piece also. It is really well-worth reading, for it draws out lessons on our metabolism that are really not well-understood.

Conventional wisdom about metabolism - pah!

Traditionally, metabolism has been thought of as the energy we expend each day, which comes from the food we eat. Evolutionary biologists often think of metabolism as an organism's energy budget, which must be balanced in order to perform essential tasks like growth, reproduction, and bodily maintenance. Humans have a higher metabolic rate and larger energy budget compared to other apes, due in part to our larger brains and longer lifespans.  

A better way to think of things is calories in, plus activity (such as food gathering and preparation, breathing, digestion, pumping blood, movement, and all of the rest of it), plus the utterly essential housekeeping and repair of the 38 trillion or so cells in your body, plus other bodily processes (sweating, excretion, etc.), plus environmental conditions (hot, cold, etc.), all accounting for the energy that we actually burn and store.

What causes metabolism to change over the course of a lifetime?

One factor may be the number of mitochondria in our cells - the energy-producing structures within cells, and their number and activity can affect metabolism. Older people have fewer mitochondria, which may contribute to the decline in metabolism that occurs with age. Additionally, older people tend to have less muscle mass, which can also lead to a decrease in metabolism.

Implications

Cooperative food production, which includes hunting and gathering, was a key factor in the evolution of Homo sapiens, and may have led to changes in our biology, including our metabolism. We catch the animal, butcher it, bring it home, prepare the fire, cook it, and eat it, together. We ground-forage, find honey, harvest fruits and tubers, and bring them home, and prepare them - together. This behaviour requires we deprioritise our immediate needs for calories in favour of the group - and we see this still today. We prefer to eat together, rather than eating alone. And when we celebrate, we do so together - with a meal. Eating is not merely for calories - eating is social. We break bread together, for this is wired deeply into our social brains.

Our metabolism gradually changed from single forager plant, fruit, and nut eating to cooperative hunting and food gathering. This cooperative approach to obtaining food placed a premium on intelligence, and over time brain size began to increase. Cooperative foraging also (probably) led to the development of farming.

One other interesting thing: Pontzer’s work shows metabolism is not consistently higher in men than in women, as previously believed, nor is it necessarily affected by BMI.

Rat diets

There is a particular diet used for feeding laboratory rats for experimental studies of the effects of obesity. This lab diet is known, ironically, as the “Western” diet: it is a high-fat, high-sugar emulsion, apparently with the consistency, texture, and taste of a dense cheesecake[i]; (I’ve never tried it!).

Rats apparently go wild for it, and will eat it to the exclusion of many other activities. Rats don’t easily satiate to the western diet as they would to the separate sugar and fat components of the diet. The mixture is truly the thing. This diet causes changes in the rat brain and body which are also seen in human clinical obesity. These effects are generally malign: hypertension and diabetes follow, among other changes. [ii]

Policy changes that target sugar alone are misguided. Humans, and for that matter, rats, do not freely consume diets that consist solely of sugar; rather, the critical thing is how the sugar is prepared and mixed with other ingredients. Sugar taxes are not by themselves solely the proper means of reducing sugar consumption. Instead, what is needed is a cleverer policy mix, focused on limiting sugar and saturated fat mixes in the diet. Reducing high levels of fat and sugar in food - which make an almost magically-palatable substance when consumed in the correct proportions together – will be key. But bringing other diverse, fibre-rich foods back into our diets is also vital.

The brain networks that control eating are complicated, of course…

Notable also: A new medication effective for weight loss

Semaglutide is a medication effective for weight loss. It mimics the effects of the hormone GLP-1 (involved in appetite control and glucose metabolism). Several controlled studies have now shown semaglutide can lead to significant weight loss in individuals with obesity, and can also improve markers of metabolism such as insulin sensitivity. I expect it will become widely available in future years - but it comes with the caveat that sustainable weight loss also requires lifestyle changes.

I’ll do a piece again on why aerobic exercise is good for the body and brain; but don’t forget weight- and resistance-training - they’re especially good for combating frailty and muscle loss - also have a look below at previous pieces here

Daniel Lieberman - Exercised

and

Let’s Walk (a lot more): Why We Need to Walk, And Now, More Than Ever

---

[i] An illustrative finding: Kanoski, S. E., & Davidson, T. L. (2011). Western diet consumption and cognitive impairment: links to hippocampal dysfunction and obesity. Physiology & behavior, 103(1), 59-68. There are many others, all similarly negative regarding the long-term consumption of the ‘Western diet’: see.

[ii] Some illustrative findings: Dobrian et al. (2000). Development of hypertension in a rat model of diet-induced obesity. Hypertension, 35(4), 1009-1015 (http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.335.6599&rep=rep1&type=pdf); Marques et al. (2016). High-fat diet-induced obesity Rat model: a comparison between Wistar and Sprague-Dawley Rat. Adipocyte, 5(1), 11-21 (https://www.tandfonline.com/doi/full/10.1080/21623945.2015.1061723); Totsch et al. (2016). Total western diet alters mechanical and thermal sensitivity and prolongs hypersensitivity following complete Freund's adjuvant in mice. Journal of Pain, 17(1), 119-125 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4817348/).