Doubly Labelled Water is Less Useful For Weirdos

ExFatLoss recently posted an article where he said that he had taken the “gold standard” test of metabolic rate using the “Doubly Labelled Water” method. His results seem a bit suprising. The test says he is burning a bit north of 4600 calories per day whereas he claims to be eating around 2800 calories per day. CICO guys would say this should result in a loss of around a half pound per day, but as you can see from his weight loss chart (below) this did not happen.

WHAT IS HAPPENING?!?!?!??!?!??!?!

I find the Doubly Labelled Water method interesting. I think it works reasonably well on normies. ExFatLoss is not a normie. His diet consists of cream, butter and beef tallow with JUST enough beef to supply basic protein needs (probably a lot less than you think). He describes his lifestyle thusly:

“I am a sedentary, introverted, night owl programmer. My day job is sitting in front of a computer typing, and my hobby is also sitting in front of a computer (and typing this nonsense). I hate cardio. I hate the outdoors. I exercise about 60 seconds per day with my x3 bands, when I don’t have a rest day. I take a lot of rest days.”

The Doubly Labelled Water method

In the Doubly Labelled water method, you drink some water where both the hydrogen and oxygen are Labelled with a naturally occurring (ie safe) isotope. The idea is that the labelled hydrogen is only lost through the body as water when you urinate or in the breath, sweat, etc. The labelled oxygen leaves through that route but ALSO is released as CO2 when it is consumed by your metabolism in the Krebs cycle. Don’t worry, we’ll step through the whole thing.

The labelled oxygen winds up leaving faster than the hydrogen since it has two escape routes. Since the second escape route of oxygen involves being used in your metabolism it is an indirect indicator of metabolic rate. The difference in the rate of loss of labelled hydrogen and labelled oxygen can be used to approximate total caloric burn over 10-14 days.

DNL Steals The Hydrogen

So the Doubly Labelled Water method works pretty well in normies. But there is a known problem where metabolic rate appears lower if the individual is doing a lot of De Novo Lipogenesis. This is because hydrogen molecules are transferred from NADPH to the growing fatty acid by the enzyme Fatty Acid Synthase. So hydrogen has a second escape route as well: being stored as fat.

This has real life consequences. Consider the guru-fattening sessions of the Massa. (Pasquet, 1992) Men ritually overeat MASSIVE quantities of sorghum bread and milk to fatten up to get a wife. Different than modern America. During the Guru Walla, resting metabolic rates as measured via traditional gas exchange (strap a mask on your face and measure oxygen consumed and CO2 released) increase by 45%. But the Doubly Labelled Water method tells a totally different story.

Subject 1 increases his caloric consumption from ~2500 calories per day to ~6600. This is about 70% of calories from carbohydrate and increasing carbohydrate consumption is known to increase your metabolic rate. This increase in metabolic rate is reflected in the measured RMR. However, the total energy expenditure (TEE) of subject one as measured with Doubly Labelled Water sees an apparent DROP of energy expenditure from 3300 calories per day down to 2200. If he’s eating 6600 calories per day and only burning 2200 calories he SHOULD BE gaining 1.25 lbs of fat per day, but he’s only gaining a measly half pound.

The group as a whole has an RMR increase of 45% but an apperent change of zero when measured by doubly Labelled water.

Why? The labeled hydrogen has a second escape route: it is being turned into fat. This decreases the DIFFERENCE in the rate that oxygen and hydrogen are being lost and leads to a low apparent metabolic rate.

CONCLUSION: Measuring the metabolic rates of things doing a lot of DNL is a bad use case for Doubly Labelled Water.

So Why The Increase?

In ExFatLoss’s case we see a rate measured that is HIGHER than expected by Doubly Labelled Water. The most likely reason for this is that the labelled Oxygen has a THIRD escape route that we haven’t thought about yet. Let’s start at Krebs.

Here is the Krebs cycle. This is at the core of how you convert calories into energy (ATP). I’ve highlighted in green the three places where water is consumed.

The oxygen from the water that is incorporated as fumarate is converted to malate pops off as water again after citrate, so that’s not too interesting.

The oxygen molecule that comes in as water to convert cis-Aconitate to D-isocitrate has to go through a full round of Krebs before exiting as CO2 when D-isocitrate becomes aKG. I’m reasonably sure that I’m correct and I’ve highlighted in green where that oxygen resides at each molecule in the cycle. (There’s a small error – I switched to the double bonded O after they flipped the drawing of the molecule, but that should be inconsequential).

This is the classic second exit for oxygen! Water was incorporated into the Krebs cycle, left it’s hydrogens behind and exited as CO2.

But what about the third water, the so-called “ninja” water because it is hiding. It’s the only thing on the diagram that is INSIDE the circle and it’s not bolded like the other waters.

What happens to the oxygen from the ninja water is a bit complicated but it ends up as one of the “end” single bonded oxygens in citrate.

Now that oxygen CAN BE bubbled off as CO2 and that is what often happens in normies. ExFatLoss is, however, NOT a normie. He is self-described fat person on a weight loss journey who is eating a ketogenic diet. This is results in not running Krebs very efficiently. Fat people have low succinate dehydrogenase activity and so they don’t run the full Krebs cycle very efficiently.

Furthermore, pyruvate is normally supplied from glucose, so in a ketotic state ExFatLoss is probably doing a lot of recycling of citrate back into pyruvate.

The process of recycling citrate as pyruvate looks like this. The citrate is exported from the mitochondria and the acetyl-CoA is dumped to (presumably) do some DNL and the pyruvate can re-enter the mitochondria.

If we take a look at the step where citrate becomes oxaloacetate something interesting happens. The enzyme acyl-citrate lyase (ACL) splits off the acetyl group like so:

THE OXYGEN LEAVES AS INORGANIC PHOSPHATE!! Or at least it does half the time: citrate is symmetrical, so the phosphate can grab either O.

This Really Happens

Just to confirm that this is real, I found this:

The δ18O of body water has been shown to vary linearly with the mean δ18O of local meteoric water. This conclusion also holds for the bone phosphate. Thus, δ18O(PO3−4) values of unaltered fossil bones from humans and domestic pigs can be used to reconstruct the δ18O values of local meteoric waters during the life-times of the mammals.

Oxygen isotopes in mammal bone phosphate (Longinelli, 1984)

Bones are made from phosphate. It’s weird to think that your bones are largely made of oxygen, but they are.


So the Doubly Labelled Water calculation gets doubly tricky since the labelled hydrogen gets lost into the growing fat but also since the oxygen gets lost into your bones and/or other sinks of phosphate. If you’re losing hydrogen it makes the Doubly Labelled Water metabolic rate calculation low and if your losing oxygen it makes it look high.

I don’t think ExFatLoss is breaking the laws of thermodynamics. I think he’s “weird”, metabolically speaking. I think Doubly Labelled Water works better on normies than on weirdos. If your a weirdo I would think twice before shelling out $1000 for the Doubly Labeled Water test.

Longinelli, Antonio. “Oxygen Isotopes in Mammal Bone Phosphate: A New Tool for Paleohydrological and Paleoclimatological Research?” Geochimica et Cosmochimica Acta, vol. 48, no. 2, 1984, pp. 385–90,

Pasquet, P., et al. “Massive Overfeeding and Energy Balance in Men: The Guru Walla Model.” The American Journal of Clinical Nutrition, vol. 56, no. 3, 1992, pp. 483–90,

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