Hyperbaric Hyperbole

An Israeli study came out last week that has been described as rejuvenation via hyperbaric oxygen. I’m not taking it very seriously, and I owe you an explanation why.

  • The main claim is telomere lengthening. I used to think of telomeres as the primary means by which aging is programmed, but since the Danish telomere study [Rode 2015], I think that telomeres play a minor role.
  • I think that methylation age is a far better surrogate than telomere length. The study doesn’t mention methylation age, but reading between the lines…
  • I think the study’s results can be explained by elimination of senescent white blood cells. This might explain the observed increase in average telomere length, even without expression of telomerase. 
  • Are there signs of senolytic benefits in other tissues? That’s the big question going forward.

A study was published in the Aging (Albany) last week claiming to lengthen telomeres and eliminate senescent cells in a test group of 20 middle-aged adults using intermittent hyperbaric oxygen treatment. It was promoted as age reversal in popular articles [for example], apparently with the encouragement of Tel Aviv University.

Telomeres as a surrogate marker for aging

Several years ago, I was enthusiastic about the use of telomere length as a measure of biological age. Telomeres shorten progressively with age, and I thought this mechanism provided a good candidate for a mechanism of programmed aging. But when the Rode study came out of Copenhagen (2015), I saw that the scatter in telomere length was too large for this idea to be credible.

I came to think that telomere shrinkage plays a minor role in aging. Around the same time, I became enthusiastic about methylation clocks. Methylation changes with age are correlated far more strongly with less scatter.

So I think that methylation is plausible as a primary cause of aging, and telomere shrinkage, less so.

Telomere length vs age, new data


The Treatment

The air we breathe is only 21% oxygen. Breathing pure oxygen, five times as concentrated as in air, is a temporary therapy (hours at a time, but not days) for people who have impaired lungs. But prolonged exposure to pure O2 can injure the lungs and other tissues as well. Oxygen is highly reactive, and the body’s antioxidant system is gauged to the environments in which we evolved, so oxygen therapy is not to be taken lightly.

Hyperbaric Oxygen Therapy (HBOT) is oxygen at double full strength. The patient breathes pure oxygen at twice atmospheric pressure. If you just put a tube in your mouth with that much pressure, you wouldn’t be able to hold it, or to exhale. But the body can withstand high pressures as long as it’s all around, not just inside the lungs. If you SCUBA dive, at 30 feet below the surface the ambient pressure is two atmospheres, and SCUBA tanks adjust to feed air into your mouth at a pressure that is matched to the surrounding water.

(Incidentally, pressure varies a lot with altitude, so that in Denver it’s 20% lower than New York. Two years ago, I trekked in the Himalayas at 17,000 feet, where the air pressure is only half the standard (sea level) value, and of course there is only half as much oxygen.)

HBOT needs to arrange higher ambient pressure, not just in the oxygen tank. The patient has to be enclosed in a chamber where the ambient pressure is twice atmospheric pressure. Pure oxygen is expensive enough that the ambient air is just normal air at high pressure, and the patient is given oxygen to breathe from a tank. The patient can be in a pressurized room or lying in a personalized chamber.

HBOT has been around for a century, and standard medical uses are for detoxification, gangrene, and chronic infections.  More recently, HBOT has been used with success for traumatic injury, especially nerve damage. There are studies in mice in which HBOT in combination with a ketogenic diet has successfully treated cancer.

In the new Israeli study, subjects received 90 minutes of HBOT therapy 5 days a week for 12 weeks. For 5 minutes of every 20, patients breathed ordinary 21% air. The intermittent treatment was described as inducing some hypoxia adaptations. Apparently, the body adjusts to the high oxygen environment, and then it senses (relative) oxygen deprivation for those 5 minutes.

How does it work?

There is no accepted theory for how HBOT works, so I feel free to speculate. The primary role of a highly oxidative environment is to destroy. That’s probably how HBOT treats infections, since bacteria are generally more vulnerable to oxidative damage than cells of our bodies. Another thing that HBOT does well is to eliminate necrotic tissue, and I wouldn’t be surprised if it turns out to be an effective cancer treatment, since tumor cells thrive in an anaerobic environment. But the body also uses ROS (reactive oxygen species) such as H2O2 as distress signals that dial up chemical protection and repair. This is akin to hormesis, and I’m inclined to think that when HBOT promotes nerve growth, it is via a distress signal.


Authors of the new study make two claims: that telomeres are lengthened in several classes of white blood cells, and that senescent white blood cells are eliminated. Let’s take them in reverse order.

Elimination of senescent cells has been a promising anti-aging therapy since pioneering work of van Deursen at the Mayo Clinic. A quick refresher: telomeres get shorter each time cells replicate, and in our bodies, some of the cells that replicate most (stem cells and their offspring) develop short telomeres late in life that threaten their viability. Cells with short telomeres go into a state of senescence, in which they send out signals (inflammatory cytokines) that increase levels of inflammation in the body and can also induce senescence in adjacent cells, in a chain reaction. Senescent cells are a tiny proportion of all cells in the body, and Van Deursen showed that the body is better off without them. Just by selectively killing senescent cells in a mouse model, he was able to extend their lifespan by about ~25%. But to do the experiment, he had to genetically engineer the mice in such a way that the senescent cells would be easy to kill selectively. Ever since this study, the research community has been looking for effective senolytic agents that could kill senescent cells and leave regular cells alone (without having to genetically engineer us ahead of time).

The new Israeli study demonstrates that senescent white blood cells have been reduced. (Red blood cells have no chromosomes, so they can’t have short telomeres and can’t become senescent in the same way. They just wear out after a few months.) The effect continued after the 60 hyperbaric sessions were over, suggesting that HBOT kills the cells slowly, or damages them so that they die later. Apparently, the reduction was measured by separating different cell types and counting them. There was a great deal of scatter from one patient to the next.

The first claim is that average telomere length was increased in some populations of white cell sub-types. Again, there was a great deal of scatter in the data, with some of the subjects decreasing telomere length and others. For example, when they say that B cell telomeres increased by 22% + 40%, I interpret that to mean that the mean telomere length increased by 22%, but the combined standard deviations from the before and after measurements was 40% of the original length. Hence, a great deal of scatter.

Aside about statistics (With apologies — this from my geeky side)

First, what does that mean 22% + 40% ? How can that be statistically significant? Answer: The standard deviation of a set of measurements is a measure of the scatter. It tells you how broadly they differ from one another. If you’re looking for the average of that distribution, you can be pretty sure that the average isn’t out at the edges, so the uncertainty in the average is a lot smaller than the standard deviation. How much smaller? The answer is the square root of N rule. The “standard error of the mean”, or SEM, is the standard deviation divided by the square root of the number of points, or √N. So the 40% standard deviation gets divided by the square root of the number of subjects in the study, √26=5.1, and “22% + 40%” should really be reported as 22% + 8%. The mean is 22% and the uncertainty in that 22% is 8%.

The way this group did the statistics was based on

  • Finding the average telomere length among 26 subjects after the study
  • Dividing by the average telomere length among 26 subjects before the study

First they average, then they divide.

But it’s well-known (to statisticians) that the most sensitive test is to reverse the operations. First divide, then average. In other words, compare each subject’s telomeres after the study with the same subject before the study. If you do the statistics this way, then the original scatter among the different subjects all cancels out. You can start with subjects of vastly different telomere lengths, and it doesn’t matter to the statistics, so long as each one of them changes in a consistent way.

If you average first (before dividing), the scatter among the initial group imposes a penalty in statistical significance, even though that has nothing to do with effectiveness of the treatment.

So this raises the question: Why did the authors do the statistics this less-sensitive way? They hint at an answer: “repeated measures analysis shows a non-significant trend (F=4.663, p=0.06)” They seem to be saying that the test which normally gives a better p value, in this case gives a worse p value.

That can only happen if the the people who had the longest telomeres at the end of the study were not the same as the people who had the longest telomeres at the beginning.

Here’s what I think is really going on

Telomerase is the enzyme that increases telomere length. We think of telomerase as anti-aging, and supplements such as astragalus and gotu kola and silymarin are gobbled up for their telomerase activation potential. When we think of longer telomeres as a result of a study, we imagine that telomerase has been activated.

But in this case, I think that the average has gone up simply because the cells with short telomeres have been killed off. The authors are telling us that there are less senescent cells as a result of the treatment. Senescent cells are the ones with the shortest telomeres. At the beginning, the average telomere length is an average of a wide range of cells with long and short telomeres. At the end, you have the same long telomeres in the average, but the shortest ones are gone, so the average has increased.

I’m suggesting that telomerase has not been activated. There has been no elongation of telomeres, but the average length has increased because cells with the shortest telomeres have been eliminated.

It’s only a hypothesis, but it might help explain why the people who had the longest average telomere length at the beginning were not the same as the people who had the longest average telomere length at the end. The senescent cells that were being eliminated had no relationship to the telomere length in other cells.

Next steps

One thing I’d like to know is whether the HBOT treatment affected methylation age by any of the Horvath clocks. I’ve written to the authors with this question, and haven’t received a response. Maybe they did the methylation testing and didn’t report the results because they were negative—just a guess.

But even without reprogramming methylation, the therapy can be valuable if it is eliminating senescent cells generally, and not just in white blood cells. An easy first test would be whether inflammatory cytokines in the blood decreased after the treatment. Confirmation would come from the kind of test van Deursen did, assaying senescent cells in different tissues.

If hyperbaric oxygen can be shown to decrease methylation age, that would be a promising finding. If not, but the treatment has general senolytic effects (not just in white blood cells), it may yet have value as an anti-aging treatment. Maybe the authors already know the answers to these questions; if not, they should be busy finding out.