Dr Otter Tackles the Alkaline Diet
Casebook #1 - Bohr Shift and the Alkaline Diet by Dr Otter MD
Our virtual mailbag has been virtually overflowing for the last couple of weeks with questions from you, loyal readers, about diving physiology. We’ve always said we’ll give you access to cutting edge information about the scientific aspects of Freediving, but since the unexpected departure of Terry Tarpon late last year, the Freedive-Earth team has been comprised mostly of halfwits and layabouts, certainly no-one with the gumption to tackle the thorny problems you’ve been posing. That is, until now. Ladies and gentlemen I’m pleased to introduce to you, all the way from…er… Eurasia it’s Dr. Otter MD!
With glasses like these, and a magic wand to boot, it’s pretty obvious that whatever she doesn’t know about diving physiology isn’t really worth knowing. So, on to the first question: Dear Dr Otter, I’ve always been told that acidic blood is good for freediving because the Bohr shift makes more oxygen available for the dive… so why does everyone spend so much time trying to alkalinise their blood beforehand?Confused, Bali This is a really good question, confused. You’re absolutely right, the Bohr shift does make more oxygen available to the body as you can see from this graph:
We can see that for a given partial pressure of oxygen, say 6kPa, the saturation of Haemoglobin with oxygen is lower (68% vs. 80%) when the partial pressure of CO2 (pCO2) is higher:
This is a good thing: Oxygen bound to haemoglobin is no use to us - we want the oxygen to be released to the tissues. CO2 makes it easier for this to happen, not because of something magic about CO2, but because of the effect of a lower (that is, more acidic) pH on haemoglobin.
pH is a measure of the acidity or alkalinity of a solution and is determined by the concentration of Hydrogen Ions (written as [H+]). A low (acidic) pH means a high concentration of H+ and vice-versa. So what does that have to do with CO2? Well, CO2 reacts with water in the blood in a way that releases hydrogen ions:
CO2 + H20 <=> HCO3- + H+
See the extra H+? That’s what makes CO2 an “acidic gas”. When it reacts with water in the blood it releases Hydrogen ions and lowers the pH of the solution. This in turn makes haemoglobin more keen to give up oxygen to the tissues. So far so good.
So if acid is so good for breath-holding, why on earth do some freedivers spend so much time trying to alkalinise their blood?!
Still a good question! The answer lies partly in the effect that a lower pH has on the respiratory centres of the brain (which is, as we’ll see, a bit of a dodgy reason for trying to alkalinise) and partly in the effect of repeated long breath-holds on the body over time (which, as we’ll see, is probably a much better reason).
Acid and the Respiratory System
It’s widely taught that high levels of CO2 trigger the urge to breathe, which is true, but it’s important to understand that it’s actually the pH of the blood that we’re really sensitive to, not the concentration of CO2 itself. As an example, look at what happens in a condition called Metabolic Acidosis where acid from another source: ingestion of strong acid perhaps, or a problem of cell metabolism, results in a lower concentration of CO2 in the blood, but still drives the urge to breathe:
|Normal Range||Metabolic Acidosis|
|pO2||11.3-12.6 kPa||12.6 kPa||Normal|
|pCO2||4.7-6.0 kPa||2.3 kPa||Low|
What’s happened here is that the digestive tract or the cells which are under stress in the body have started to release acids into the blood. The body is very very sensitive to changes in the pH of the blood and so it does everything in its power to keep the pH in the normal range. The easiest way for the body to compensate for metabolic acidosis is to “get rid” of excess hydrogen ions which would otherwise make the blood acidic. The easiest way to do that is to “blow them off” (stop sniggering you at the back) by getting rid of CO2 from the blood through the lungs). As every freediver knows, the best way to do this is to hyperventilate and that’s why, when I see these patients in the Emergency Room of my little riverbank hospital, they’re always puffing away like trains, even though there’s nothing at all wrong with their lungs. When you hold your breath for a short time (i.e. not long enough to seriously effect the partial pressure of oxygen), you create a different situation, called Respiratory Acidosis which, if you’re following ok, you can probably guess looks a bit like this:
|Normal Range||Metabolic Acidosis||Respiratory Acidosis (short time)|
|Urge to Breathe||High||
See how the pH is still low, and the urge to breathe still high? But the problem now is with the buildup of CO2, not with some other sort of acid in the blood. This situation is also very stimulating for the respiratory centre and generates the urge to breathe during a breath-hold. It’s clearly pH that’s driving the urge to breathe, not CO2. As the hold becomes longer, the concentration of CO2 in the blood will rise and the pH will become lower and lower until it gets into the region of around 7.00. Very high acidities like this cause a range of disturbances to the brain and the functioning of pretty much every cell in the body. A pH lower than 7.00 is really not compatible with life for more than a few minutes and once you get into the territory of 6.90 and below, it’s pretty much game over for everybody, no matter how good a diver you are.
All this, by the way, is completely independent of the amount of oxygen you might have in the blood at the time. It’s just a factor of the pH. So, it makes sense, in theory, to start your dive with as high a pH as possible to give you the “space” to get to really high concentrations of CO2 by the end of the dive without blacking out or dying from acidosis. Or does it? The idea that if you start with a higher pH you’ll be able to hold your breath for longer might sound familiar - that’s exactly what we’re trying to achieve when we hyperventilate. By alkalinising the blood we might stave off the urge to breathe for a bit, but really we’re penalising ourselves in the long term because, as we saw right at the beginning, a higher pH makes oxygen (which we haven’t really considered until now) much less available. In the long run, alkaline blood makes oxygen a limiting factor. Hmmm. More concerning than that for enthusiasts of alkalinisation, though, is this: As we’ve already seen, the body is very very sensitive to changes in pH and will do everything possible to maintain it within the narrow range of 7.35-7.45. You might be able to get it up to 7.80 by hyperventilating or down to 7.00 with a long breath-hold, but you’ll be lucky if you can manage either of those things for more than about 20mins. Any changes that take place over a time period of days or weeks will be effectively countered by the kidneys which differentially excrete acid and alkali to maintain the pH of the blood within the normal range, as we’ll see in a minute.
How Does Diet Affect pH in the Body?
Given everything we’ve said so far, the short answer is, really it doesn’t. But that isn’t the whole story. The basic idea of an “alkaline diet” for freediving is that by eating foods which form an alkaline residue in the body after they’ve been metabolised, you reduce the “acid load” from diet and increase the body’s stores of alkali (sometimes called “base”) which means that you’re in a better position to deal with acid generated by breath-holding. Take a look at the CO2 equation again:
CO2 + H20 <=> HCO3- + H+
The double arrow means that the equation also goes in the other direction:
H+ + HCO3- <=>H2O+ CO2
HCO3- is a “base” (an alkali) called bicarbonate. Sodium Bicarbonate (NaCO3) or “baking soda” is the active ingredient in pills for indigestion or “heart-burn” - caused by an excess of acid in the stomach. It works by “neutralising” the acid to something harmless, like water. That’s one of the ways that the kidneys deal with acid in the blood. Tubular cells in the kidney can secrete bicarbonate from the urine back into the blood where it is used to “neutralise” blood born acid, turning it into CO2 which is then breathed out by the lungs. The other mechanism involves the excretion of acid into the urine in the form of ammonium (NH4). Ammonia (NH3) is produced in large quantities in the body as part of the breakdown of proteins and reacts with hydrogen ions like this:
NH3+ + H+ <=>NH4+
The NH4 is then excreted in the urine. So… if you eat an 8oz steak and 2 hours later measure the pH of your urine, you’ll almost certainly find that it’s more acidic than normal. That’s because at least part of the protein that’s broken down in the digestive tract and metabolised in the cells ends up as a mixture of ammonia and acidic compounds called “keto-acids”. That’s why red meat is considered to be “acid forming” in the diet. The kidneys have to deal with the toxic ammonia and excess acid it contains by excreting it as ammonium or producing bicarbonate to turn it into CO2. In short, whatever acid you generate or put into your body has to be balanced by alkali or excreted from the kidneys or the lungs. That includes the acid hidden in fat and protein-rich foods like meat, eggs and cheese and the acid generated through exercise and breath-holding.
So what’s the bottom line?
You can’t really affect the pH of your blood by changing your diet, but you can minimise the acid load that the kidneys and the lungs have to deal with and provide the body with the “base-load” that it needs to neutralise the acids in your diet and those that you produce by breath-holding and exercise. This has got to be a good thing, particularly if you’re training every day. All these processes require energy and create oxidative stress in the cells. Minimising the work your body has to do in dealing with these things will, in theory at least, help your training. As far as a single breath-hold goes, there’s some evidence that alkaline supplements can help to reduce the acidosis involved, but we should bear in mind that
- too much alkalosis is a bad thing for a freediver because it reduces the effect of the Bohr shift
- it’s actually not possible to push up the pH of the blood very far with your diet anyway and
- even if you’ve got extra bicarbonate available for a single breath-hold, all it can do is turn some hydrogen ions into CO2 which has nowhere to go... because you’re holding your breath.
In summary, alkalinisation is probably a good thing in the long run in keeping your body healthy under heavy training, but won’t have a huge effect on a single breath-hold. Got a question for Dr Otter? She’s on the case. Post a comment below or email us at email@example.com. References: Schwalfenberg GK. The alkaline diet: is there evidence that an alkaline pH diet benefits health? J Environ Public Health. 2012; 2012:727630. Epub 2011 Oct 12. Webster MJ, Webster MN, Crawford RE, Gladden LB. Effect of sodium bicarbonate ingestion on exhaustive resistance exercise performance. Medicine and Science in Sports and Exercise. 1993;25(8):960–965. Catch you next time! Dr Otter