But before I go there, let me urge you to just buy The Art and Science of Low Carbohydrate Living, and read pages 164-165. Phinney and Volek have the best description of this that has probably ever been written, and you should really just read it from them. If I could copy these pages verbatim and paste it here, I would. Seriously, it’s only a few bucks and it’s quite literally the book you want to own if you’re interested in low carb ketogenic diets.
OK, while you wait for your book to arrive, let’s dig in…
What ketostix measure
First off, we need to understand what ketostix actually measure, and more importantly, what they don’t. Generally speaking, ketostix measure excess ketones in your urine. They are considered excess, because they are removed from your serum and shunted to your urine by your kidneys. Their caloric content is thereby wasted.
Of the three types of ketones (acetate, acetoacetate, and beta-hydroxybutyrate) produced by your body, ketostix only measure acetoacetate. This is extremely important to understand, because it turns out that your body produces different quantities of these different types of ketones depending on how long you’ve been in ketosis. If you’ve been in ketosis for a while, you’re going to see a reduction in the “intensity” of what you register on your ketostix for two reasons:
- A change in the relative volume of the ketones produced/present in your body
- A reduction in the volume of ketones in your urine as your kidneys reduce the amount they secrete
Both of these are covered below.
Changes in the types of ketones you produce
When you first start your ketogenic diet and you are not yet fully ketoadapted, your kidneys actively excrete two types of ketones into your urine: beta-hydroxybutyrate (not technically a ketone, but it is generally referred to as one in the literature) and acetoacetate. These ketones are created in the liver in a roughly equal ratio (Note: Technically, acetoacetate is created by the mitochondria of liver cells, and from this beta-hydroxybutyrate is created and acetate is produced as a side product. (source)).
When you start restricting carbs, Phinney and Volek assert that at first your muscles use both beta-hydroxybutyrate and acetoacetate for fuel, but after awhile, they begin taking the acetoacetate and converting it to beta-hydroxybutyrate, and returning that to your serum. As mentioned before, beta-hydroxybutryrate is not technically a ketone. It is more of a “ketone reserve. When plasma acetoacetate concentrations begin to decrease, more of it is produced from beta-hydroxybutyrate” (source).
As a result of the continued conversion of acetoacetate to beta-hydroxybutyrate, the serum and urine volume of acetoacetate (the only ketone detected by ketostix) is significantly reduced. Your takeaway point is this: If you stay on your keto diet for long enough, the primary ketone circulating in your serum (and consequentially, present in your urine) is not detected by your ketostix.
Reduction in kidney excretion of ketones
Besides the change in relative volumes of serum ketone types, as you become keto-adapted your kidneys naturally down-regulate the volume of ketones they excrete into your urine. So even if you still had high-serum acetoacetate (which can be detected by ketostix), you’d still be seeing lighter colors on your ketostix because your kidneys are simply not excreting as much of them into your urine as they were previously.
The precise reason for this downregulation is not described by Phinney and Volek, but they speculate it is to prevent wasting the minimal amount of calories contained in urine ketones. Ultimately suggest that it is the result of kidney adaptation to sustained carbohydrate restriction (164-5) and leave it at that.
Even though we lack the sophisticated understanding of why kidneys downregulate ketone secretion into urine, we know that it happens and we know that the volume of acetoacetate in your serum drops as you become more fully ketoadapted.
So quite literally, after a few weeks into your low carb ketogenic diet, you’ll likely start seeing a change in your ketone levels as measured by ketostix. This is completely normal and does not mean you are having trouble with your diet. It doesn’t even suggest that you aren’t in ketosis anymore, because it can’t. Ketosticks are unable to measure the type of ketone that constitutes the majority present in your blood and urine at this point.
This doesn’t mean you have no way to monitor your ketosis: If you really want to know something important about ketones, you’d measure your serum beta-hydroxybutyrate levels. This is possible by using a portable device like the Abbot Laboratories Precision Xtra Glucose and Ketone Monitoring System. If you go this route, you’ll also have to buy their very expensive and proprietary Precision Xtra Ketone Test Strips to use with it. And when I say expensive, I’m serious. You’re looking at ~$5.00 each time you test.
So you can spend the money and keep an eye on your ketone levels by measuring serum levels, or you could just not spend money on ketostix and expensive testing gizmos. As long as you eat 20g of carbs a day (or less), you are pretty much guaranteed to stay in ketosis.
- The Art and Science of Low Carbohydrate Living
- Precision Xtra Blood Glucose and Ketone Monitoring System
- Precision Xtra Ketone Test Strips