A very common question for those of us who eat a keto diet is “Is this [whatever] keto?”. Along with it comes comments like “you mean you can’t eat bread [or whatever]”.
You can google almost any food with the phrase Nutrition Facts to get the carb values. For instance “google banana nutrition facts” returns:
The total carbohydrates for a medium banana is 27g. I subtract off fiber because your body doesn’t digest fiber (although the microbiom in you does) so the next carbs is 23.9g. That’s more than a day’s worth of carbs in one medium banana. So no, bananas don’t fit in keto numbers.
It’s down to carbs
In the end, it’s as simple as the number of grams of carbohydrates. There are no hard and fast definitions of what constitutes Low Carb and what constitutes ketogenic. The technical answer is what amount of carbohydrates you can eat in a day/meal that keep you in ketosis.
What is Ketosis?
But that begs the question of what constitutes being in ketosis. There’s also no accepted range of numbers or measurement methods. But the idea is that your body is using ketones for fuel. Even that is not an absolute since everyone uses some mixture of ketones and glucose. If you do an extended fast your body will generate glucose from your liver which converts fat to glucose in a process called Gluconeogenesis (GNG).
The easiest thing to measure and track are carbohydrates using a scale and an app like Cronometer which can show you the total number of carbs in your day.
Ketones can be tested by three methods; blood, urine and breath. Each of these tests measure chemicals produced as a byproduct of ketosis. There’s some correlation between these three measurements but even that is not absolute.
Nutritional Ketosis Measures Blood Ketones
A widely accepted method is to use blood ketones and measure them against a standard. Except that there is no standard. Often cited is Stephen Phinney’s definition of “nutritional ketosis” illustrated below:
This says that blood ketones in the range of 0.5 to 3.0 mmol are optimal for brain and muscles.
Is this Food Keto?
Using this definition any food which you eat over a meal/day that drops your blood ketones below 0.5 mmol is not keto. But there’s no easy mapping of how many carbohydrates it takes to push someone to below that number. A commonly asserted amount is 20 grams of carbohydrates per day will take a person out of nutritional ketosis. For most people this number will be adequate. For others it may be too high and for others it may be too low.
The Only Way to Know is to Measure
The only way to know for sure is to measure the effect of a particular food or activity on your ketone levels. And testing isn’t cheap at around $5 a test strip. I’ve used the Precision Xtra meter for my measurements but I never actually mapped carb amounts to ketone levels. I mostly tracked the ketone levels vs days of fasting.
Cheaper Way to Measure
A cheaper way to measure is urine test strips. They are around $5 for 50 strips. You pee on a strip and compare the color of the strip to a scale. They work well for most people (at least at the beginning) but are affected by urine concentrations (which is a function of your level of hydration). They are also slow to react to diet changes. They can indicate what your level of ketosis was hours ago.
Another Expensive Way to Measure
I also bought a breath ketone measurement device, the ketonix. It is fairly expensive but can be reused.
The trouble is there’s a messy mapping from breath to blood ketones. Here’s the scatter diagram from breath to blood ketones with the best fit curce. Going up from 0.5 mMol to the line shows that that’s something like 2.0 PPM.
The ketonix has a USB interface and the data can be downloaded to your computer. Here’s a screen shot of one capture showing the level at 5.4 ppm which would be around 1 mMol.
In the end, the best way to be in ketosis is to eat a very limited amount of carbohydrates such as less than 20 grams a day.