Ketosis – advantaged or misunderstood state? (Part I)
As Verdant Herbals approaches its first birthday in about a month, I figured it was as good a time as any to put together some thoughts on a subject I get asked about with great frequency. (For those wondering when I’ll get to Part X of The Straight Dope on Cholesterol, the answer is, “hopefully before the end of the year.”) A few months ago I was planning a post along the lines of “the 10 things you need to know about ketosis,” but I’m now thinking that might be putting the proverbial cart before the horse. So, let’s start with a more fundamental set of questions. In part I of this post I will see to it (assuming you read it) that you’ll know more about ketosis than just about anyone, including your doctor or the majority of “experts” out there writing about this topic.
Before we begin, a disclaimer in order: If you want to actually understand this topic, you must invest the time and mental energy to do so. You really have to get into the details. Obviously, I love the details and probably read 5 or 6 scientific papers every week on this topic (and others). I don’t expect the casual reader to want to do this, and I view it as my role to synthesize this information and present it to you. But this is not a bumper-sticker issue. I know it’s trendy to make blanket statements – ketosis is “unnatural,” for example, or ketosis is “superior” – but such statements mean nothing if you don’t understand the biochemistry and evolution of our species. So, let’s agree to let the unsubstantiated statements and bumper stickers reside in the world of political debates and opinion-based discussions. For this reason, I’ve deliberately broken this post down and only included this content (i.e., background) for Part I.
What is ketosis?
Ketosis is a metabolic state in which the liver produces small organic molecules called ketone bodies at “sufficient” levels, which I’ll expand upon later. First, let’s get the semantics correct. The first confusing thing about ketosis is that ketone bodies are not all – technically — ketones, whose structure is shown below. Technically, the term ketone denotes an organic molecule where a carbon atom, sandwiched between 2 other carbon atoms (denoted by R and R’), is double-bonded to an oxygen atom.
Conversely, the term “ketone bodies” refers to 3 very specific molecules: acetone, acetoacetone (or acetoacetic acid), and beta-hydroxybutyrate (or beta-hydroxybutyric acid), shown below, of which only 2 are technically ketones. (The reason beta-hydroxybutyrate, or B-OHB, is not technically a ketone is that the carbon double-bonded to the oxygen is bonded to an –OH group on one side, technically making B-OHB a carboxylic acid for anyone keeping score.)
Now, back to the real question at hand. Why would our body make these substances? To understand why or when the body would do this requires some understanding of how the body converts stored energy (the food we eat or the energy we store in our body, i.e., fat or glycogen) into phosphate donors. For a refresher on this process, please refer to the video in this post, specifically the section from 2:15 to 13:30.
The ATP issue
As you may recall, about 60% of the energy we expend, say 1,800 kcal/day for someone consuming 3,000 kcal/day in weight balance, is purely devoted to keeping us alive by generating enough ATP (“energy currency”) to do 2 things: allow ion gradients to function and allow muscular relaxation. So, obviously we can’t tolerate – literally even for one minute – insufficient ATP production. In fact, one of the most potent toxins known to man (cyanide) exerts its effect on this process by inhibiting the electron transport chain which generates the bulk of the ATP our body produces. Even the most transient interruption of this process is fatal.
Take home message #1: No ATP, even for 1 minute, equals no life.
The brain issue
The brain is a particularly greedy organ when it comes to energy requirement. To put this comment in perspective consider the following: though our brain represents only about 2% of our body mass, it accounts for about 20% of our energy expenditure. (In children, by the way, this may be closer to 40-50% of basal metabolic demand.) So, beyond the ATP issue, above, there is a substrate issue with the brain as neurons derive most of their energy from glucose. While there is emerging evidence that neurons can also oxidize fatty acids directly in small amounts and may even prefer lactate (over glucose), these two substrates do not approach the levels of consumption by neurons that glucose does. So, for the purpose of this discussion, let’s just focus on the need of the body to provide glucose to the brain.
You’ll recall, from the point I made above, that my brain requires about 400 to 500 kcal of glucose per day (100 to 120 gm). You’ll also recall (from the video, above) that I can store about 100 to 120 gm of glucose in my liver. While I can store much more in my muscles, (on the order of about 300 to 350 gm), because muscles lack the enzyme glucose-6-phosphatase, glucose stored in muscle as glycogen is unable to re-enter the bloodstream and is meant for the muscle and the muscle alone to use. In other words, muscle glycogen is a stranded asset of glucose in the body to be used only by the muscle.
So, if I’m deprived of a dietary source of glucose, I depend solely on my liver to release glycogen (a process known as hepatic glucose output, or HGO). How long can HGO supply my brain with sufficient glucose? It depends on a few things that impact both the “source” and the “sink” of glucose. Other competing sinks for glucose (e.g., activity level, thermogenic needs) and sources (e.g., glycerol and gluconeogenic amino acid availability) can make a difference for a while. But, in a state of starvation we’ve only got about one to three days before we’re in trouble. If our brain doesn’t get a hold of something else, besides glucose, we will die quite unceremoniously.
Take home message #2: No glucose for 24-72 hours equals the need for something else the brain can use instead (that is not fat or protein, since neurons can’t oxidize fat and the last thing we want to do is start muscle wasting at a geometric rate).
The Krebs Cycle
This poses a real evolutionary dilemma. We need an enormous amount of energy just to not die, but the single most important organ in our body (also quite energy hungry in its own right) can’t access the most abundant source of energy in our body (i.e., fat) and is, instead, almost solely dependent on the one macronutrient we can’t store beyond a trivial amount (i.e., glucose). Obviously our species wouldn’t be here today, blogging for example, if this were the end of the story. But, to understand how we survived requires one more trip down biochemistry memory lane. In the figure below (also included and described in the video) I gloss over a pretty important detail.
How, exactly, does our body take pyruvate (from glucose) or acetyl CoA (from fat) and generate so much ATP? The answer lies in the beauty of the Krebs Cycle, which feeds into a process called the electron transport chain (or ETC), I alluded to above. Since the adage ‘you can’t get something for nothing’ is as true in biochemistry as it appears to be in life, to get all that ATP (i.e., stored energy in the form of the phosphate bond), we need to give up something. What the ETC does give up, as its name suggests, is electrons. Through a series of redox reactions the ETC trades the stored energy held by electrons going from higher to lower energy states in exchange for the chemical energy stored in the bonds of the third phosphate group on an ATP molecule.
To think of it another way, if you start with stored energy – glucose or fat, for example, which if burned in calorimeter will give off varying amounts of heat – and you’re willing to convert their carbon, hydrogen, and oxygen molecules into another form with less energy – water and carbon dioxide which, if burned, produce very little heat – it’s a fair trade! The ETC is simply the vehicle that allows our body to make the switch.
In a car, by contrast, it’s much simpler. The engine combusts the hydrocarbon (e.g., gasoline) directly and in one flash liberates the heat contained within the hydrogen-carbon and carbon-carbon bonds in exchange for carbon dioxide, water vapor, and a few other things.
If you take a look at the figure, below, you’ll get a sense of the moving pieces involved in this cyclic transfer process. Molecules shuffle back and forth, around the cycle, and kick off spent carbon (carbon dioxide, termed “waste”) and reducing agents (e.g., conversion from NAD+ to NADH) for the ETC.
Under conditions of abundant glucose (and sufficient insulin sensitivity) the brain is primarily converting glucose to pyruvate (left side of figure). Pyruvate is then shuttled into the mitochondria and converted into acetyl CoA with the help of a very important enzyme called pyruvate dehydrogenase (PDH). I’m going to come back to this enzyme, in part II of this series, because this is where the story gets very interesting. Acetyl CoA (which is also a direct byproduct of fatty acid breakdown) is then combined with oxaloacetate and so begins the Krebs Cycle, which generates all the reducing agents to feed the ETC and generate massive amounts of ATP.
Where do the ketones come in?
In the absence of acetyl CoA (several ways this can happen, including substrate shortage, as I’m describing here) we evolved a cool trick. Our liver can make – out of fat or protein, though we muchprefer to use fat so we can spare our protein and prevent severe muscle wasting – something called beta-hydroxybutyrate, one of the 3 ketone bodies I described above.
B-OHB and acetoacetate (see figure below from this paper by Cahill and Veech, 2003) are produced by the liver from long and medium chain fatty acids and released into the bloodstream.
Acetoacetic acid and B-OHB live in reversible equilibrium (on the left), but once acetoacetate is converted to acetone (on the right) there’s no going back.
Now take a look at the figure below, from this 2001 paper. This is another rendition of the figure above showing the Krebs Cycle, but here you can see where B-OHB and acetoacetate enter the picture.
The reason a starving person can live for 40-60 days is precisely because we can turn fat into ketones and convert ketones into substrate for the Krebs Cycle in the mitochondria of our neurons. In fact, the more fat you have on your body, the longer you can survive. As an example of this, you may want to read this remarkable case report of a 382 day medically supervised fast (with only water and electrolytes)! If we had to rely on glucose, we’d die in a few days. If we could only rely on protein, we’d live a few more days but become completely debilitated with muscle wasting.
The graph below, also from the Cahill and Veech paper, shows the blood chemistry of a person starving for 40 days. Within about 3 days, a starving person’s level of glucose stops falling. Within about 10 days they reach a steady-state equilibrium with B-OHB levels exceeding glucose levels and offsetting most of the brain’s need for glucose. In fact, the late George Cahill did an experiment many years ago (probably would never get IRB approval to do such an experiment today) to demonstrate how ketones can offset glucose in the brain. Subjects with very high levels of B-OHB (about 5-7 mM) were injected with insulin until glucose levels reached 1 mM (about 19 mg/dL)! A normal person would fall into a coma at glucose levels below about 40 mg/dL and die by the time blood glucose reached 1 mM. These subjects were completely asymptomatic and 100% neurologically functional.
The last point I’ll make on the starving patient is that, as you can see in the figure below, the glucose level normalizes at about 65-70 mg/dL (about 3.7 mM) within days of fasting, despite no sources of exogenous glucose. Why? Because with so much fat being converted into B-OHB and acetoacetic acid by the liver, a significant amount of glycerol (the 3-carbon backbone of triglycerides) is liberated and converted by the liver into glycogen. As an aside, this is why someone in nutritional ketosis – even if eating zero carbohydrates – still has about 50-70% of a normal glycogen level, as demonstrated by muscle biopsies in such subjects.
Take home message #3: We evolved to produce ketone bodies so we could not only tolerate but also thrive in the absence of glucose for prolonged periods of time. No ability to produce ketone bodies = no human species.
Last point of background: Everything I’ve just presented is based on data from starving subjects. As most of you reading this know, if one restricts carbohydrate intake, typically to less than about 20-50 gm/day (dependent on timing and carbohydrate composition), and maintains modest but not high protein intake (because protein is gluconeogenic – i.e., protein in excess will be converted to glycogen by the liver), one can induce a state referred to as “nutritional ketosis” with similar physiology to what I’ve just presented without resorting to starvation. Why you’d do this is something I will discuss later.
One other housekeeping issue: Ketosis versus DKA?
In an earlier post I explained the difference between nutritional ketosis (NK) and diabetic ketoacidosis (DKA). If this distinction is not clear, I’d suggest giving this earlier post a quick skim for a refresher. DKA is a pathologic (i.e., harmful) state that results from the complete or near absence of insulin. This occurs in the setting of type 1 diabetes or very end-stage type 2 diabetes, and often as the result of a physiologic insult (e.g., an infection) where the patient is not receiving sufficient insulin to bring glucose into their cells. A person with a normal pancreas, regardless of how long they fast (including the fellow I reference above who fasted for 382 days!) or how much they restrict carbohydrates, can not enter DKA because even a trace amount of insulin will keep B-OHB levels below about 7 or 8 mM, well below the threshold to develop the pathologic acid-base abnormalities associated with DKA. Let me reiterate, it is physiologically impossible to induce DKA in anyone that does not have T1D or very, very, very late-stage T2D with pancreatic “burnout.”
Embarrassing admission: I remember exactly where I was sitting in a clinic at Johns Hopkins in 2002 explaining to (admonishing, really) a patient who was on the Atkins diet how harmful it was because of DKA. I am so embarrassed by my complete stupidity and utter failure to pick up a single scientific article to fact check this dogma I was spewing to this poor patient. If you’re reading this, sir, please forgive me. You deserved a smarter doctor.
In Part II of this post I’ll tackle the questions I know folks still have on their mind (below). Until then, re-read this post to make sure you really understand this physiology. You’re already 10 steps ahead of the next person.
Is there a “metabolic advantage” to being in ketosis?
Are there dangers of being in ketosis?
What are the most important things you need to know about getting into (or staying in) ketosis?