Understanding ketosis

Nutritional ketosis is a controversial topic both amongst the public and health professions alike. However, on this page I’m going to avoid the arguments (low-fat diet proponents are somewhat dogmatic) and try and provide a simple overview of what ketosis is, and how it is attained.

Common misconceptions

When I started out, I mentioned my low carb diet plans to three people who worked in health professions. Two of them voiced immediate concern, stating variously that a ketogenic diet would damage my kidneys leading to possible kidney failure, that the ketones are produced by the kidneys, or that ketones are toxic. All of which are either false and/or lack any credible evidence.

  • Ketones are produced in the liver, not the kidneys. There is no evidence that nutritional ketosis places any undue burden on the kidneys in a healthy individual.
  • Nutritional ketosis is often confused with ketoacidosis, an uncontrolled and potential lethal combination of high blood sugar and runaway ketone production that can occur in Type 1 diabetes. Nutritional ketosis is a perfectly healthy metabolic state. Even those on a normal western diet can enter this state after a period of fasting – even overnight.

What are ketones?

When talking about ketosis, the terms ketones and ketone-bodies are sometimes used interchangeably. Ketones are hydrocarbon molecules that can be used as an energy source by many of our organs instead of fatty acids or blood sugar (glucose). The three primary ketones found in our bodies are



Nail varnish remover (incidentally, this is the source of the Atkins diet dog-breath jokes, as we can excrete acetone in our breath).



This is produced by the liver, but will break down to acetone within a short space of time unless it is converted to B-OHB



Also called B-OHB, this ketone becomes the predominant one in serum after adaptation.

The liver produces all three ketones in varying degrees, although the first, acetone, is also produced through decarboxylation (the left hand side of acetoacetate is a carboxyl group – see here for carboxyl groups in fats) of acetoacetate, either spontaneously, or through enzymatic action.

Our muscles can also convert the second, acetoacetate into the third, B-OHB, but more on this in a while.

What is ketosis and keto-adaptation

Keto-adaptation is a complex process whereby our body undergoes various metabolic changes in order to primarily burn fats instead of carbohydrates. It usually starts to occur within a few days of carbohydrate restriction, but doesn’t fully complete for around a month or longer, depending on the individual. The early stages can be tested by using Ketostix (a tradename) urine test strips.

Our cells burn three main fuels – glucose, fatty acids, and ketone-bodies. Various organs prefer certain fuels over others, or are unable to process certain fuels at all. This fuel partitioning is a complex and dynamic system.

  • Skeletal Muscles – Happy to burn glucose (or glucose stored internally as glycogen), ketones, or fatty acids – truly a jack of all trades.
  • Liver – Primarily burns fatty acids and glucose
  • Heart – When at rest, the heart will mainly burn fatty acids, even in non ket-adapted individuals, and when under exercise load it will actually prefer lactate over glucose (lactate being created by muscles under load)
  • Brain – The brain has no way to directly burn fat, and so relies on glucose or ketones for fuel.

The typical western diet, high in carbohydrates, forces our bodies (particularly our brains), to run primarily on glucose. Our constant consumption of more carbohydrates stimulates the release of insulin, which in turn inhibits our ability to mobilise fat from our adipose tissue. It’s a vicious cycle and plays a major role in explaining the current obesity epidemic.

In contrast, a low carbohydrate diet forces the body to find an alternate fuel source for the brain. To counter the falling blood glucose levels, ketones are produced in the liver via a process called ketogenesis. This can be a permanent, perfectly safe and sustainable, state that causes Acetyl-CoA (Acetyl Coenzyme A) in the liver to mediate the breakdown of fatty acids to produce the ketones acetoacetate and beta-hydroxybutyrate (B-OHB). Any excess acetoacetate breaks down to acetone which is then mostly excreted in the breath and urine (giving you that odd, sweet smell).

The acetoacetate and B-OHB are released into the blood stream where they are readily oxidised (burnt for fuel) by organs and muscles – at least in the initial days of keto-adaptation.

This initial phase can cause a loss of energy and lethargy for a few days – this is quite normal. The lower carb intake causes your body to dump sodium (salt), and excess water into your urine. As your sugar reserves slowly dwindle, your liver will start to create ketones to keep things going. Your brain and muscles are quite happy burning ketones, so don’t worry.

Over the coming weeks the muscles burn less and less ketones and instead start to convert the serum (blood) acetoacetate into B-OHB and releasing it back into the blood stream. B-OHB then becomes the predominant serum ketone. At this stage, Ketostix test strips become misleading because they only test for acetoecetate.

Why does muscle tissue begin converting acetoacetate to B-OHB and not using it for fuel? Because B-OHB becomes the primary fuel for your brain. To use Dr Stephen Phinney’s words your brain’s fuel becomes an elegantly choreographed shuttle of fuel from fat cells to liver to muscle to brain.

Our muscles like to burn fatty acids directly, and after a few weeks of adaptation and a prolonged reduction in insulin levels (due to the restriction of carbohydrate), our adipose tissue regains its ability to undergo lipolysis. That is to say, the breakdown of fatty tissue in order for it to be burnt as fuel. Insulin is an important inhibitor of lipolysis – the hydrolysis of triglycerides back into free fatty acids – so it’s easy to see why less insulin flooding our system makes it easier to burn fat.

Human Energy Storage

Once adaptation is complete, your body has ready access to a much bigger fuel tank. If you read about carbohydrates, you will remember that our body’s glucose fuel tank maxes out at around 2000 calories. The keto-adapted individual has ready access to (in my case when I was still 200lb) 60lb of fat – and fat contains around 3500 calories per pound, so that’s, let’s see, 210,000 calories!

For many of us, our bodies are like the truck, above – we keep filling it with glucose, and every time we overfill it, we convert the glucose to fat and store it in the huge tanker. Wouldn’t it make more sense to run our truck straight off the huge fat tank instead of the smaller glucose one?

The advantages of keto-adaptation

There are many anecdotal reports of improved attention/concentration, but some are so common they are worth mentioning along with advantages that have been scientifically proven.

  • Efficient access to a vast energy reserve of body fat
  • Better regulation of blood glucose – in the absence of significant dietary carbohydrate, the liver can maintain a more steady supply of blood glucose through gluconeogenesis
  • Increased insulin sensitivity, allowing the muscles and adipose tissues to take up excess glucose (blood sugar) more readily, vastly reducing ectopic storage of fat (storage of fat in areas other than adipose tissue – e.g the liver). In the case of the liver, this reduction in ectopic fat that improve conditions such as non alcoholic fatty liver disease)
  • Fat and protein as the main diet provide improved satiety (feeling full) – in fact you will feel less hungry in general. There are lots of studies from both sides of the diet fence (low-fat vs low-carb) that will argue opposing views – but I can only speak from experience. I have spent a lifetime eating a bowl of fruit/muesli for breakfast and within an hour or two I was ravenous for more food. Since switching to a low carb diet, I often start with a cup of broth, and only feel the need for a modest snack around mid-day. My blood sugar levels are more consistent, and I believe this has had a marked effect on a previously roller-coaster appetite.
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