Why Is My Blood Sugar High on Keto? The Real Reasons

You have cut carbohydrate to the bone, you are in solid ketosis, and then you check your blood sugar and it reads higher than it did when you were eating bread and pasta. It feels like a betrayal, and plenty of people take it as a sign that keto is doing something wrong. In most cases it is doing something quite normal, and the explanation is more interesting than alarming. Let me walk through why glucose behaves this way on a low-carb diet, when a raised reading is benign, and the few situations where it genuinely warrants a look.

First, set your expectations

A lot of the worry here comes from a mismatch between what people expect and what a healthy body actually does. Your blood needs a certain small amount of glucose circulating at all times, mostly for tissues that lean on it, and the body defends that level whether you eat carbohydrate or not. So fasting glucose on keto does not drop towards zero; it tends to settle into the normal range, and for some people it sits at the upper end of normal or a touch above. A morning reading in the high 5s mmol/L (roughly 100 to 110 mg/dL) after months of clean low-carb eating is a common and usually unremarkable sight. The number on its own tells you very little; the pattern over time tells you almost everything.

Physiological insulin resistance, the glucose-sparing effect

This is the big one, and it has an unfortunate name. After a while on keto, your muscles become reluctant to take up glucose. They are running happily on fat and ketones, so they politely decline the glucose on offer and leave it in the bloodstream for the tissues that really need it, chiefly the brain and red blood cells. The result is a fasting glucose that reads a little higher than you might predict, alongside low insulin. Researchers call this physiological, or adaptive, insulin resistance, and it is a smart rationing strategy rather than a fault. It looks superficially like the insulin resistance of type 2 diabetes, but the cause is opposite: here glucose is being conserved for the organs that depend on it, not piling up because the system is failing. If you want the fuel-switching picture behind this, the piece on being fat-adapted on keto covers how the body makes the change.

The dawn phenomenon

If your highest reading of the day is the one you take first thing in the morning, before you have eaten anything, you are probably meeting the dawn phenomenon. In the pre-waking hours the body releases cortisol and growth hormone to get you ready for the day, and these prompt the liver to release a little glucose into the blood. Everyone does this; it is ordinary human physiology. On keto, where your baseline glucose is low and steady, that early-morning nudge can stand out more than it would on a higher-carb diet, so your 7am fasting figure can be the peak rather than the trough. Testing again a couple of hours later, once you are up and moving, often shows a lower number.

Where glucose comes from when you barely eat carbs

People sometimes assume that eating no carbohydrate should empty the tank, so a normal or raised reading feels impossible. The liver has it covered through gluconeogenesis, making glucose from protein and the glycerol backbone of fats. Importantly, this is largely demand-led: the body makes roughly what it needs rather than flooding the blood because you ate a steak. That is why very high protein meals do not send most people’s glucose soaring the way a bowl of rice would. Gluconeogenesis keeps your essential glucose supply topped up quietly in the background, which is another reason fasting levels hold steady instead of collapsing.

The everyday things that push a reading up

Before you read anything into a single high number, run through the ordinary culprits, because they move glucose far more than most people realise.

• A hard workout. Intense or heavy training triggers a stress response that releases glucose on purpose, to fuel the effort. Testing straight after a tough session can show a surprisingly high figure that means nothing worrying.
• Poor sleep and stress. Cortisol raises blood sugar, so a broken night, a looming deadline or general life pressure will lift your reading. This is worth taking seriously as a pattern; there is more in the guide to keto, stress and cortisol.
• Coffee. Caffeine can nudge glucose up in some people, so a fasting test taken after your first espresso is not truly fasting.
• The cold. A chilly morning or a cold measuring hand can give a higher or simply unreliable fingerstick.
• The meter itself. Home glucometers carry a margin of error of around 10 to 15 per cent, and readings bounce from one finger-prick to the next. One high value in isolation may be nothing more than measurement noise.

When a higher reading actually deserves attention

None of the above should be used to wave away a genuine problem, so here is where to draw the line. A one-off high reading in context, after exercise or a bad night, is not a concern. A fasting glucose that is consistently and substantially high, creeping well above the normal range day after day, is worth investigating, particularly if you also feel the classic symptoms of high blood sugar: unusual thirst, frequent urination, blurred vision or persistent fatigue. The most useful test here is HbA1c, which reflects your average glucose over roughly three months and cuts straight through daily noise; a fasting insulin reading alongside it tells you whether your insulin is low, as you would expect on keto, or inappropriately high. If you have type 2 diabetes and are using keto to manage it, these readings and any medication changes belong with your doctor, and the evidence for that approach is set out in the piece on keto and type 2 diabetes.

What to actually do

For most people the answer is to stop staring at single numbers and look at the trend instead. If you insist on measuring, be consistent: same time, same conditions, before coffee and before exercise, and judge the average rather than the outliers. There is a wider point about how much home monitoring is really worth to a healthy person, which the guide to measuring ketosis gets into. Make sure you are sleeping enough and not running yourself into the ground, since stress and poor rest do more to your glucose than an extra few grams of carbohydrate ever would. And if the trend genuinely troubles you, or symptoms appear, get an HbA1c and a fasting insulin done rather than trying to interpret a twitchy glucometer on your own. In the great majority of cases, a slightly elevated fasting glucose on keto is simply your body being sensible with its fuel rather than a warning light.

This is general information about the ketogenic diet, not medical advice. Keto does not suit everyone; if you are pregnant, on medication, or managing a condition such as diabetes, speak to a doctor or dietitian first.