What are ketone supplements?

In one line

Ketone supplements deliver ketones, the fuel the body makes from fat, directly by mouth, briefly raising blood ketones without fasting or changing the diet.

What are ketone supplements?
Drinks and powders said to raise ketones without dieting
'Ketone drinks' and 'ketone powders' are sold with the promise of raising blood ketones without a low-carb, high-fat diet. This post explains what ketone supplements are made of, what happens in the body after taking them, and how this differs from ketones raised by diet.
In short
Supplements that deliver ketones directly
Ketone supplements deliver ketones, a fuel the body normally makes itself, directly by mouth. Even without fasting or cutting carbs, blood ketones rise soon after taking them. There are two main types: 'ketone salts' and 'ketone esters'.
Background: ketones and BHB
A backup fuel the liver makes from fat
Ketones are a fuel the liver makes from fat when glucose runs short, and it sends them to the brain, heart and muscles. There are several ketones, and one of them is beta-hydroxybutyrate (BHB for short). Ketone supplements put this BHB into the body.
Ketones the body makes
They rise only with long fasting or very few carbs
For the body to make lots of ketones on its own, a person has to fast for a long time or eat very few carbohydrates, and ketones rise slowly. This state, with ketones built up in the blood, is called 'ketosis'. Ketone supplements skip this process and put ketones in directly.
Ketone salts
A salty powder of BHB attached to minerals
Ketone salts are BHB attached to minerals such as sodium, potassium and calcium. So raising ketones with ketone salts also means taking in a lot of minerals. They taste salty.
Ketone esters
A bitter drink of BHB attached to butanediol
Ketone esters are BHB attached to a substance called 'butanediol', and they are usually taken as a drink. In the gut they split into BHB and butanediol, and the liver turns the butanediol into ketones as well. They taste bitter.
How high do ketones go?
For the body's own form, esters reach almost three times higher
Half the BHB in ketone salts is a left-right flipped (mirror-image) form of the one the body makes. When the same amount of BHB was given, ketone esters raised the body's own form of BHB in the blood almost three times higher than ketone salts. The two forms added together were similar for both.
The effect is short
Back to normal within a few hours
The body's own form of BHB raised by a ketone supplement does not last long. With ketone esters it peaked about 15-30 minutes after drinking and returned to normal within two to four hours, depending on the amount. The flipped form of BHB from ketone salts, however, stayed in the blood for more than eight hours.
Blood glucose
As ketones rose, glucose dropped slightly
After taking a ketone supplement, blood ketones rose and at the same time blood glucose dropped slightly. When many studies were combined, this change was larger with ketone esters than with ketone salts.
Stomach upset
More than two in three who took ketone salts
In a study that gave ketone salts before cycling exercise, more than two in three people reported stomach or gut discomfort. In this study, ketone salts also lowered blood glucose.
Common misconception
Does drinking ketones put you in a fat-burning state?
No. After ketone drinks, 'fatty acids', the pieces fat breaks into when it is used as fuel, actually went down in the blood. In a study that raised BHB by putting it into a vein, the body also slowed the release of fat and cut its own ketone production. Blood ketones are high, but this is different from diet-induced ketosis, in which the body pulls fat out to use.
Summary
Supplements that briefly add ketones directly
Ketone supplements attach BHB to minerals (ketone salts) or butanediol (ketone esters) and raise blood ketones for a few hours without dieting. Even with ketones up, the body actually releases less fat, so this differs from diet-induced ketosis. Related terms: ketones, ketogenic (low-carb, high-fat) diet, MCT oil.