Ok. Ketones are what your body produces so that you can use fat as fuel when you run out of glucose. Low-carb diets feature ketosis which has initial effects including constipation and headaches.
Supplementing with ketones may skip this sudden crash stage and so eliminate such effects.
Where the weight-loss comes in is in a suppression of appetite, which evidence suggests may occur in people who have fasted before taking the supplements (makes sense, if you imagine that hunger is your body recognising it needs to get fuel from somewhere).
It appears that studies around weight-loss are still in the early stages, but this article suggests it's probably not a good idea:
Despite the potential appetite-curbing effects of ketone supplements, their potential weight loss benefits are unknown.
Therefore, ketone supplements cannot be recommended for weight loss at this time. In fact, some evidence suggests that they may even hinder it.
Ketones Inhibit Fat Breakdown
The purpose of the ketogenic diet for weight loss is to produce ketones from stored fat as an alternative fuel source. But if your ketone blood levels become too high, your blood can become dangerously acidic. To prevent this, healthy people have a feedback mechanism that slows down production of ketones if they become excessively high.
In other words, the higher your blood ketone levels are, the less your body produces. As a result, taking ketone supplements may prevent body fat from being used as fuel, at least in the short term.
Ketones Contain Calories
Your body can use ketones as a fuel source, meaning they have calories. They contain about four calories per gram, the same number of calories as carbs or protein.
A single serving of exogenous ketone salts typically contains less than 100 calories, but to maintain a state of ketosis, you’ll need several servings each day. That’s because the effect of ketone supplements lasts only a few hours and thus requires repeated doses throughout the day to maintain a state of ketosis.