Exogenous Ketones Side Effects: Salts, Esters, and Safety
Exogenous ketones promise the metabolic state of a ketogenic diet in a bottle: drink a shot, and blood ketones rise within minutes. They are marketed for endurance, mental clarity, and appetite control. The science is genuinely interesting, but these products come with side effects that labels rarely emphasize. Our editorial team summarizes what is known.
What exogenous ketones are
Ketone bodies — mainly beta-hydroxybutyrate (BHB) and acetoacetate — are fuels the liver produces during fasting, prolonged exercise, or carbohydrate restriction. "Exogenous" simply means they come from outside the body, in a drink or capsule, rather than being made internally.
There are two main categories on the market. Ketone salts bind BHB to minerals such as sodium, potassium, calcium, or magnesium. Ketone esters link BHB to an alcohol, most commonly (R)-1,3-butanediol, creating a compound that raises blood ketones more powerfully. A third group, ketogenic precursors such as 1,3-butanediol alone or medium-chain triglycerides, are converted to ketones by the liver.
Ketone esters can push blood BHB into the range of several millimoles per liter, similar to days of fasting. Salts typically produce smaller and shorter rises, often below 1 mmol/L. This difference in potency also shapes the side effect profile.
Importantly, taking exogenous ketones while eating carbohydrates does not put you into nutritional ketosis in the dietary sense. Blood ketones rise temporarily, but the body's own fat-burning ketone production is actually suppressed during that time.
Digestive side effects
Gastrointestinal complaints are the most consistently reported adverse effects of exogenous ketones in research. In a study by Stubbs and colleagues comparing ketone esters and salts, GI symptoms were common, especially at higher doses.
- Nausea and vomiting: more frequent with esters and when taken on an empty stomach.
- Diarrhea: particularly with ketone salts, likely related to the mineral load drawing water into the gut.
- Abdominal cramps and bloating.
- Belching and reflux: esters have an extremely bitter taste that many people find hard to tolerate.
- Flatulence: often reported with MCT-based precursors.
During exercise, GI distress can directly harm performance. Several cycling studies reported that athletes who took ketone salts or esters during time trials experienced stomach upset that offset any theoretical energy benefit.
Starting with a smaller serving, taking ketones with a small amount of food, and avoiding them for the first time on race day are common-sense measures. Many sports dietitians recommend testing any new product extensively in training.
Individual tolerance varies a lot. Some people notice nothing at all, while others feel sick after a single serving, so a cautious trial is essential.
Mineral load from ketone salts
Ketone salts deliver BHB bound to minerals, and the amount of those minerals is not trivial. To raise blood ketones meaningfully, a person may need multiple servings, each containing hundreds of milligrams of sodium or other cations.
| Product type | Main side-effect concern | Typical BHB rise |
|---|---|---|
| Ketone salts (Na, K, Ca, Mg) | High mineral intake, diarrhea, possible blood pressure effects from sodium | Small and short-lived |
| Ketone esters | Nausea, bitter taste, intoxication-like feeling at high doses | Large, several mmol/L possible |
| 1,3-butanediol | Metabolized like alcohol; may cause drowsiness and impaired coordination | Moderate |
| MCT oil | Cramping, diarrhea, extra calories | Mild |
For people with high blood pressure, heart failure, or kidney disease, a large sodium or potassium load is a real concern. The American Heart Association suggests limiting sodium, ideally toward 1,500 mg per day for many adults, and a few servings of a sodium-based ketone salt can consume a large portion of that budget.
Potassium-based salts present a different risk for people with impaired kidney function or those taking potassium-sparing diuretics or ACE inhibitors, since blood potassium can rise to unsafe levels. Calcium- and magnesium-based salts may cause loose stools and can interact with certain medications.
Always check the "Supplement Facts" panel for mineral content per serving. Products marketed as "BHB" can differ substantially in their mineral makeup.
Effects on blood sugar and performance
Exogenous ketones tend to lower blood glucose acutely. This happens partly because ketones may reduce liver glucose output and partly through effects on insulin. For healthy people this is mild, but it matters for those using glucose-lowering medication.
Performance effects have been disappointing in many trials. An early study by Cox and colleagues in elite cyclists suggested a benefit when ketone esters were combined with carbohydrate. However, subsequent research — including work by Leckey and colleagues — found that ketone esters impaired time-trial performance in some athletes, partly due to GI symptoms.
Some data suggest ketones might blunt the body's use of muscle glycogen or alter acid–base balance, since BHB is an acid. Ketone esters in particular can cause a mild drop in blood pH and bicarbonate, which may not suit high-intensity efforts.
Research on recovery, cognitive function, and appetite suppression is ongoing. Some trials report reduced hunger after ketone esters, but effects on long-term weight loss are not established.
Who should be cautious
People with type 1 diabetes or insulin-dependent type 2 diabetes need particular caution. While exogenous ketones do not cause diabetic ketoacidosis on their own, combining them with insulin deficiency, illness, or SGLT2 inhibitor medications could complicate the clinical picture and confuse ketone monitoring.
Those taking drugs that lower blood sugar should be aware of the risk of hypoglycemia. Monitoring glucose and consulting a physician before use is advisable.
Ketone esters based on 1,3-butanediol can produce an alcohol-like feeling at high doses. They should not be combined with alcohol or used before driving. Pregnant or breastfeeding women, children, and people with liver disease should avoid these products because safety data are lacking.
Athletes subject to drug testing should note that ketones are not prohibited by WADA. However, as with all supplements in the US market, product purity varies, and third-party certification is advisable.
Conclusions of the editorial team
Exogenous ketones reliably raise blood ketones, but their practical benefits for most athletes remain uncertain. Side effects — chiefly digestive — are common and can undermine performance.
Ketone salts carry a substantial mineral load that matters for people with blood pressure, heart, or kidney issues. Ketone esters are more potent but harder to tolerate and pricier.
If you decide to try them, test cautiously in training, read the mineral content, and talk to a clinician if you take medications affecting blood sugar or electrolytes.
Related topics worth exploring include the ketogenic diet for athletes, MCT oil and its effects on energy, and electrolyte balance during endurance training.
References
- Stubbs BJ, Cox PJ, Evans RD, et al. On the metabolism of exogenous ketones in humans. Front Physiol. 2017;8:848.
- Cox PJ, Kirk T, Ashmore T, et al. Nutritional ketosis alters fuel preference and thereby endurance performance in athletes. Cell Metab. 2016;24(2):256–268.
- Leckey JJ, Ross ML, Quod M, et al. Ketone diester ingestion impairs time-trial performance in professional cyclists. Front Physiol. 2017;8:806.
- Evans M, Cogan KE, Egan B. Metabolism of ketone bodies during exercise and training: physiological basis for exogenous supplementation. J Physiol. 2017;595(9):2857–2871.
- Clarke K, Tchabanenko K, Pawlosky R, et al. Kinetics, safety and tolerability of (R)-3-hydroxybutyl (R)-3-hydroxybutyrate in healthy adult subjects. Regul Toxicol Pharmacol. 2012;63(3):401–408.
- Margolis LM, O'Fallon KS. Utility of ketone supplementation to enhance physical performance: a systematic review. Adv Nutr. 2020;11(2):412–419.
