Since the 19^th century, lactate has been regarded as a simple waste product of cellular metabolism. In fact, it is still by many people believed to bring about that tickling feeling of burning muscles you get during exhaustive exercise. Yet, within the last two decades, the view of lactate has changed tremendously. Today, lactate is considered to be a key source of energy and an important signaling molecule. It is even regarded to be the driver behind several of the health-promoting effects of physical activity such as appetite control and weight loss, as well as improved learning and memory. However, postdoc Jens Lund and colleagues have identified a major methodological flaw that puts significant parts of previous research findings into question.

Last night, postdoc Jens Lund and colleagues at the Novo Nordisk Foundation Center for Basic Metabolic Research at University of Copenhagen published the paper ‘The Anorectic and Thermogenic Effects of Pharmacological Lactate in Male Mice are Confounded by Treatment Osmolarity and Co-administered Counterions' in Nature Metabolism. The paper is the result of Jens Lund’s PhD project funded by the Danish Diabetes Academy (DDA).

The Overlooked Confounding Factors

Many studies have linked lactate to numerous beneficial effects within e.g. the brain and other organs. However, a large proportion of these claims are based on research where rodents or humans have been treated with concentrated solutions of ‘sodium lactate’, a commercial metabolite salt consisting of lactate bound to sodium. Often, these studies have not taken the counterion (sodium) and the treatment hyperosmolarity into account, but just assumed that any observed effect of ‘sodium lactate’ was due to lactate – and not the sodium (or the combination of the two ions). However, the new paper demonstrates that treatment osmolarity and co-administered sodium can be major confounding factors. 

- Our findings highlight a major flaw in the study of lactate and call for a re-evaluation of the effects attributed to lactate in studies where ‘sodium lactate’ has been administered without controlling for treatment osmolarity and the co-administered sodium. This methodological flaw might also affect the study of other signaling metabolites like ketones, bile acids and fatty acids, which are also commercially available as counterion-bound metabolite salts, says Jens Lund.

The appetite-suppressing and body weight-lowering effects of ‘sodium lactate’ are not driven by lactate

Upper half: Illustrates an experiment in which diet-induced obese mice where treated for one week with daily subcutaneous injections of either ‘isotonic saline’, 2 g/kg ‘sodium L-lactate’, or iso-osmolar 2 g/kg ‘sodium D-lactate’. As shown, treatment with ‘sodium D-lactate’ completely mimicked the hypophagic and anorectic effects of ‘sodium L-lactate’.
Lower half: Illustrates an experiment in which diet-induced obese mice were treated for two weeks with daily subcutaneous injections of ‘sodium L-lactate’ or iso-osmolar ‘sodium chloride’. As shown, such injections of ‘sodium L-lactate’ increase blood lactate in a manner similar to exercise, but the weight loss seen in response to ‘sodium L-lactate’ is uncoupled from this L-lactatemia and mimicked by iso-osmolar treatment with ‘sodium chloride’. Similar results were obtained in mice lacking the lactate receptor, GPR81.

Lactate Does Lead to Weight Loss – But Not for the Reason You Think

Lactate is constantly metabolized and circulates at high concentrations in the blood. Thus, to mimic the exercise-induced surge in blood lactate, large amounts of exogenous lactate has to be administered. This has often been done by dissolving and injecting ‘sodium lactate’ (a powder) in volumes of water that are suitable for proteins and peptides but are very low for metabolites like lactate. Consequently, many studies have treated mice and rats with lactate solutions that are very concentrated and therefore have osmolarities that are more than ten times higher than the osmolarity of tissue fluids.

- Our new paper confirms previous observations that treatments of ‘sodium lactate’ does indeed suppress food intake and increase energy expenditure and fat oxidation, thus leading to a weight loss in diet-induced obese mice. Yet, through mouse studies with proper controls (iso-osmolar solutions of e.g. sodium chloride, sodium D-lactate and mannitol), in combination with behavioral analyses, we demonstrate that these seemingly beneficial effects are not caused by lactate per se, but instead driven by the stress and the malaise caused by injecting hypertonic solutions, says Jens Lund.

The study also indicates that changes in gene expression in cell studies can be confounded by co-administered sodium. Thus, the study found that the increase in expression of uncoupling protein 1 in brown fat cells by ‘sodium lactate’ could be mimicked by the same doses of ‘sodium chloride’. 

Confounding Depends on the Route of Administration

The new paper also highlights that route of administration is an important factor to consider when studying metabolite salts. As such, the metabolic effects of the Krebs cycle signaling metabolite, succinate, has usually been studied by administering large amounts of ‘disodium succinate’ in drinking water. Several groups have reported that such treatment profoundly attenuates weight gain in mice fed a high-fat diet.

Yet, these previous studies have not controlled for the sodium that is also added to the drinking water. Therefore, Jens Lund and colleagues included a control group of mice that ingested water with the same amount of sodium added as that in the group receiving water supplemented with ‘disodium succinate’. The results from this experiment clearly showed that the anti-obesity effect of ‘disodium succinate’ is independent of the co-administered counterion - and thus driven by succinate itself. Thus, confounding from the co-administered sodium and the treatment osmolarity is probably much less of a problem when metabolite salts are administered orally compared to when they are injected and/or infused. This makes sense given that the gastro-intestinal tract is designed to handle ingested solutions with varying osmolarities and sodium contents.

Danish Diabetes Academy Funded Study

It has been known for a couple of decades that muscle secretes signaling molecules and the same is true for brown fat and many other tissues. Lactate is released in large amounts from muscle during exercise and from brown fat in response to cold exposure. Yet, the physiological function of this secreted lactate is less clear. Moreover, blood-borne lactate has been reported to reach the brain to improve appetite control, blood sugar regulation and learning and memory. Given this, Jens applied for a DDA PhD scholarship in 2018 with the aim of exploring lactate’s metabolic effects and role as a signaling molecule between peripheral organs (muscle and brown fat) and the central nervous system. As illustrated by the new paper, Jens Lund ended up revealing an overlooked confounding issue in the field of metabolite research, rather than uncovering novel hormonal effects of lactate.  

'The Anorectic and Thermogenic Effects of Pharmacological Lactate in Male Mice are Confounded by Treatment Osmolarity and Co-administered Counterions', Nature Metabolism, 13 April 2023

Contact Information

Jens Lund, Postdoc
Novo Nordisk Foundation Center for Basic Metabolic Research, Faculty of Health and Medical Sciences, University of Copenhagen

E-mail: jens.lund@sund.ku.dk