For overweight people, diet-based weight loss interventions are a critical factor in restoring health. But dieting alone without concurrent exercise—which is the reality for many—results in loss of skeletal muscle along with the fat. And that can be very problematic.

Some studies indicate that loss of fat-free mass (aka muscle) accounts for between 25%–30% of the total weight lost following an intensive dietary intervention (Turicchi J. et al. Am J Clin Nutr, 2020; Chaston TB, et al. Int J Obesity, 2007). Given the importance of skeletal muscle in metabolic regulation, daily physical function, and overall health, it is vital to find ways to prevent muscle loss while dieting.  

Krill oil—a rich source of omega-3 fatty acids and astaxanthin—can help. A new placebo-controlled study from researchers at the University of Glasgow’s School of Human Nutrition shows that daily supplementation with 1.14 g of krill oil (4 capsules per day, giving 764 mg/day of EPA, and 376 mg/day of DHA), minimized the loss of fat-free mass (FFM) in the context of an 8-week, alternate-day fasting program.

There was a notable difference between the groups in terms of muscle loss. The subjects taking krill oil showed almost no change in FFM from baseline, despite significant total body weight loss. In contrast, those on the placebo showed a mean 1.2 kg drop in FFM.

“While losing weight is often a good thing, one of the unintended consequences is that we don’t only lose fat, but we also lose muscle. In this study we have found that krill oil can help to preserve muscle mass and strength as we lose weight,” says study co-author Stuart Gray, PhD. “As maintenance of muscle is very important for our overall health and quality of life, these are extremely exciting findings.”

Medication-Free

The trial involved a total of 41 overweight but otherwise healthy Glaswegians (25 women and 16 men), randomized to take either krill oil or a vegetable oil placebo while following an alternate-day fasting regimen for 8 weeks. The krill oil supplement not only provided DHA and EPA but also 312 mg choline, and 400 mcg astaxanthin, both of which are found naturally in the oil.

The authors note that the placebo was a blend of olive oil (extra virgin, cold-pressed), refined corn oil, refined palm kernel oil, and medium-chain triglycerides, which they claim is “similar to the proportional distribution of fatty acids in a normal British diet.”

All participants were non-smokers, and none were taking any prescription drugs, dietary supplements, or OTC products prior to enrollment. Notably, none were on GLP-1 agonist drugs, and none were following special diets.

The participants took their assigned supplements daily for four weeks before beginning the fasting program, which consisted of alternating 24-hour fast days and ad libitum feeding days. In total, there were 28 fasting days and 28 feeding days.

The fasts were not total abstentions from food. On these days, the patients were permitted to eat a single protocol-defined 500 kcal midday meal (roughly 60 g carbs, 22 g protein, 15 g fat, and 10 g fiber), comprised of common store-bought ingredients. They were also allowed up to 50 kcal worth of banana, blueberries, melon, or grapes, and they could freely drink water, coffee, tea, and non-caloric beverages, including diet sodas.

On ad lib feeding days, they could eat whatever they wanted based on their usual dietary habits, and in their usual portion sizes. But the clinical team encouraged them to drink a lot of water and avoid compensatory over-eating.

Muscle Maintenance

Not surprisingly, the dietary intervention did result in significant weight loss which was essentially equivalent between the two groups. The krill oil group lost a mean of 4.6 kg (± 1.4 kg) from baseline over the 8-week intervention, and the placebo group dropped 4.5kg ( ± 1.9 kg).

But there was a notable difference between the groups in terms of muscle loss. The subjects taking krill oil showed almost no change in FFM (−0.2 ± 0.9 kg) from baseline, despite the significant total body weight loss. In contrast, those on the placebo showed a mean 1.2 kg drop in FFM (± 2.0 kg). The difference is statistically significant (Alblaji M, et al. Obesity. 2025).

FFM showed a functional correlate in terms of hand grip strength, as measured by a handheld hydraulic dynamometer. There was little change in grip strength among the krill oil subjects, but a mean 0.9 kg (± 0.7 kg) decline in grip strength in the placebo group. Those in the krill group also showed a trend toward faster performance of the Chair Rise Test, an indicator of leg muscle strength.

Reduced Inflammation

The researchers measured several serum biomarkers, and found notable reductions in key inflammatory markers among both groups. However, the krill oil subjects showed a greater reduction in TNF-α (−1.4 ± 0.2 pg/mL versus −0.9 ± 0.5 pg/mL), as well as a bigger drop in c-reactive protein (−51.4 ± 25 ng/mL versus −33.5 ± 12.6 ng/mL).

There’s room for questioning the true clinical significance of these differences given that the placebo contained refined corn and palm oils, both of which are considered inflammatory by some researchers. The presence of these oils could potentially have blunted the net anti-inflammatory effect of weight loss in the placebo group.

That said, this is not a reason to doubt the anti-inflammatory potential of krill oil.

“Long chain omega-3 PUFA supplementation during body weight loss may positively influence muscle function by exerting a more pronounced anti-inflammatory effect compared to body weight loss alone.”

There were no major differences between the krill oil and placebo groups in terms of glucose regulation, but there was a meaningful difference in systolic blood pressure. While both groups showed significant and beneficial decreases from baseline, the effect was much greater in the krill versus the placebo group (−9 ± 6 mmHg versus −4 ± 4 mmHg).

Multiple Benefits

In short, addition of daily krill oil to an intensive dietary intervention for weight loss confers multiple benefits, most notably a preservation of FFM.

“Given that body weight loss, and EPA and DHA supplementation are both associated with reductions in inflammatory markers, it can be expected that long chain omega-3 PUFA supplementation during body weight loss may positively influence muscle function by exerting a more pronounced anti-inflammatory effect compared to body weight loss alone,” the authors write.

Earlier studies pointed in this direction, with several trials showing that omega-3 supplementation could improve body composition by reducing fat and increasing FFM. But these studies looked at supplementation alone, in the absence of other weight loss interventions.

Krill oil contains naturally-occurring choline and astaxanthin, which are not found in fish oils. This, too, weighs in favor of krill for maintaining muscle.

This new study, a collaboration between Dr. Gray’s Glasgow-based team, and a group at the Qassim University, Buraydah, Saudi Arabia, is the first to look at the impact of krill oil on body composition and muscle function in the context of a strict diet-based weight loss intervention.

The data suggest that krill oil is highly effective in this context, but it is not a quick-fix. “The benefits from supplementation with krill oil might be expected only when supplementation is longer than 4 weeks and the intake of long chain omega-3 PUFA is reasonably high,” the authors write.

The authors posit several potential mechanisms through which krill oil supplementation could preserve muscle during weight loss. Given that chronic systemic inflammation contributes to a deterioration of lean muscle mass and strength, it could be that the omega-3s from the krill oil prevent this by attenuating inflammation. There’s also some evidence that omega-3s promote muscle protein synthesis, improve mitochondrial function, and enhance neuromuscular function.

Could similar effects be obtained with fish oil-based omega-3 supplements?

While no one has yet done a head-to-head comparison in the context of muscle preservation, there are data indicating that because more than half of the long-chain fatty acids in krill oil are in a phospholipid form, they are therefore more bioavailable than the same fatty acids in fish oils. Presuming that efficacy is largely dependent on bioavailability, this could make krill a more advantageous choice (Ulven SM, Holven KB. Vasc Health Risk Managment. 2015).

Dr. Gray’s Glasgow team points out that krill oil contains naturally-occurring choline and astaxanthin, which are not found in fish oils. This, too, weighs in favor of krill for maintaining muscle. That’s because choline, as a precursor for acetylcholine, is important for maintaining muscle strength. In rodent studies, astaxanthin was able to reduce muscle atrophy and fibrosis—a finding that merits confirmation in human future studies.

Would krill oil have the same muscle-sparing effect in a weight-loss intervention that also included exercise in addition to caloric restriction? This, too, is a worthy subject for future research.

The Glasgow team contends that the question does not in any way compromise the significance of their findings because the truth is, many overweight people who are willing to diet may not be willing to exercise. Thus, their study design “reflects real-world adherence to the alternate-day fasting diet and enhances the external validity of our study, and as such, we consider this a strength.”

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