Preserving Muscle While Losing Weight: What Amino Acid Supplements Really Do

When people lose weight, it’s not just fat that decreases, but also lean body mass (LBM) — a portion of which is skeletal muscle. Maintaining as much LBM as possible is important for strength, metabolism, and long-term health. The authors of a narrative review in Nutrients looked at whether amino acid supplements — primarily BCAAs and essential amino acids (EAAs) — help better maintain muscle mass across different weight-loss scenarios: diet and exercise, bariatric surgery, and incretin therapy (GLP-1 and tirzepatide).

• Format: narrative review with analysis of mechanisms (mTOR/MPS), preclinical and clinical data.
• Goal: To understand when and which supplements are appropriate if it is difficult to get protein from a daily diet due to a calorie deficit.

Background of the study

Weight loss almost always involves losing not only fat, but also lean mass (LBM), which is critical for metabolism, strength, mobility, and maintaining results. That's why today we talk not only about "how many kilos have gone", but also about the quality of weight loss - how well LBM is maintained. The basic strategies are well known: a high-protein diet and regular strength training reduce LBM loss against the background of energy deficiency. Against this background, there is growing interest in amino acid supplements as a targeted support tool during weight loss.

Mechanistically, the focus is on leucine and BCAA/EAA: leucine activates mTORC1 and stimulates muscle protein synthesis, and can also reduce breakdown; HMB (a leucine metabolite) is also discussed in clinical practice. However, the overall data are still heterogeneous and depend on the population, dose, duration and context (athletes/non-athletes, young/old, initial protein level in the diet). Hence the request for a summary analysis - when exactly and which formulas are appropriate.

A special "real" context is bariatric surgery and incretin therapy (GLP-1/tirzepatide). Here, the total volume and variety of food often decrease, and the share of LBM in total weight loss can be significant (with strict restrictions - up to ~45%). Experts recommend focusing on ~1.5 g protein/kg of "ideal" weight and simultaneously monitoring micronutrients; when it is difficult to "get" protein from food, small-volume EAA/peptide mixtures can potentially come to the rescue.

Finally, what does the overall data show? Preliminary: EAA/peptide formulas help better preserve LBM precisely when dietary protein is insufficient, especially when combined with resistance training. BCAAs alone produce variable results, and when total protein is already sufficient, the effect is minimal. Larger, more standardized studies are needed to assess the clinical significance of preserved LBM and refine optimal protocols.

Why is this important?

In the real world, the proportion of LBM in total weight loss can be significant. In large programs and studies, it varies, but in severe deficits it can sometimes reach ~45% (usually less than the proportion of fat). With GLP-1/tirzepatide therapy, 20-40% of the weight lost can be LBM - with functional indicators usually not deteriorating, and muscle quality (less fat in muscle) even improving. However, monitoring and nutritional support are needed.

What the data says

Most studies agree: if the total protein is normal, the addition of BCAA specifically makes a modest or zero contribution. If there is not enough protein (severe deficiency, early postoperative period, pronounced decrease in appetite for GLP-1), then EAA / hydrolysates help to “reach” the threshold for stimulating muscle protein synthesis (MPS).

• BCAA: More of a "signal" (leucine turns on mTORC1), but without a full complement of EAAs, MPS is not maintained for long. Review conclusion: isolated BCAAs are inferior to complete proteins/EAA when dietary protein is equal.
• EAA/hydrolysates: quickly absorbed, appropriate where protein is not gained from solid food (early period after bariatrics, significant loss of appetite, low energy “quota”).
• Whole proteins (whey/soy): Contains BCAAs along with all the EAAs and is often preferred as a base solution.

Where supplements make the most sense

1) Bariatric surgery
In the first weeks, caloric intake and food volume are severely limited - it is difficult to get enough protein. Here, EAA/hydrolysates + protein-rich meal replacements help to maintain LBM better compared to a diet without such support.

2) Incretin drugs (GLP-1/tirzepatide)
Appetite decreases, mTOR signaling may be partially "muffled", and protein gain is hampered. Experts recommend a guideline of ~1.5 g protein/kg of "ideal" body weight; if this doesn't work, EAA/leucine-enriched formulas around strength training can help "finish off" the anabolic threshold (~2.5-3 g leucine per meal). There are few direct RCTs with BCAA/EAA with GLP-1 yet, but logic and early data favor this approach.

3) Classic "weight loss" through deficit + training
With a moderate deficit (≈−500 kcal/day), a high-protein diet and regular strength training are often enough; supplements provide a minimum "on top". The more severe the deficit and the worse the food tolerance, the greater the benefit from fast-digesting amino acids.

How it works (mechanisms)

Even in energy deficit, leucine can turn on mTORC1 and trigger MPS, but all essential amino acids are needed for stable synthesis. There are some specific features (slower gastric emptying, hormonal/incretin shifts) against the background of GLP-1 therapy and after bariatrics, due to which the forms of protein/amino acids and the time of intake begin to play a big role.

• mTORC1↓ with GLP-1 therapy - observation from recent studies; clinical significance is not yet clear, but decreased appetite makes protein supplementation critical.
• EAA/hydrolysates require minimal digestion and quickly increase the amino acid pool - useful for very low calorie diets.

Practical guidelines (if you are losing weight)

The base without which the supplements won’t “take off”:

• sufficient total protein (with intensive weight loss, the guideline is ~1.2-1.6 g/kg; with GLP-1 - up to ~1.5 g/kg of “ideal” weight),
• strength training 2-3 times a week,
• control of micronutrients (iron, B12, fat-soluble vitamins) while reducing the volume and variety of the diet.

When supplements are appropriate:

• after bariatrics / at the start of GLP-1, when food “doesn’t go down” → EAA/hydrolysates in small portions 2-3 times a day;
• if protein is not “obtained” from food → meal replacement with a high EAA content;
• BCAAs can be used around training, but as a supplement, not a replacement for complete protein/EAA.

Limitations and what's next

This is a narrative review: no meta-analysis and heterogeneous protocols in the included studies. There are almost no head-to-head population RCTs on GLP-1/tirzepatide - the authors call for head-to-head trials of BCAA vs EAA vs hydrolysates with rigorous assessment of mass and function (not just DXA/BIA).

Conclusion

Amino acid supplements are a situational tool, not a magic powder. With moderate deficiency and a good diet, their contribution is modest; with severe dietary restrictions (earlier postoperative window, pronounced decrease in appetite for GLP-1), EAA/hydrolysates help maintain LBM. Always start with the base: protein, strength training, sleep, micronutrients - and add supplements where you "don't get enough" with regular food.

Source: Cannavaro D. et al. Optimizing Body Composition During Weight Loss: The Role of Amino Acid Supplementation. Nutrients. 2025;17(12):2000. doi:10.3390/nu17122000.