Why muscle gets harder to keep after 40, and what leucine has to do with it.

You did not get lazy. Your muscle got harder to talk to.

Anabolic resistance, in plain words

Muscle protein synthesis is the repair-and-build process that a meal switches on. With age, the same meal switches on less of it. Researchers call this anabolic resistance. A 2016 systematic review concluded it can be overcome when exercise and amino acid nutrition are combined, but that the dose, and specifically the leucine content, has to be high enough to clear a threshold (PubMed).

How high? A 2023 systematic review found that the amount of leucine ingested predicted the muscle response after exercise in older adults but not in younger ones, and it did not settle on a single number (PubMed). So we will not give you one either.

What happens when you get the dose right

In a two-week study, older adults eating protein at the recommended daily allowance added 4 grams of leucine to each of three meals. Both resting and post-meal muscle protein synthesis improved (PubMed).

In a 2015 study of sixteen women with a mean age of 66, 3 grams of leucine-rich essential amino acids stimulated muscle protein synthesis comparably to 20 grams of whey protein. The authors wrote that the whey offered no trophic advantage over the amino acids (PubMed). That study is the basis for the line on our box.

The other half: how much protein people actually eat

The current Dietary Guidelines for Americans set a protein goal of 1.2 to 1.6 grams per kilogram of body weight per day. Federal intake data puts women aged 40 to 69 at roughly 66 grams a day on average, which works out around 30 to 45 grams short of that range. Nobody is adding two chicken breasts a day to fix it.

Where a stick fits

Rejuvenate contains no protein. It is a precise serving of essential amino acids, leucine-rich, with 1,000 IU of Vitamin D3, at five calories. One serving activates muscle comparable to 20g of protein†. Two a day, hours apart, gives muscle two separate signals instead of one bigger one. It is the light option for the meal that was mostly carbs, or the afternoon that had no meal at all.

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Sources

1. Anabolic resistance to exercise and amino acid nutrition in ageing: systematic review. Am J Physiol Endocrinol Metab 2016. PubMed
2. Leucine dose and post-exercise muscle protein synthesis: systematic review. Physiol Rep 2023. PubMed
3. Casperson SL et al. Clin Nutr 2012. PubMed
4. Bukhari SSI et al. Am J Physiol Endocrinol Metab 2015. PubMed
5. Dietary Guidelines for Americans 2025 to 2030; NHANES 2021 to 2023 intake data.

† These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Rejuvenate contains no protein.