You can’t get a prescription for it, and insurance companies don’t recognize it as a disease, but sarcopenia, the gradual loss of muscle during aging, is a condition that many of us will confront at some point in our lives (see prior blog for a general description of sarcopenia, the unusual models that teach us about it, and how caloric restriction protects mice from it). Loss of muscle strength and function puts a person at an increased risk for age-related morbidities and mortality. Yeah, it’s a big deal.
While scientists and doctors recognize the dangers of sarcopenia, what happens at the cellular level in humans is still being debated. Simplistically, to maintain muscle mass the amount of muscle breakdown, or “catabolism”, and synthesis, or “anabolism”, must be equal. With sarcopenia, the rate of muscle catabolism exceeds the rate of anabolism. The general consensus among experts is that the rate of muscle protein synthesis decreases during normal aging, while muscle protein breakdown does not change. The rate of muscle protein synthesis is not constant; it increases in response to so-called anabolic stimuli. Anabolic stimuli can take the form of pharmaceuticals, such as testosterone or other steroids, macronutrients, such as dietary protein, and exercise, such as strength training. As we age, our ability to increase muscle protein synthesis in response to anabolic stimuli is reduced, a physiological phenomenon termed “anabolic resistance”.
Here’s some evidence in support of this phenomenon: A group of researchers in Scotland took 44 healthy men and gave them different amounts essential amino acids, such as leucine, which is a frequently used anabolic stimuli in the lab. They found that equal amounts of essential amino acids did not stimulate muscle protein synthesis equally in older and younger subjects; older subjects were resistant to the leucine stimulus, which may contribute to the loss of muscle mass during aging. This result was independently verified in separate study by a different lab. In another study, researchers tried varying intensities of strength training, and got similar results: there is less muscle protein synthesis in older versus younger men in response to the same amount of strength training. Worse yet, making older men inactive from bed rest further decreased their response to essential amino acids. Why does this happen? Well there are many possibilities with supporting data. Reduced anabolic cellular signaling, reduced muscle or gut uptake of amino acids, and reduced blood flow to muscles are all feasible hypotheses for why older individuals have anabolic resistance.
Regardless of the mechanism for anabolic resistance, there are several proven interventions known to overcome anabolic resistance. The first potential solution is very simple: eat more protein as you get older. The amino acids that are the building blocks of protein act as pro-anabolic factors and stimulate muscle growth. Protein is not the only nutrient or supplement that can help muscle protein synthesis. Fish oil may help as well. After eight weeks of fish oil supplementation older men and women increased their muscle protein synthesis rates in response to essential amino acids and insulin. The exact reason that this occurs is unknown but the authors speculate that it’s because omega-3 fatty acids increase the anabolic signaling response and reduce inflammation. There are vegetarian sources of omega-3 fatty acids that have many benefits such as flaxseed oil, but it’s still unknown whether they have the same effect as fish oil on anabolic signaling.
The last intervention is my old standby: exercise. When older subjects perform one bout of strength training, they display elevated levels of muscle protein synthesis for the next 3 days. Even low intensity treadmill walking can enhance the anabolic effects of insulin in older subjects. In fact, some researchers think that physical inactivity itself, not aging, is the real culprit in anabolic resistance. Remember that these studies don’t show that strength training alone can overcome anabolic resistance, but that strength training and proper nutrition is enough to restore normal muscle protein synthesis rates in older subjects.
You may be inspired to eat more protein, take fish oil supplements, and exercise based on what I’ve written, but there are a few caveats from these studies. First, they were mostly tested with men. Second, they were done in older adults (>75 yr old typically). And third, they were done in a relatively small number of people (typically 10-50, a small number to make general recommendations to a diverse, large population) because acute measurements of protein breakdown and synthesis in humans are extremely difficult and time intensive. So while we can generally recommend a healthy diet and appropriate exercise to overcome anabolic resistance, consult with your medical provider before making any significant lifestyle changes.