What Causes Anabolic Resistance?
This Week’s Research Highlight
Background
We have known for some time that people progressively lose muscle mass as they get older, and the rate of this decline accelerates with age.
People in their 50s lose roughly 0.5-2.0% of their muscle mass per decade. In their 60s, this relative loss increases to 4-5%, and in their 70s it approaches 10%. Over time, this contributes to disability and even mortality, due to devastating falls and subsequent fractures.
The reason why we lose muscle as we age is due to a phenomenon called anabolic resistance.
Normally, after consuming protein from food, your body breaks it down into constituent amino acids, which are transported to skeletal muscle to be used for muscle protein synthesis. Randomized trials show that in young healthy adults, consuming 20 grams of protein maximizes muscle protein synthesis for around 4-6 hours. But in older people, this dose is much less effective.
Over time, this blunted response to dietary protein means less muscle building, more muscle breakdown, and ultimately muscle loss.
But why exactly does this happen? And can we do anything about it?
Study
There are many ongoing theories surrounding the etiology of anabolic resistance. But one major factor that likely plays a role is physical activity. Older people are generally much less active than younger counterparts, and we know that muscle disuse reduces the ability of skeletal muscle to use amino acids from the diet. Which actually makes sense from a biological standpoint, right? Muscle is metabolically expensive, why maintain it if you’re not using it?
Ironically, some of the most persuasive evidence for this explanation does not come from research in older subjects. Instead, researchers take healthy young people and impose physical inactivity upon them. Dutch researcher Luc van Loon and his group took healthy young men and equipped them with full leg casts. After five days, rates of muscle protein synthesis after eating dropped by 50% in the cast legs, compared to the free limb. Immobilization, in effect, appears to induce anabolic resistance, even in youthful muscle.
So does this mean, conversely, that exercise could rescue aged muscle from anabolic resistance, and restore healthy anabolic signaling like that of a young person?
To test this hypothesis, van Loon and colleagues recruited 24 healthy elderly men (average age of 74) and 24 young controls (average age 21) who were matched in body weight. Within each age group, subjects were randomly assigned to either an exercise group or a rest group.
All participants visited the lab in the morning. The rest group was asked to just lie down on their back for a half hour, while the exercise group performed a 30-min standardized exercise session, at an intensity customized to each individual based on previous testing.
Then, both rest and exercise participants consumed 20 grams of a special milk protein which was labeled with a tracer so that the researchers could follow the metabolic fate of the amino acids. Blood samples and muscle biopsies were collected to observe how the protein was being handled by the subjects’ bodies, as well as rates of muscle protein synthesis.
Findings
If we look at muscle protein synthesis rates in the rest group, you can see that the elderly subjects did not mount as robust a response to the milk protein, compared to the younger subjects.
MPS rates at rest
- Young: 0.0620 ± 0.0065%/h
- Old: 0.0560 ± 0.0039%/h
MPS rates post-workout
- Young: 0.0719 ± 0.0057%/h
- Old: 0.0727 ± 0.0040%/h
However, exercise enhances muscle protein synthesis in both young and old participants, and the age-related difference is abolished.
So, what we see here is that physical activity makes skeletal muscle more sensitive to amino acids. Notably, this effect lasts longer than you might think, reverberating far beyond just one meal.
One study found that 45-minutes of moderate aerobic activity enhanced muscle protein synthesis rates in older adults for up to 16 hours afterwards. This suggests that just committing to daily movement — sort of like taking a pill every day — could be a powerful treatment for anabolic resistance. And it doesn’t have to be super hard exercise to achieve this benefit.
FInally, if we take a closer look at that particular study, we can gain some insight as to why exactly exercise has this impact. Compared to rest, the exercise group experienced greater blood flow to their muscles, which facilitated delivery of amino acids.
This appears to be mediated by the hormone insulin. Insulin is best known for its role in blood sugar, but it also stimulates amino acid delivery to muscles through its vasodilatory effects. As we get older, we tend to become resistant to the effects of insulin, both with respect to glucose and protein metabolism.
Regular exercise, thankfully, can restore healthy insulin signaling, and perhaps in turn overcome anabolic resistance.
Random Trivia & Weird News
🕸️ Spiders are able to avoid getting tangled in their own webs — by spinning different types of silk.
Spiders emit silk from their abdomen, which many species use to construct webs for ensnaring prey. These webs are coated with droplets of glue, making them terribly sticky (if you’ve ever accidentally wandered through an orb weaver’s web, you are all too aware of this adhesive property).
But spiders reside inside these webs themselves — how do they manage to stay immune from their own trap?
Well, not all spider silk is sticky. If you were to closely examine a spider web, you’d see that the webs are made up of strong structural lines (frame and spokes) radiating from the center, with thinner elastic threads spiraling into the center of the web. Only the latter threads are imbued with glue, and the spider knows where they are and how to avoid them.
In this respect, spider webs are a more complex trap than they appear at a glance. Makes me grateful that spiders aren’t bigger than they are…
Photo credit: FarmersAlmanac.com
Videos We Loved This Week
- Jeffrey Friedman: Obesity causes & treatments– The end of the beginning. Via Cold Spring Harbor Laboratory.
- Mark Mattson: Understanding and counteracting brain aging. Via Brain Ponderings.
Products We Like
Legion Whey + Protein Powder
We just discovered this brand and we have become big fans already. This is whey protein isolate made from truly grass fed milk. The products are very “clean” (no artificial sweeteners, flavors, dyes) and are analyzed for purity.
I particularly like them because they come in a wide array of interesting flavors, like fruity cereal and coconut cream, so you’re bound to find something palatable.
humanOS Catalog Feature of the Week
The Simple Food Diet
This week, we’d like to highlight one of the courses from our Ideal Weight Program, developed by our good friend Stephan Guyenet.
Simple food is exactly what it sounds like – food that is prepared and consumed as close as possible to its natural state, with limited added fats and sugars. These types of food enable you to stay satisfied and stave off hunger, while lowering energy density. Lean protein, importantly, is a pillar of this approach.
In this course, Stephan breaks down the scientific rationale for the Simple Food Diet, what to eat, vital food preparation tips, and other information crucial to lasting success on this plan. For more information, please refer to our How-to Guide for the Simple Food Diet, and the other materials from the Ideal Weight Program.
Wishing you the best,
