Why is my endurance fading?
Thinking About A Personal Health Record - Part IV: Understanding declining V̇O₂max in active adults and aging athletes.
Quick Summary
Welcome back! The idea of this series was to think about Personal Health Data in a structure that works for you. Because nutrition is a big part of personal health, and because many people already (sort of) understand nutrition data, I started Part I with Food. I identified three phases in American food science: food as fuel, food as a consumer product, and then food as an ideological and tribal battleground.
Part II used nutrition data as an example of how you could keep records, even in a simple notebook, and use it develop very useful information that supports Thinking About Your Own Health.
Part III filled out the hypothetical notebook further, with various biomarker data which might help you figure out how you’re doing at regulating your own nutrition, metabolism, and other aspects of health.
If you’ve been with me through that series of articles about self-assessment of nutrition and metabolism, and you wondered what happened – you’re right, I left you hanging for a while.
The truth is I was a bit stuck! Nutrition and metabolism fit kind of easily into the bigger idea of personal health data. But what other kind of data might you want to track? Then I got to thinking about why my friend’s running loop was taking him a bit longer than it used to, even though he trains hard and competes with younger runners. So here in Part IV the idea is to tackle a different kind of biomarker that is a great barometer of health, because it depends on the optimal functioning of a number of different physiological systems – the V̇O₂max. It is sensitive to aging, but also to the integrated performance of heart, lungs, blood, muscles, metabolism, joints, nerves, kidneys, brain – pretty much everything. If you devote a page of your health notebook to keeping track of your V̇O₂max, or some other marker of aerobic fitness, then you set yourself up for success at catching problems early if something is not quite right. Address the problem and exercise to fight off the decline! You’ll still get older, but I hope you’ll age more slowly as a result.
I’ll be trying out some new media release approaches as well – if you want audio or video instead of reading this article, here’s the link to the podcast.
V̇O₂max: Lungs on Fire
Think about the last time you ran a mile. For some it’s an unhappily buried memory, four laps around the football field for the elementary school fitness test. Other people knock out dozens of them every day. But if you ran that mile as absolutely fast as you possibly could -- then you experienced your lungs on fire, breathing fast and deep yet still “hungry” for more air. That sensation, whether you hate it or love it, gives you an idea of the concept of “V̇O₂max.”
Maybe your smartwatch estimates V̇O₂max, or maybe you’ve even had an accurate metabolic exercise test, sucking every breath through a Darth Vader mask. These tests try to measure your oxygen consumption during exercise. To a physicist, exercise is a kind of “work.” Lifting heavy things, moving your body through space – that’s all work, and energy measurement works the same for steam engines and for athletes. Our human bodies burn oxygen to do work, and measuring the oxygen consumption during work is a story that begins just over 100 years ago.
Before that, exercise physiologists had been focused on the nutritional requirements of work – the calories consumed for energy. But chemical weapons -- the horrible suffocating poison gases like chlorine, phosgene, and mustard gas first used during WWI (1914-1918) -- ignited a global scientific drive to understand how gases transitioned between the lungs and the bloodstream. The development of sealed rubber gas masks enabled dramatic progress in the understanding of how the tiny spongy air sacs in human lungs pass oxygen into the blood.
In 1923 the English physiologists A.V. Hill and Hartley Lupton published the discovery of V̇O₂max. Let’s break it down: V is volume of gas, and V̇ with a dot means volume per unit of time. O₂ is oxygen gas. So the V̇O₂ is a direct measurement of your body’s capacity to transport and use oxygen. What Hill and Lupton observed was that as a runner worked harder and faster, oxygen use increased, up until it reached a point beyond which oxygen use stopped increasing. It’s not that the runner didn’t need more oxygen, it’s that they couldn’t get more oxygen. There was a cap on the oxygen consumption, and that cap is the V̇O₂ max.
With the ability to make this rather obviously important measurement, Hill and Lupton also showed that during really intense exercise the energetics of oxygen can shift to an “oxygen debt.” If you’ve sucked wind with a 100% effort to run those laps even when your body was begging you to stop, then you have an idea. If you’re a beast, you can perform a little bit of extra work that requires even more oxygen than can be supplied in real time. Now, if you will excuse the pretty lousy analogy and science, a common way to think about that oxygen debt is that it is secured by an “IOU” in lactic acid, liberated during activity and then paid back in oxidative recovery once exertion slows or stops.
When you put non-athletes on a treadmill, many will be exhausted before they reach their V̇O₂max —their legs or motivation may quit first. But athletes generally become familiar with their limits, and one of the most valuable assets for an elite athlete (maybe especially for middle distance runners like milers) is the ability to operate near that limit; their training lets them exercise at V̇O₂max for longer stretches in a race. Stop training, and the V̇O₂max falls. Get back in shape, and it goes up.
Figure from their 1924 paper “Muscular exercise, lactic acid, and the supply and utilisation of oxygen.—Parts IV-VI” Hill, Long, and Lupton used a wearable Douglas bag containing all the gas that the runner would breathe during an experiment, allowing analysis of inspired O2 and expired CO2 during exercise. The work was funded by the Industrial Fatigue Research Board, itself born directly from WWI Ministry of Munitions research. Proc. R. Soc. Lond. B.97:84–138 http://doi.org/10.1098/rspb.1924.0045
Why does V̇O₂max fade as we age?
Lately V̇O₂max has gotten a lot of attention as a predictor of longevity. A fit older person is less likely to die. But no matter how fit you are, if you’re over 50 and you’re reading this it’s quite likely that your 18 year old self would have sizzled you in a four lap race. Though anti-aging and longevity influencers would like us to believe that aging is optional, the facts seem to be pretty clear: even for adults who remain physically active, V̇O₂max generally declines with age. Most adults will see V̇O₂max drop by about 30% from their 20s to their 60s, and by 10% per decade as years go on. Endurance training slows this slide and helps you stay ahead, but for everyone, the loss is real: marathon times slow, recovery takes longer, and everyday activities feel more taxing. Because V̇O₂max depends on the combined peak functioning of your heart, lungs, blood, muscles, and more, this decline can happen for a number of reasons. If any part of the oxygen-burning chain is not operating at peak performance, it can affect the V̇O₂max. If your fitness isn’t what you expect, consider these factors.
Pumping Capacity of the Heart
Delivery of oxygenated blood to working muscles is a primary determinant of V̇O₂max. If there is something that impairs heart pumping function (clogged coronary arteries, weak heart muscle, leaking valves, or other common conditions) this can severely limit oxygen delivery. A patient with severe heart failure, short of breath just sitting in a chair, waiting in the hospital for a transplant might have a value of less than 14 mL/kg/min. At the opposite extreme, an Olympic rower might be able to use 70-80 mL/kg/min of oxygen, and a crucial part of his high V̇O₂max is his trained heart’s elastic ability during exercise to fill with an extra large blood volume and to pump it at very high rates through the circulation, and his healthy blood vessels to dilate and accommodate more flow. The very highest V̇O₂max test results ever recorded are above 90 mL/kg/min, and belong to elite cyclists, cross-country skiers, and long distance runners.
Metabolic function
Metabolic disorders like insulin resistance and type 2 diabetes can lower V̇O₂max, sometimes before any other symptoms have appeared. In studies of people who are at risk for diabetes but don’t yet have the diagnosis, a lower-than-expected V̇O₂max appeared to be an early clue of their metabolic disorder, decreased insulin sensitivity. This reflects a series of cellular mechanisms that connect defective insulin signaling to inefficient oxygen utilization in skeletal muscle.
Skeletal muscle is the main site for insulin-stimulated glucose disposal, accounting for 80% of glucose uptake after a meal. Since glucose is the primary fuel for oxidative energy to perform exercise, the blunted glucose uptake caused by insulin resistance translates directly to decreased energy availability, limiting muscular work and reducing V̇O₂max.
Insulin resistance is also associated with mitochondrial impairment. The number of mitochondria in muscle fibers is reduced, and muscles are less capable of generating energy from oxygen at high exercise intensities, lowering V̇O₂max.
We can burn carbohydrates like glucose, and we can also burn fat for energy. Insulin resistance shifts muscle metabolism away from carbohydrate oxidation and towards more reliance on fat. Because mitochondrial dysfunction limits fat oxidation, neither fuel source is used efficiently, increasing lactic acid production, triggering earlier fatigue, and blunting maximal aerobic performance.
While the heart gets all the glory for pumping blood, the blood vessels through which blood moves are just as important. Insulin resistance leads to dysfunction of the endothelial lining of blood vessels, so that they cannot relax to accommodate more blood flow when needed. The density of capillaries, tiny blood vessels in skeletal muscle, is also diminished, reducing the surface through which oxygen and nutrients diffuse from the blood into muscle cells. Studies show reduced blood flow to muscle in people with diabetes, limiting the extraction and utilization of oxygen and further reducing V̇O₂max.
Muscles’ Capacity to Use Oxygen
Beyond delivery, the muscles' capacity to extract and utilize oxygen from the blood may also limit the V̇O₂. Even if more oxygen is delivered, the oxygen diffusion capacity into muscle cells has limits, and these limits get worse with deconditioning, or increase with endurance training.
Oxygen-carrying capacity of the blood
is diminished if there are not enough red blood cells or hemoglobin. “Blood doping” by cheating Tour de France cyclists has been used to increase their oxygen-carrying capacity. A drop in your red blood cell capacity could reduce your V̇O₂max; even a little decrease within the “normal” range may be significant.
Tissue Buffers of (“Lactic”) Acid Debt
During strenuous exercise, the fluid inside muscle cells becomes acidic. We’ve got buffer chemicals in the blood and tissues which resist this acidification, bringing the pH back towards normal (around 7.4) and the efficiency of these buffers determines how much acid can be absorbed without distress. In even early stages of kidney disease, for example, the buffers are less effective, leading to faster fatigue and greater discomfort. Deconditioning also reduces tissue buffers, while training increases the local buffering power of muscle proteins, allowing for greater exercise tolerance.
The vicious cycle is real
Whether impairment begins with a problem in the heart, the lungs, the metabolism, an injury, or simply from life circumstances that lead to a sedentary lifestyle and deconditioning, deteriorating exercise tolerance can further worsen cardiorespiratory fitness and V̇O₂max. Just as conditioning can make these systems work better, a period of reduced physical demand on muscles, whether due to disease, injury, immobility, or aging leads to reduction in muscle capillaries, muscle cell mitochondria, and muscle function – setting up a negative cycle of deconditioning. Over time, the decline in V̇O₂max is not just a passive consequence of aging but an active process of de-adaptation.
Fighting the Decline: The Power of Activity
I am skeptical about the claims of longevity influencers who want to sell you a remedy for aging. Aging is real. But in spite of age and genetics, just about anyone can improve their V̇O₂max by physical activity and exercise. How?
Move your body! Regular aerobic movement (such as brisk walking, cycling, swimming) can boost insulin sensitivity, help muscles use fuel more efficiently, and can raise V̇O₂max. Even a beginning exercise program can quickly increase VO2max by 10% in people with metabolic syndrome.
Balance nutrition with exercise. Matching caloric intake to energy expenditure can be extraordinarily hard, but that doesn’t mean you shouldn’t try. Metabolic disease often starts with overnourishment. Even for those who are not overweight, there may be important gains available by improving the quality of nutrition and reducing or eliminating your Kryptonite – whether it’s popsicles, chocolate chip cookies, doughnuts, ice cream, or alcohol.
Be Your Own Health Maintenance Engineer: Using your health notebook to keep track of simple, meaningful health biomarkers —such as waist circumference, glucose levels, hemoglobin A1C, and yes, V̇O₂max itself—can warn you early to make changes, long before disease strikes.
The Takeaway for Everyone
Understanding V̇O₂max isn’t just for elite athletes or aging runners—it’s a vital sign for anyone who wants to live well. All energy systems must be “go!” Heart, lungs, blood, muscles – each system must be working well to achieve a great V̇O₂max. To combat the age-related slide, it’s really important to keep the fuel and reserves just right. Insulin-resistance can lower VO₂max, and you’d like to catch that and fight back! Your tools are movement and food, but the most powerful tool is your curiosity. Thinking About Your Own Health!