What Happens to Our Bodies as We Train for Marathons?

Can you break down what happens to our bodies as we gradually train to run longer distances?

Saviola: The biggest changes I see in myself are in my heart rate and how I adapt to heat. My running coach has me training in zone 2, which involves keeping your heart rate below 148 bpm. If during training my heart rate exceeds 148, I slow down and walk. Ultimately, my heart rate stays lower over a longer period of time, and it’s gotten to where I look down at my watch and see my pace is 8:30 per mile and heart rate at 125. As far as adapting to heat, when summer hits I do sweat tests and notice I’m losing a liter or more of water. Running in heat more and more, you ultimately adjust. I also use saunas and take extra hot showers right after runs to boost that adjustment. But over time I adapt to heat and keep a steady heart rate at harder exertions while training consistently. 

Nemkov: So, Anthony mentioned zone 2 – which is the second of five zones of intensity that we can train in. There are different definitions of the boundaries of these zones ranging from relative perceived effort (RPE), to heart rate, to blood lactate. While heart rate correlates with lactate and is easier to measure, lactate levels are really the most precise to measure exertion from a metabolic perspective.  It used to be thought of as metabolic waste, but it’s not – it’s a useful fuel source. These levels increase during high exertion, meaning your body is producing lactate faster than it can metabolize it. When we reach a certain intensity or start to fatigue, lactate really starts rising. This point is called the lactate threshold, and it increases as we become more fit.

When we work out in zone 2 (of 5), we’re at an intensity range just beneath our individual thresholds, like Anthony mentioned, where lactate is efficiently consumed by the body as quickly as it’s produced. This trains your body to deal with exertion and will ultimately help keep your heart rate down and increase endurance in higher intensities. Also, the types of skeletal muscle and amount of mitochondria in that muscle will determine how much lactate it consumes. Zone 2 training is thought to help reprogram your skeletal muscle to better process lactate. Fat too – better athletes burn fat more and for longer. All of these are results of zone 2 training.

Saviola: My coach describes training like it’s a candle: Whenever your heart rate goes over 148-150, you’re going to burn faster. So I stop and walk until I hit a lower heart rate and start running again. I find it helps with speed too. 

Nemkov: That’s because you’re training your lactate threshold to be higher. When you run a mile, you can go at a faster pace without fatiguing as quickly. 

How does altitude play a role in training?

Saviola: I ran Leadville in August of last year, which is a 100-mile race that starts at 10,200 feet and climbs to 12,600 feet. It was interesting to witness people adjust. A lot of runners experienced GI issues from it. I’ve been told you’re supposed to live high and train low, but I always thought it would be the opposite. 

Nemkov: The concept behind this is that if you live at high altitudes, your body needs to adapt to lower oxygen, which is similar to when you get fatigued. When you go down to low altitude, you can push yourself harder because you're used to working with a lower supply of oxygen. There is a lot of research going into these strategies now to optimize fitness training.

What does overtraining mean, and how can we know if we’re doing it?

Nemkov: There is a lot of research going into understanding overtraining. Part of it is energy storage. If you're using glycogen (your stored glucose reserves) and not properly building it back up, you’ll fatigue more quickly.  There are some markers that might suggest it as well. One might be the presence in the bloodstream of proteins that should be in skeletal muscle like creatine kinase, which could indicate skeletal muscle damage. In cyclists, incompletely burned fat increases in their blood as they fatigue because the mitochondria in the skeletal muscle start working less efficiently. Interestingly, this is the same marker of fatigue we see in long COVID patients at rest. 

Saviola: I feel like training can mean you’re always riding the line of injury. You hear or see people saying that they don’t feel like they’re getting better, and taking breaks with less mileage can help. But running is so personal, and we are all so different in how we adapt and train. For one person, a training plan might be perfect, but another might want to go longer or faster, and another may need more rest. 

Are some people genetically more capable of endurance running than others?

Nemkov: There are studies looking at the genetics of elite athletes, and there are specific microbiome populations discovered in elite athletes too. I think that genetic component will help discern the top of the top. If your VO2 max, or the maximum amount of oxygen your body can use during exertion, is 80, it’s probably genetic; 40-55 (depending on your age) can be trained. If you’re trying to run a marathon, it’s not about genetics; it’s about getting your body to adapt. Exercise is an ancient thing our bodies do: We used to do it to survive, and now we do it to thrive.

How can we utilize food/drinks/supplements to properly fuel for endurance races?

Saviola: It’s important to train your gut too! During training it’s important to put food in your belly and run with it to see how you react. Before shorter distance races, I usually carbo-load for stored energy. But for ultras, it can feel a bit like weighing you down; you need more real food sources and to space them out. So fueling is race-dependent and personal. As far as fueling goes during the race, it’s important to test that out too. People eat goos, electrolyte chews, candy. Goos are popular but don’t work well for me personally, especially during ultras. I’m more deliberate about fuel source and time of intake.  

Lastly, Anthony, how did you get into ultramarathoning and endurance running?

Saviola: I was working on my postdoc at Scripps Research Institute, and I started running just a little bit. My partner is a runner. She wanted to do a marathon, and I didn’t know what it was, and it was so much fun. I love the community behind it. I wanted to do another event, so I signed up for a 50K trail race in Colorado and had a blast. I realized how beneficial it is after a long stressful day at work to check out and run. I’m just a curious person, and I'm excited to see how far I can push it. I’m also a chicken, and I don’t like skiing and mountain biking, but I can push it while running, and it doesn't scare me. It allows me to challenge myself mentally and physically. 

Photo: Anthony Saviola completed the Cocodona 250-mile endurance race in Arizona, which includes almost 40,000 feet of elevation gain, on May 11.