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Training With Cancer

Training With Cancer: How Seth Marcus Came Back From The Brink

Written by
February 22, 2021
Updated July 18, 2023

When Seth Marcus began the Onnit 6 Challenge, he was still undergoing treatment for Hodgkin’s lymphoma. That sometimes meant lifting kettlebells only hours after a chemotherapy session—running to the bathroom to puke between sets, and then getting back to his mat before the rest period was up.

Just completing the workouts would have been enough for most people, but Marcus managed to gain eight pounds over the course of six weeks, and won the whole contest. “Truthfully, [the training] was more therapeutic than it was challenging,” says the 33 year-old Denver man with a smile. “Exercise is the best medicine.”

You Never Think It Could Happen To You…

Marcus wasn’t the type to take life for granted. The CEO of his own startup media company, he was also a business coach, musician, yogi, outdoorsman, and, as his friends would tell you, the gracious host of many a pool party and backyard barbecue. “Until August 2019, I was in great shape,” he says. That’s when he was diagnosed with cancer.

It had started as stomach pain after dinner at a friend’s house, so, at first, Marcus suspected he had food poisoning. But the agony built throughout the night and into the next day until he was forced to go to the hospital. There, doctors examined him and discovered a growth in his chest. He was sent to the Mayo Clinic in Rochester, Minnesota, for special treatment.

“It doesn’t serve me to think about why it happened,” says Marcus. “I ate my fruits and vegetables, and exercised regularly. Cancer just hit me out of nowhere. My mom was devastated. But she’s a doctor herself, and we knew that I had a good chance of beating it.”

While he hoped for the best, Marcus was enduring the worst. The cancer ulcerated his stomach, causing unbearable pain. With no appetite to eat, Marcus had to be force fed through a feeding tube, a procedure he says he “wouldn’t wish on anybody.” A little more than a month after being diagnosed, he had lost 30 pounds, his bodyweight dropping to a gaunt and ghostly 126.

“They don’t know why it became a digestive issue,” says Marcus, “but Hodgkin’s tends to be very inflammatory, and it shows itself in differing ways. They told me mine was one of the most unique cases the Mayo Clinic had ever seen.”

Nevertheless, the chemotherapy treatments were effective, and Marcus began to get better. By February 2020, the growth in his chest was no longer visible on a PET scan, and his condition stabilized. Able to eat on his own again, Marcus’ doctors encouraged him to take in as many calories as he could to help him put on weight, but he didn’t want to eat indiscriminately.

“I had been taking so many medications that I did not subscribe to ethically,” says Marcus. “It was hard to sit there and take chemo and listen to doctors say they just want to see me put weight back on and they don’t even care if it’s fat. As long as I got back up to 155 pounds, they didn’t care if I was drinking milkshakes all day. ‘Yeah,’ I thought, ‘If I do that, I might weigh 155 again, but I’d be in worse shape than I was when I was in the depths of the cancer.’ I didn’t want to trade cancer for being pre-diabetic! I thought, ‘I have to take this on myself.’”

Marcus was cleared to return to Denver that March, though he remained on a chemotherapy regimen. He hired a nutritionist, and researched diets that have shown promise with cancer patients, ultimately leaning toward a low-carb approach. “I started using lots of Onnit products to help with low-carb eating,” Marcus says. He found that MCT Oil added flavor to his coffee without adding sugar, and Krill Oil supplied healthy fats.

Marcus didn’t want to gain weight by food alone. He wanted to regain the muscle he’d had before he got sick, and that would require some form of strength training. The Covid-19 pandemic swept the country that month, closing gyms in its wake, but Marcus still had his backyard. “My roommate, Adam, who had been a huge support for me throughout my illness, found out about the Onnit 6 Challenge,” says Marcus. “He said, ‘Why don’t we do it together? It will make for a great story when we’re done.’”

Building Disciplines

The Onnit 6 (O6) Challenge is a six-week fitness transformation contest where the goal isn’t just a better body but a fuller life. Participants choose an Onnit 6 workout program—streamable video workouts led by Onnit trainers that can be done from home—and aim to establish healthy habits they can sustain long-term. Winners are the individuals (one male and one female) who demonstrate the greatest improvements physically, mentally, and spiritually. Grand prizes include free Onnit products, access to all O6 programs for life, and $6,000 in cash. The contests are held several times annually.

Marcus and his roommate signed up for the spring kettlebell Challenge, beginning in April 2020. Though he was still undergoing nauseating treatments, Marcus was determined to build himself back up, and he established a schedule that kept him focused. “Starting is always the hardest thing in any commitment,” he says, “so I built my own disciplines to stay on track with my recovery.”

Marcus got up every day at 7:30 a.m. to meditate. At 8, he and his roommate did their Onnit 6 training. Then he’d have coffee with MCT Oil, or eat a bone broth soup, and try to get some work done. Lunch was often a Cobb salad with eggs and avocado for extra fats, and dinner would be salmon with asparagus or grass-fed steak and broccoli. Along the way, Marcus kept a journal.

“You need healthy tools that let you express yourself and release the pressure valve, emotionally,” says Marcus. “We tend to relive negative emotions and events in our life, and we don’t pay attention to how that directly affects our current state. If you’re sitting there dwelling on bad news, you still have those negative chemicals running through your body. But writing in a journal really helps get them out. Seeing words form on the page helps you move forward. So, if I felt myself relapsing to a dark place and feeling negative, I could go back and read how I was the day I last felt that way and it would make me feel like that’s an emotion of the past now. Thinking about it that way, I don’t have to choose to stay in that negative moment. I can choose to be positive again.”

Gaining Muscle… On Chemo?!

Marcus downplays the rigors of performing his kettlebell workouts on chemo. Yes, he was often in pain, but he had his roommate to hold him accountable and encourage him, as well as the Onnit Tribe—the Facebook group that Onnit 6 Challengers join to lend support during the contest. In spite of the intensity of the training, his O6 sessions were often the highlight of his day.

“It always sucks to work out in the morning, because you’re tired and you don’t want to,” says Marcus. “But once the endorphins start flowing—and especially once you’re finished—you feel empowered and great for the rest of the day… I’m a big believer in just showing up,” says Marcus with a laugh.

O6 workouts offer three levels of difficulty. Since he was still on the mend, Marcus mainly chose exercises from the most basic tier, Level 1, sometimes pushing to Level 2 when he was up to it. He never needed to regress to easier moves than what the program offered, and he was able to hang in there for all its challenging training protocols, including EMOMS, circuits, and Tabatas.

Marcus’ strength came back quickly. He started with a 25-pound kettlebell and was swinging the 35-pounder by the end of six weeks (none of the O6 kettlebell workouts require heavier weight than that). When the program began, he weighed 145 pounds. By the end, he was 153—nearly the size he had been before he’d gotten sick. “Onnit 6 brought back the definition I had in my shoulders and my abs,” says Marcus.

While his physical comeback was inspiring, Marcus impressed the O6 Challenge judges even more with his humble (and relentlessly positive) attitude. He never asked for special treatment or sympathy, and even seemed somewhat unable to recognize just how far he’d come.

“When I got home from the Clinic,” says Marcus, “I asked people how they were doing, and they’d say things like, ‘Oh, things aren’t great with the wife,’ or, ‘Business isn’t going well, but it’s nothing compared to what you’ve been through.’ I found that they would belittle their own emotions. But I’ve learned that everybody has problems, and the more we can be transparent and support each other, the more we’ll remember that everybody’s fighting battles that we know nothing about. My battle was very public and obvious and intense, but I hope I can inspire people to be more open about their own weaknesses and struggles. At the end of the day, hard is hard. It’s not meant to be compared.”

Marcus won the spring kettlebell Challenge, and finished his chemo treatments in July 2020. He has been healthy ever since, and continues to train according to the Onnit 6 template. He says he’s almost back to the shape he was in before the illness.

“I didn’t have any idea I’d win the Challenge,” says Marcus. “I didn’t even think about winning it… I was just happy to have it.”

All images and video courtesy of Seth Marcus.

Learn more about the Onnit 6 Challenge HERE.

Sean Hyson
Sean Hyson is the Editor in Chief of Onnit. A Certified Strength and Conditioning Specialist (C.S.C.S.), he is the author of The Men's Health Encyclopedia of Muscle, and the e-book The Truth About Strength Training (
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