How do cyclists physically survive the Tour de France? We asked a physiologist and former pro rider

The Tour de France is probably the most physically demanding major sporting event in the world. Over 23 days and 21 stages, riders burn an average of roughly 5,000 calories per day, which is almost twice as much as a pro marathon runner might burn in a single race, and all the while they must maintain a sharp mind, marking their opponents and working with teammates to go fast as efficiently possible before their bodies cave in.

Cycling a grand tour is as much a fight against one’s own body as it is against opponents. Quite literally, cyclist’s bodies begin fighting back after the first week. Riders begin to lose muscle, their immunes systems tank, and they practically have to force feed themselves in order to maintain the baseline system functionality to climb, let’s say, Alpe d’Huez.

Yes, cycling has a doping problem, but it’s hard to blame athletes who are putting their bodies through very real trauma for looking for some kind of shortcut. Humans shouldn’t do their bodies what Tour riders do to theirs. And indeed, it’s hard to believe some riders aren’t aliens, with their bird-like frames and gaunt features.

There are few people on Earth who can possibly comprehend what riders will be going through this July. I found one: Dr. Stacy Sims, an environmental exercise physiologist and nutrition scientist at the University of Waikato in New Zealand. Dr. Sims was a pro women’s cyclist who later went on to work with men’s pro cycling teams like Saxo and Dimension Data during the Tour de France, advising team chef Hannah Grant on nutrition.

I spoke with Sims about the hell of Grand Tour riding, how to avoid the dreaded “bonk”, and what’s really happening in those disgusting photos of cyclists’ legs we see every year.

How safe is it to ride a grand tour, really? What are the biggest dangers?

Dr. Stacy Sims: Three weeks of that kind of intensity — first you have to take into account that they have some kind of genetic predisposition that allows them to be selected for that sport. They have some kind of DNA abnormality or anomaly that kind of self selects them to the sport. But that said, it is putting yourself through the ringer for three weeks.

It’s kind of like the first week their body can handle the stress. It’s going to be hard, they’re going to have an elevated metabolism and poor sleep because you have the traveling, you have the racing, you have the poor food that goes with it — because unless you travel with a chef you’re never guaranteed that you’re going to get the food that you really need and want.

But then after the first week you really see the body start to bottom out. And you see people start to drop out, getting sick, more crashes because there’s less reaction time available, because their bodies enter so much stress. And then as you get to the third week you’ll see a little bit of a rebound, because the body is trying to compensate for this continued stress.

Photo by Doug Pensinger/Getty Images

You can kind of put into the idea of fight or flight and the famine situation, where the body is all of a sudden like, “OK, I have this increased stress, and I have to deal with it because it’s not going away.” And you’ll see some of the less seasoned riders who don’t have as much training history will drop out earlier, or fall and be off the back a lot earlier, and those who have more training history and can handle the stress can probably stay in the peloton or be off the front.

From a physiology point of view, you’re going to be in this continuous state of catabolism: losing muscle mass, losing fat mass. They’ll start off a little bit heavier than they should for the Tour, because they need that weight to lose.

And then when you start adding in all the environmental considerations, if they’re not acclimatized to the heat, that increases dehydration status and inflammation, poor abilities to recover. And then when they get into the cold and the altitude, they have that compensation they have to worry about, less oxygen available, and the fitter you are, the more you suffer at altitude.

The cold as well, getting hypothermic and not being able to control the gears because your hands are frozen. These have some consideration into crashing and attenuating your immune response to exercise.

Illness seems to be something that take a lot of rider out, especially around that first rest day. Why is that? What causes that bottoming out?

SS: So for each successive stress day, you have an increase in cortisol, you have increased inflammation, you have your protein breakdown, which is your catabolism. You’re depleting your fuel stores. And your immune system is very reliant on having glutamine available, which is a key amino acid which is also in the gut. Also having some protein available for white blood cell regeneration and stimulation.

You also have the need for a reduction in inflammation in order for the immune system to do what it needs to do, because the response to inflammation, again, is an immune response. So if your body is dealing with body and muscle inflammation, it can’t necessarily deal with virus or bacterial infections that come into play.

So as you are continuing to put your body under this stress, after about seven days you’re at this tipping point, so that’s usually when the rest day occurs.

But then in my experience working with a lot of the pro riders, all they want is sugar — sugar, sugar, sugar — because they’re in such a depleted state, they’re craving carbohydrates. So instead of trying to help repair, they’re just looking for that quick hit of carbohydrate and quick hit of sugar, and the more they have of the sugar aspect, the worse it is on their gut microbiome, which is also tied to the immune system.

Because if they’re eating high sugar diet, then it’s going to reduce the growth of the bacteria that protects from fatigue and inflammation, and helps your immune system, and the more it’s going to grow the bacteria that relies on simple sugar and the metabolites of stress. So it’s a multi-tier effect of why they bottom out.

So diet is critical then. How do you tailor nutrition for cyclists, and how varied does that get from rider to rider?

SS: When I was working with Team Dimension Data, and my partner Hannah Grant, we had the ability to actually get in and change what they were eating. Working with her, she’s a professional chef, and I would take the science and we would actually time the food and make it so that their body was able to get what it needed when it needed it.

When you start taking a look at overall recovery, it’s more than that 30 minutes post stage. It’s how are you going to facilitate that recovery five hours later, after the transfer, or on the transfer. So we looked at having a higher protein with some complex carbohydrate, because we wanted to really get in and get that catabolic state down, and we wanted to reduce sugar cravings, and you can do that with protein and complex carbs.

Photo by Doug Pensinger/Getty Images

Also bringing cortisol down, helping bring your metabolism down, your core temperature down to baseline. Because the faster you get down to baseline, the better it is for your immune system and your gut. The other aspect that people don’t really think about, is when you are exercising at high intensity, your intestines are under this severe heat and hypoxic stress, and when it’s under this heat and low oxygen stress, then you’re also going to have a perturbation in the gut microbiome.

So we’re also looking at having them eat really good prebiotic based foods, leafy greens and stuff. And then cultured foods as well — so sauerkraut, some of the guys liked kombucha, having them eat a lot of non-fat greek yogurt, just for the higher probiotic aspects.

We didn’t have any upper respiratory tract infections, we had better joint elasticity according to the massage therapist, less muscle soreness, less DOMS [delayed onset muscle soreness]. So overall the guys felt the difference, and so they were able to adhere, because they could see the difference and feel the difference.

So did everyone eat the same thing, or did you tailor the nutrition plan to specific riders?

SS: The older riders are very set in what they do over the years, but the younger riders buy into it. And we were pretty specific to position as well, because if you’re sitting in the peloton and you’re not doing much, your output isn’t as high, and your stress response isn’t as high. But if you’re at the front, then it’s a completely different ride. So we would look and see what was happening, when the mountain stages were coming up, if it was going to be hot or not, and really looking at tailoring position and terrain to what they needed.

So does diet change with type of rider, too? Say it’s a sprint stage and you you’re going to have a rider who will need to be effective in the last 200 meters of a stage.

SS: Overall diet not so much of a change, but on the bike, what are they using on the bike? Making sure they are really well fueled, and doing — say there’s a critical sprint, or it’s the bottom of the hill where they have to get their hill climber up to the front, so they have to go full gas at the bottom. So then you’re looking at what’s in their bottle, maybe some beet juice and beta alanine for vasodilation. So we look at some of the legal ergogenic aids to put in their bottle to give them that extra bit when they need it right when they need it.

Every year during the Tour de France you see the “bonk.” Can you explain what the bonk is?

SS: That comes from poor recovery or inadequate glucose regeneration and glycogen. We technically say it’s “low energy availability,” but what happens is that they’re not taking in enough to put everything back in their muscle and their liver, partially because they’re in this breakdown state — their body is using more in the immediate rather than storing it. So as you get further and further into the race, and you have less and less storage, they have toe eat more and more, and physically they just don’t have the appetite. Like after your gut has been under this extreme stress for seven to eight days, your appetite is completely gone. So it’s more force feeding both on the bike and off the bike.

So as you get more and more into this depleted state, it becomes this fine balance of getting riders to take in enough to be able to perform, but also knowing that they have no appetite, and they’re dehydrated and you have this repercussion aspect. They’re going to hit the wall, they’re going to bonk, and then it’s all over. [Laughs] I didn’t do my job properly.

What are the steps you take as a rider to recover? And I mean after the Tour is over, you’ve ridden all 21 stages successfully. What is your body like at the very end?

SS: This is a little bit more individualistic. It depends on training history, what the work output was during the Tour — were you a domestique, or were you the GC contender, the sprinter.

But absolutely stopping off the bike is not a great idea, because your body has been so used to training for so many months and months, and racing.

So recovery is just real easy facilitation of blood movement and muscle contractile strength. And if you want to, you can do some swimming or some walking, but not just completely stopping, otherwise it’s a too much of a shock to the body, where your body’s like, “Woah what’s going on, all of a sudden I have no input of muscle contraction, I have no increased blood flow.”

What would be the consequence of stopping riding altogether after a grand tour?

SS: This is when all the injuries and niggles come out. It’s as if someone had an injury and becomes bedridden. Your joints and tendons and everything will start to get a little bit tight from the fact that you’re not having increased blood flow and that range of motion every day. So some of the scar tissue will start to really adhere, trying to repair some of the micro-trauma and inflammation factors that have happened.

Photo by Chris Graythen/Getty Images

You lose a lot of your plasma volume, the watery part of your blood, because you need some stimulus to keep that, so your overall blood volume shrinks. You have a decrease in your red cell count as well, you have a decrease in your immune factors as well. So when your body is used to having some increased blood flow, and increased heart rate, and increased blood circulation through everything, and then it stops, the repercussions are as if you are bedridden.

You mentioned that third week bounce back. What is exactly is happening then? Is it that your body chemistry has changed?

SS: It’s not so much the chemistry, it’s just that you get to a point and it becomes about survival. So you are getting a little bit fitter as you’re riding, so that extra fitness comes into play as well, and you’ll have less of a stress response because your body has kind of learned that stress response.

And what I mean by that is, let’s take it out of the cycling context. Say you have a final exam, or you have a big presentation, and you’re nervous and you get that big adrenaline rush. And your heart rate goes up, and your pupils dilate, and you’re ready to go. And you have that same response every time you have something major to do. But over time, if you were to have that every day, you don’t have as big a response because your body has learned what that is.

It’s the same thing with riding for those two weeks. Your body is starting to attune to that stress and understand what that is, so when you get into that third week, it’s not as strong of a response to that stress.

That is what the bounce back is, because you don’t have as much cortisone being released, not as much adrenaline being released. The body is more efficient in carbohydrate utilization, more efficient in fat utilization. The inflammation response isn’t as great, so everything has kind been attenuated and habituated to that response.

If there were to be a four-week grand tour, who knows what would happen. You would probably see some guys really take off in that last week because of the residual fitness and the ability of the body to learn what that stress is. It’d be pretty interesting actually.

If you ride all three grand tours, and the 10-day races, and Classics — you put your body through a cycle of stressing and relaxing — what is the cumulative damage of that, say, 20 years after you’re done? Or 40 years?

SS: That’s a research question that has been around, and people are doing a little bit of longitudinal research, but the confounding factor there is the fact that the sport isn’t clean. If the sport was clean, then we’d have some really good data to say yes or no. Like if you look at runners who have been racing competitively and don’t have the same drug induced aspect that cycling does, then there’s some really good research to show that the hardcore racing is protective — so your cartilage is protected, your heart is protected — so you’re aging at a slower rate. But with the earlier aspects of all the drugs that were involved in cycling, the data for the grand tour and the longitudinal effects on the health, it’s not there, until we can clean up the sport really well.

For some of the younger riders, some of the results are coming out really well except for bone density. So the long term effect on bone density is horrendous for riders because it’s such a strong stress and there isn’t any kind of gravitational pull or bone stress from all the riding, that even in the short span of a season, they can go from normal bone density to osteopenic, which is right before osteoporotic. So they have to build bone off the bike. And the consequence of that — seasonal, seasonal, seasonal — is a high incidence of osteoporosis and risk of fracture.

With a high sweat rate and a high work output, riders lose a lot of calcium, because you lose calcium in sweat, and if you don’t have enough dietary calcium, then the body starts building on the bone. And every six months, your bone remodels, so if you think about the season being six or seven months, you have six or seven months of high calcium output and drawing off the bone, in that six or seven months you’re going to get really low bone density.

You see those pictures Tour riders’ legs, and they have knotted veins and it looks like knives are coming out of their legs. It looks alien. What’s happening there?

SS: They’re very vasodilated, so all their blood is in their legs and their muscles, and they’re super, super lean, and you have these surface veins and you have these deep veins, and the surface veins have kind of popped out because they’re so dilated. And with not very much body fat — or almost none — over the legs, it’s just really apparent. So it’s kind of like a body builder being super pumped before they come out on the stage.

What is the effect of altitude? When you’re riding in the Alps, what sort of changes does that bring on the body?

SS: Yeah, altitude is awful. [Laughs]

I say that because when you’re working at a high output at altitude, and you’re super fit, your body relies heavily on oxygen, and with a low oxygen availability and partial pressure of oxygen in that environment, the fitter you are, the more you suffer. Your body also uses a lot more carbohydrate and your respiration rate and heart rate are also elevated. So if they’re going to altitude without any pre-acclimatization, or even heat stress to increase total blood volume and your red blood cell production, then they’re going to suffer.

This is the way I watch the Tour: I look at teams that are based primarily in the heat or those who are based in the mountains, and then I can say, “OK, those teams are going to do really well when we get to the Alps.” And then those that are based in Belgium, or they’re not in any kind of environmental extreme, you can pretty much guarantee that they’re going to suffer when you get to the Alps. They might be OK in the cold, but because of the altitude, their body just can’t perform at that top percent that it needs to.

These guys are working at a very high level of aerobic capacity. We always talk about your lactate threshold, or your VO2 max. Say two people have a VO2 max of 85, and one can work at 90 percent and one can work at 80 percent, then the percent who works at 90 percent is going to suffer more than the one who works at 80 percent, because they’re relying so heavily on that oxygen intake.

How hard is it — say, for someone who likes to bike — how hard would it be for someone to get to a place where they could even just ride a grand tour course. Not race or anything else, but be physically able to ride, compared to how long it would take to train for, say, a marathon?

SS: It depends on your baseline fitness.

Most people take about six to eight months of continuous coached training to get there. If you’re not looking to go fast and race it, it’s very doable because you have the long slow plod, right? And anyone could accomplish that distance if they have enough motivation and enough food.

As to race it? That’s a different story, because that comes into years of base training and getting that top end speed, and knowing the food and recovery aspects that you need.

But it’s doable. You should do it!

Uh.