In this interview, ultrarunner Zach Bitter explains his nutrition approach for training, emphasizing a high‑fat, low‑carbohydrate foundation complemented by protein. He treats carbs as a tactical tool—similar to caffeine—used only when a performance “gas” is needed. Bitter stresses that his diet is not static; he periodizes carbohydrate intake based on training demands and performance feedback. Drawing on over ten years of training data (more than 50,000 miles) and countless races, he monitors workout quality and race outcomes. When low‑carb periods lead to diminished high‑intensity performance (e.g., hill intervals), he introduces the minimal effective dose of carbs to restore power without abandoning fat adaptation. The core decision rule is performance‑first: if metrics suffer, he adjusts carbs; otherwise, he stays fat‑adapted.
Chapters
Let's talk more about nutrition because, like, you obviously have the race nutrition is one thing, but the training being such a big part in the whole preparation of this, you've really much focused on low carb, high fat. Can you talk a little bit more about the actual nutrition strategy for you during training? Because I know it's not a— you're not following the same, the same approach day in, day out. You fluctuate with ketosis at certain stages, you take in more carbs in certain peak stages than in other stages? Would love to hear a bit more about that.
Yeah, it's, it's a really good question, and it's one that I think it's, it's a little slippery for people to get their hands on sometimes because they'll ask, like, well, what do you eat in a day? And then my response is usually, well, which day are you talking about? Because, like, if you look at what I ate during that race at the Pettit Center or what I ate the next day, it's completely different. So, and those are the two ends of the extreme, you know, there is some consistency in the middle. The way I describe it is I follow a version of a high-fat, low-carb diet where fat is definitely the foundation of my nutrition. Protein is a pretty big foundation of my nutrition, and then I use carbs as a tool to essentially hit the gas when I really need to. I view carbs probably the same way as a lot of people would maybe view caffeine, where if you think about it, if I'm drinking a cup of coffee in the morning, I'm going to get a nice little energy burst from that, and it's going to do what it's going to do if I do that on routine. Now, if I decided one afternoon to have a second cup of coffee, I could probably get another energy burst from that. Now, compare that to, say, someone who's drinking 4 cups of coffee in the morning routinely. You give them a 5th cup of coffee in the afternoon, they might hardly even notice that. I'm working from that same basic principle where I'm getting fat adapted enough so I can really lean on that as the foundation of my metabolism, but I'm not doing it— I'm not sliding that scale so far over so that carbs are out of the option window either. I definitely want those as a tool when I need it. That's worked really well for me. The majority of my work is done aerobically, so I can do a lot of stuff burning high levels of fat. I just haven't seen any evidence in my own workouts in terms of hitting paces, hitting workouts that would indicate that moving from the amount of carbs I do have back up to, say, a more traditional high-carb approach of 60-70% is going to make any drastic improvements in my training that are going to get me another edge come race day. That's where my expertise on the matter is. Maybe most valuable is I do have 10 years of training data that is north of 50,000 miles and countless races that I've been able to test these things. I always use performance as my compass. If I'm doing something and my performance is suffering, my workouts are suffering, I'm going to make a change. That's just where I've gotten to this more periodized carbohydrate approach is when I've gone strict keto or zero carb in the past. I see a lot of these— I feel great during the day. I feel great on these low-intensity runs, but if I try to hit the gas or if I am doing a hill interval or something like that, I don't have that pop. To me, that's not the best route for me if that's the result I'm getting. That's when I would start to play around with a little bit of carbohydrate and just find that minimum dosage, maximum return. Spot within that.
How do you do that? How do you find that minimum dose?
What I did was I did a pretty strict ketogenic diet and did what I guess you could maybe call like a hard reset where you just reset your metabolism to be moving as much into fat metabolism as possible. I did it on an off-season because I don't want to necessarily introduce a big physiological stress on top of another big one with my training load. Once I had a little bit of flexibility in the offseason to do that hard reset, and then once I started training, I just paid attention to those things I mentioned, like how do I feel in my workouts, how does it feel on an easy run versus a workout with some speed work in it. Then it's just inching it up a little bit. Let's say I'm following a strict ketogenic diet, I'm hitting 30 to 50 grams of carbohydrate per day, and I notice I'm a little flat on those peak days. "Well, my next step then is just going to be let's up the carbohydrate a little bit and see if I can get that pop back." Once I get it back, if I'm hitting the workout I need to hit, there's no reason for me to take more of it to try to run faster than my intended goal. It falls right in the same line as like, "Well, just because you can do that interval faster doesn't mean it's in your best interest to do it faster." I find that spot where I know I need to be in terms of getting the training stimulus I'm looking for. I flex whatever nutrients I need to get me to that place. That just happens to have been a little bit of carbohydrate, but not a whole lot relative to what you're going to see a lot of people doing on a high-carb approach.
