By James Frost, founder of Flaming Phoenix. I designed Phoenix Bars to be vegan from the start, built for athletes and expedition teams who need the most calories in the least volume, which turns out to be the exact problem plant-based endurance athletes run into. I am not a dietitian, and personalised or medical questions belong with one, but this is what I have learned building fuel for this audience and working with the people who use it. Last reviewed June 2026.

A well-planned plant-based diet does not hold endurance athletes back. The research is fairly settled on that point: across systematic reviews, plant-based eaters match or slightly outperform omnivores on aerobic measures, and the carbohydrate-rich nature of the diet suits long efforts well. The single real challenge is not the one most people fixate on. It is not protein. It is calories. Plant foods are bulky, filling, and high in fibre, which makes it genuinely hard to eat enough energy when your training load is high. Solve the calorie-density problem and almost everything else falls into place. This guide is about how, and the short version is to stop worrying so much about protein and start paying attention to whether you are eating enough at all.

Why plant-based suits endurance more than most diets

It helps to start with the upside, because it is real and it is often undersold. A plant-based diet is naturally carbohydrate-rich, and carbohydrate is the fuel that actually limits endurance performance once you are working above an easy pace. Your finite glycogen stores, not your essentially infinite fat stores, are what run dry and cause the wall, so a diet built around grains, legumes, fruit, and starchy vegetables is working with your physiology rather than against it. There is a fuller explanation of why carbohydrate availability matters so much on the ultra-endurance and expedition nutrition guide.

The performance data backs this up. Reviews comparing plant-based and omnivorous athletes find aerobic capacity is maintained or improved and strength is not compromised, with one study reporting higher estimated VO2 max and longer time to exhaustion in vegan participants. Plant-based diets also tend to be lower in saturated fat and higher in fibre, antioxidants, and phytochemicals, which supports cardiovascular health and may help with the recovery and inflammation side of heavy training. None of this means going plant-based makes you faster on its own. It means you can train and race at a high level on this diet without giving anything up, provided you get the fuelling right. And the fuelling almost always comes down to one thing.

The problem nobody warns you about: calorie density

Here is the issue that sports dietitians see again and again, and that new plant-based athletes rarely see coming. Plant foods carry fewer calories for a given volume of food, and their high fibre content fills you up early. That combination, low energy density plus early satiety, makes it difficult to eat enough when your needs are high. Dietitians commonly report unintentional weight loss in athletes who switch to a vegan diet, not because the diet is flawed, but because the athlete simply cannot physically get enough food in around a heavy training schedule.

This matters more than it sounds. Falling short of your energy needs for a sustained period, sometimes called low energy availability, raises your risk of injury, illness, and lost training adaptation, and it tends to take muscle as well as fat when weight comes off. The athletes most exposed are the larger ones and those doing high volume, which describes most ultra runners and expedition athletes precisely. The standard advice from sports nutrition bodies is to eat frequently, around five to eight meals and snacks a day, to lean on energy-dense foods, to limit very high-fibre choices when you are chasing a calorie target, and to keep dense food within easy reach so eating does not depend on cooking. There is more on the principle of packing energy into less volume on the calorie-dense foods guide, and on reversing an unwanted drop in weight on the how to gain weight guide.

This is the exact gap I built Phoenix Bars to fill. Each one is 120g and carries up to 557 calories, around 4.6 calories per gram, which is close to the opposite of a bowl of vegetables or a salad that fills your stomach without paying its way. They are vegan by design, and gluten-free too, which matters if you are managing both at once. They take no preparation and travel in a pocket or drop bag, which is precisely the "keep dense food within reach" advice in physical form. They are not a replacement for a varied wholefood diet, and I would never pretend otherwise, but they are a reliable way to close the calorie gap that the wholefood diet leaves on your biggest days.

Fuelling during the effort itself

There is a second, sharper version of the calorie-density problem, and it shows up during exercise. The high-fibre wholefoods that make a plant-based daily diet so healthy are the wrong tools mid-effort. Fibre slows digestion and can trigger gastrointestinal distress when blood is diverted away from your gut, which is the last thing you want deep into a long run. The evidence-based recommendation is to switch to low-fibre, easily digested, calorie-dense carbohydrate during and immediately around hard efforts, the food equivalent of choosing white rice over a bowl of beans when the clock is running.

A dense bar fits this brief well, because it delivers a large amount of low-bulk carbohydrate without the fibre load of whole grains or pulses. Eaten cold and broken up, it goes down on the move, and made into a warm porridge with hot water it becomes soft and easy when you are tired and your appetite has flattened. If you have been relying on gels and your stomach has had enough of them, the case for real solid food is covered on the energy gel alternatives guide and the ultra-high-carb solid foods guide. For how this plays out over the specific demands of long races and expeditions, see the multi-day ultra running guide, the trail running guide, and the high-altitude mountaineering guide, where appetite suppression makes calorie density matter even more.