Protein bars are said to help build muscle and be a healthy alternative to regular candy bars.
But Colin Murray, from Wisconsin, who tried to survive on them for a week said the bars made his workouts ‘sad’ and his body ‘weak’.
Consuming more than 50 of the highly processed bars in seven days left him constantly exhausted but unable sleep and caused some unusual changes to his toilet habits.
He also said that the diet made him feel dizzy and caused so much bloating that he lost the crisp definition of his six-pack abs.
Murray is now convinced they are not much better than sugary chocolate bars and is urging people to consume them in small amounts as a treat.
Colin Murray, from Wisconsin, embarked on the daring challenge hoping that it might boost his muscle growth. He is pictured above with the first set of bars that he consumed
Mr Murray posted his diet experiment on his YouTube channel, where it has been watched more than 215,000 times.
As a calisthenics enthusiast who eats a high-protein diet for muscle growth, he used to enjoy around one protein bar a day after his workouts.
Speaking about the experiment, he said: 'I enjoy the convenience of protein bars, and they taste good, and I don't feel as guilty when I'm eating them.'
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Most bars contain between 15 and 25 grams of protein, which is around half the amount in a chicken breast.
They also have less sugar than regular candy bars. One of the most popular, a 2.12-oz Quest bar, contains around nine grams of sugar compared to 28 grams in a 1.86-oz Snickers.
But some health experts caution many protein bars are high in calories, which can lead to weight gain over time.
For example, a 2.3oz Clif Builders protein bar contains around 290 calories, compared to 250 calories in a Snickers and 218 calories in a 1.5oz KitKat.
They also contain artificial sweeteners and sugars that don't digest well and instead ferment in the stomach leading to issues like bloating, gas and diarrhea.
But because they are seen as 'healthy', many people may over-eat them and not be concerned about any health risks.
People can also eat them and not feel full, leading to someone eating more than normal and subsequently gaining weight.
Mr Murray shared this in his video, showing how he looked before the challenge (left) and afterward (right). He said the bloating caused was ruining his six-pack
Mr Murray is pictured above on the morning of day seven of the challenge. He said he was feeling low on energy and had been struggling to sleep
Dr Ernst von Schwarz, a cardiologist in Los Angeles, told DailyMail.com: 'I don't have anything against protein bars, but they are like a snack between meals and should never ever replace the standard meal.
'Many have high sugar content, that is why they taste relatively good, but they also contain many different substances that can cause bloating and gas and indigestion.'
He added that while he was working in Texas he had several patients come in with intestinal complications who had been on an all protein bar diet in an attempt to lose weight.
On the first day of the challenge, Mr Murray thought it was 'pretty nice' because he did not have to spend much time cooking.
He started with two peanut butter protein bars for breakfast at 10am, and then ate again around midday when he had two chocolate chip protein bars and a Snickerdoodle protein cookie.
For dinner, he had a peanut butter protein bar and protein cookie — but admitted he was already starting to regret the meal plan.
'I think I'm pregnant, my stomach's broken.' he said. 'I'm already starting to get sick of this food.'
The next day he started to worry about his bowel movements, telling viewers: 'I ate a lot of protein bars but nothing has come out yet, so that's a little bit alarming.'
He added: 'I haven't pooped in three days. That is a long time and to be honest it is kind of starting to freak me out a little bit.'
Protein bars contain sugar alcohols like xylitol that are only partially absorbed by the body, with the rest being fermented by bacteria in the gut — a process that causes bloating and gas.
At the same time, Dr von Schwarz suggested that because they don't smell like real food the mouth releases less saliva when they are consumed — which also hampers digestion.
Mr Murray is pictured above trying one of the protein bars while in his car
Concerned about his body, Mr Murray broke with the challenge on the evening of day three to eat blueberries and two apples — which quickly solved the constipation and led to him being able to use the bathroom.
By day four, however, he had returned to the all protein bar diet and said he was beginning to suffer from side effects.
'I've been tired this entire day and my stomach still makes that noise,' he said, 'I normally always have a six-pack [too], but since I've been doing this diet, I'm like always bloated. I also smell'.
'Other than that though, I'm feeling really good.'
By day four, he also found himself sitting down to eat several protein bars at once rather than the occasional two or three that he was having for meals.
Describing how he is feeling on day five, he said: 'I have this kind of like a restless feeling but I am also exhausted at the same time, I really don't know how to explain it. I can't sleep.'
By day six, he said he had 'never been more bloated in my life' and said he was struggling with workouts.
He kept up his calisthenics fitness routine while eating the bars, with calisthenics involving using your own body weight to build muscles
'It was a sad workout,' he said, 'I felt really weak. Normally I'm a lot stronger. Could it be the protein bars? Probably, probably.'
Without fiber to slow digestion, the nutrients are rapidly absorbed by the body rather than the slow release effect that happens when you eat whole food protein sources like chicken and red meat.
Dr von Schwarz said this is the reason that people often don't feel full after eating them, which prompts them to eat more food and consume more calories.
On the last day of the challenge, he only ate one protein bar in the morning and then nothing for the rest of the day — until he could have real food again.
By the end he said: 'But the more I learn, the more guilty I feel. It feels like I've been poisoning myself for the past like five years, cuz I've eaten these a lot — especially this week.'
He added: 'Processed food [like protein bars] really just kills your energy, is kind of what I'm noticing... when you eat processed food it really messes you up.
'This past week, I haven't gotten sleep, I haven't been able to do anything... They should be an every now and again snack.'
Throughout the challenge, he also kept up with his calisthenics fitness routine, a type of exercise that uses your body weight and gravity to strengthen muscles.
The video was published at the end of October last year, with viewers congratulating him for his efforts but saying they were unlikely to follow suit.
One commented, 'there's no whey I could do this!', while a second said: 'This made me so grateful for real food, thank you.'
Protein bars became a mainstay in the early 2000s, with hundreds of types now available — in a market worth $4.03billion in 2024 alone, according to estimates.