Hi Karen. I had never dieted in my life & started a very strict keto diet 3 years ago to lose 20 pounds. My boyfriend & I ate almost solely meat, fish, cheese, eggs, nuts, & fats. Within the 1st few weeks, I was so out of breath upon physical exertion (not at rest), that when we were planting trees on his 2 acre yard on a hot day, I literally couldn’t walk more than 30 feet without stopping for a rest to get my breath. I had to plop down on the lawn to catch my breath. He had zero side-effects, btw. My breathlessness was extremely odd, to say the least, because I’ve always been “athletic” & had been hiking a lot before starting the diet & while I’ve never had excellent lung capacity (even while playing several varsity sports in high school) I’ve never had trouble catching my breath after walking 30 feet !!? I knew it was ketosis because it coincided shortly after the start of my diet & that was the only thing that I had changed in my lifestyle (& it was an extremely strict diet keeping carbs under 20g). After a few weeks, my breath normalized. After 2 years of the diet, I quit keto, but recently resumed a relaxed low-carb diet. This time, I’m not using keto strips & am eating lots of low-starch veggies, so it’s not as strict, however, once again, I’m having shortness of breath every time I go hiking (again- it does not happen at rest). Fortunately, this time the breath shortness isn’t as severe, but then again, neither is my more “relaxed” diet. I think @AtomicSpaceBunny ‘s reply (i.e., “acidic blood”) is the answer: blood carries oxygen, & low pH blood doesn’t carry as much to your lungs & other organs. Good luck with your IBS & in your endeavors!! EDITED TO ADD: when i 1st experienced shortness of breath, I googled “keto acidosis”. The major symptom is rapid breathing & shortness of breath. I went so ketotic, that I think I was in keto acidosis (which isn’t healthy long term & it’s what people with type 1 diabetes sometimes experience).
Can keto cause shortness of breath?
I exactly have the same experience as OP. I am also underweight.
Thanks for all the comments, I will try the histamine thing and share my response.
Hi! Yeah that sounds rather similar to my experience. For me, the VCD is so far my best explanation. I tried the low histamine and acidic blood options and they didn’t make any difference in my case. So, just in my case, I think the VCD makes the most sense. However, I really don’t know why ketosis would cause the onset of that and make it worse. I can only guess. Good luck!! 
When you feel short of breath is it because you can’t take deep breaths? Or is it no matter how deeply you breathe, you can’t seem to get enough oxygen? Does your chest feel hollow?
I had to cut down on histamines. I am short on time, but look up histamine containing foods. There are some plants that are naturally high in histamine, but it’s mostly in protein foods that are aged or fermented: cheeses, salami, aged meat (or old), and seafood, wine.
Georgia Ede on her diagnosisdiet.com has a couple of write ups on histamine.
Hi! It was more like I couldn’t get a completely deep breath. It feels like the very top of my lungs won’t expand all the way and fill. Imagine trying to yawn and then just before you’re done inhaling, you stop short. Like something is preventing my lungs from inflating that last fraction so I don’t feel like I got a satisfying breath. I know some people on this thread have had good luck with cutting out histamine containing foods. I tried that, and it didn’t help me, so for me, it is something else. I think for me it is caused by silent reflux, which doesn’t cause heartburn but does have other effects, one of which is it can cause a vocal cord dysfunction and breathing problems. One of the triggers for silent reflux (or any reflux) is a high-fat diet. So, I think that the high fat of the keto diet was causing reflux and creating VCD. (If it were just that one symptom, I wouldn’t think I have silent reflux, but there are other symptoms too that make me think that for me, it is likely.)
So have you tried reducing overall fat and increasing protein? If so, what happened? If not, why not?
No I haven’t specifically tried that yet. The reasons are complex. I have IBS and eosinophilic colitis as well as whatever may be going on in my upper GI. It seems that the foods that help my lower GI irritate my upper GI and vice versa. It is a constant balancing act that is hard to pay too close attention to because I have 6 people in my house to feed, and, you know, life. 
Also, most sources of protein that I can eat are pretty high in fat. (I can’t tolerate soy. I do ok with Quinoa, but can’t eat large quantities of grain. Chicken and turkey are ok.)
Anyway, I’m still investigating things. I have an h. pylori test tomorrow. Then I’ll be doing a SIBO test. I’ll also be going back to my allergist to test for more potential triggers, and then I’ll be seeing a nutritionist. Hopefully, I can find enough foods that I can tolerate throughout my whole GI and not have to obsess about everything I eat. I’m tired of thinking about it, to be honest.
Yeah, pretty much. But I’ll let @Fangs and @amber respond specifically. I eat plain keto that includes some plant-based fats (red palm and kernel oils, coconut and MCT oils, occasional macadamia nuts) and my daily breakfast coffee.
When you see your doctors, be aware that bringing up keto often places a target on that as a likely cause. I personally would let them investigate before bringing forth any possible suspects (like keto).
I simply tell docs that I am done with sugar and processed food. They can rarely find fault with that and will look further than they might if “keto” is on their radar.
Just my thinking… good luck. Hope you get some good answers.
Thank you for posting. This is my first week. I already struggle with breathing problems Due to my stomach not absorbing B12. But while on this I feel terrible. And imy muscles feel like I’ve done a crazy exercise program (all day) and the breathing is hard for me. Two years ago I went to the hospital with heart palpitations but it turns out it was my larynx spasming. It was right when people were dying of Covid and I was stressed out. The doc said stress can cause the larynx to spasm. So I’m thinking from what you said it may be my larynx acting up with that stress of not having a lot of carbs now. I’ve been a sugar addict my whole life too. So I can see my body freaking out without the soda. Haha also in peri-menopause so that’s another stressor. Thanks for your post. I’m going to try the Soma breathing technique and see if that helps relax my larynx. Thanks again!
I made an account here to specifically reply to this because I actually have been dealing with the same thing! I describe it as not being able to get a full breath rather than being short of breath. I thought I was experiencing heart failure or something just from the Google searches I’d done. It scared me to think that might be the case at my relatively young age.
Things I’ve noticed - on a relatively heavy carb intake (100-200) I noticed the issue subsided or went away completely. I thought it may be asthma or allergies. This thread has been enlightening and am happy to have stumbled on it. I am still not exactly sure of how to deal with this issue as I really like eliminating as many carbs as possible but this thread gives me some data points to drill down into.
Hi JustinH,
It is such a confusing side effect. I’m still not 100% sure of the cause for me. And I noticed the same thing: when I added back 100-200 g of carbs, the brething got a lot better. Any chance you are already a thin person? I hope that isn’t too personal of a question. The reason I ask is because I am on the thin side just naturally and so I was trying to NOT lose weight on the keto diet. I still did lose about 10 lb over the course of time I was on keto, and I wonder if the breathing issue has something to do with not having much of my own body fat. I really want all the benefits that come from ketosis (which are many) without losing weight, and I haven’t figured out how to do that. It is one of the reasons I’ve gotten off keto. I still keep my carbs below “normal” and that helps my breathing, but I feel I’m missing a lot of the other benefits I had when I was keto.
My sentence about keeping my carbs below “normal” helps my breathing was confusing. What I should have said was that I do keep my carbs below normal, but not so low as to be keto and keeping above keto has helped my breathing.