If Kiyoto is really 168cm/44kg or 5'6 and 97 pounds that would give him a Body Mass Index according to the US CDC of 15.6. That would mean he would have to gain nearly 20 lbs to even be considered a healthy weight. Yes, the BMI is primarily geared towards obese Americans, but my understanding was that a BMI of around 19 was usually ideal for marathon runners.
Although it is an uncomfortable question, I do think it is relevant to question whether Kiyoto's weight is healthy. Yes, undoubtedly he is fast and the weight works for him for right now for running very fast, but is it unnaturally low that it will endanger his future running? I don't know, but following Mary Cain's travails, among many others, I think it is a question worth asking.
Like many, I have become disillusioned with the prevalence of doping among elite distance runners in recent years . I have not seen anything suggesting that Kiyoto is doping, but it would be similarly quite sad if the reality of modern competition is that you need to be an unhealthy weight to achieve elite times.
Similar questions about weight were asked about Fiona O'Keefe after she won the US woken's trials. Like many, I was disturbed by how skinny she looked in the pictures from the race, even compared to how skinny she looked at Stanford.
Shockingly, Kiyoto actually looks even skinnier than Fiona.
Agreed, no one career about speculation about weight (or doping) on Letsrun, but the reality is that Kiyoto is likely now going to be a hero and role model for young Japanese runners, so thinking and questioning whether his weight is unhealthy certainly has merit.