As far as significant weight loss, I do not believe the research supports that. The topic has come up here before, and there are a few research articles to support this.
Here is one article on the topic:
I’ll also look for another one for you that was posted here a while back.
Unless you or your client is diabetic or have a medical condition that would prohibit you from doing your cardio on a n empty stomach, I don’t see anything wrong with it (this is why checking with your physician first is always a great idea). Below are a couple more articles you could also take a look at (among many found online):
I hope this helps.
There is no simple answer, because for some populations it helps to burn more fat, for some not. Everyones metabolism is different. How and why, lot of Research is uselless and incompatible so you can not compare them. Also, a milion of processes, combinations in signaling-controling pathways in every person could be different so there is no way for simple answer without deep knowledge in biochemistry, phisyology, effects of exercise to that individual, Training adaptations, gene transcription, etc… Realy long story 🙂 Probably to some sedentary people it can help to develop metabolic flexibility and learning their body again to use fat as primary Energy substrate. Also, for some bodybuilders some light cardio at morning is helping loose fat without afecting muscle mass, becose morning cardio is only part of daily Training. Of course, at the end of the day, the most important thing is how many calories you burn in 24h, not only in the morning.
The theory and real life practice that you will have a less efective workout at morning with an empty stomach, and consequencies, is different if you speak with researcher from lab, or with a national team coach. I spent in my younger age about 15 years in Training camps of wrestlers, where national teams from different countries were training, and every day the First Training was on empty stomach, but i never noticed that it is less intensive than the Training later in the day. Or maybe, if someone is thinking that running cca 5 miles as fast as you can, than do 10 or more sprint with other guy on your shoulder, than bodyweight intensive strength and plyometric Training is not intensive. 🙂 And lot of the guys was world or Olympic medal winners. But again, adaptation developed in that kind of athletes, has nothing scimilar to average Joe.
About good Research, in my opinion, but only if you have the base knowledge, the best starting point is presentation of Jullen Zierath:
She is a chairman of Nobel Prize comitee for Physiology and Medicine, and this presentation has a tone of material for Google Search afterward. In next month after her presentation, I printed more than 1000 pages about exercise and metabolism Research, mostly connected with real life practice. Do not be afraid of the presentation title, it is not only about diabetes and obesity.