1

I am going to change my daily night diet plan, from

1 hour running after dinner 

to

1.5 hours walking before dinner

So which is better for the diet purpose? Does walking under starving(without enough energy) harmful the body?

4

Like in a lot of exercise and diet research it is difficult to find a clear-cut answer and you may even find contradictory results. Your question involves not one but two changes:

  1. Food consumption after or before exercise.
  2. Shorter exercise at a higher intensity versus longer exercise at a lower intensity.

For number 1 there is already plenty of discussion with some people being convinced that fasted cardio (and possibly also resistance training) yields better results for fat loss or body (re)composition. Others call this a myth to be debunked.

In the Journal of the International Society of Sports Nutrition a study was published (1) that concludes they didn't observe a significant difference in either approach. It was however rather small in scale. The American Journal on Physiology provides a study (2) showing differences in lipolysis, the breakdown of fat, depending on whether carbohydrates were ingested prior to exercise or not but concluded that the the overall fat oxidation was not diminished. This suggests that the net energy expenditure is of more importance than the order in which calories are burned and consumed. This meta-study reviewing 46 other publications (3) from the Scandinavian Journal of Medicine and Science in Sports implies that there is some evidence for a difference in performance and metabolic adaptations between fasted and fed exercise but calls for additional study and concludes that possible long-term results are still unclear.

So it remains unclear whether there is truly a different outcome in the use of body fat depending on exercising in a fasted versus fed state with all else being equal. If there is, the result does not seem pronounced enough to be of concern for non-athletes.

Then for number 2, here you will find a difference. 90 minutes of walking is likely to result in a different amount of calories used than 60 minutes of running. What that difference is depends on the intensity at which each is performed. So maybe it is interesting to see instead if this results in a difference in body composition. Here is a list of studies on the subject of fat utilization in cardiovascular exercise at different intensities: 4, 5, 6 and 7. In general it is found that exercise at a lower intensity relies more on the use of fatty acids for energy while exercise at a higher intensity relies on the use of carbohydrates, suppressing fat oxidation. This suggests that longer training at a lower intensity might be preferable to higher intensity training for the same energy expenditure but for a shorter time. But such studies usually look at the use of resources during the training which doesn't necessarily show the full picture. After all, depletion of glucose and glycogen stores could lead to their replenishing from fat via gluconeogenesis. The body will go looking for the needed energy and use what is available in an efficient way.

Going by this you could conclude that you'll see benefits from low-intensity training such as walking in both fasted and fed states. High-intensity training like running when fasted would rely on glucose and glycogen stores which could be replenished from fat, but would also be quickly topped up when eating after exercise. When done in a fed state it would likely take longer to reduce the levels of glucose and glycogen, already higher after eating, to the point that fat stores are used. But in the end it will depend on total calorie expenditure versus intake.

To complicate matters further there are many other factors at play. What is the nutritional composition of the food ingested? Low fat and high carb? The other way around? Primarily protein-based? Blood sugar levels and insulin response are also to be considered.

You can keep searching and searching, going deeper into the rabbit hole without really finding very conclusive answers. So to really answer your question: do what works best for you. I am convinced that the aspect of adherence to your training and diet is of a far greater importance than the question of fasted versus fed exercise. Perhaps after eating you feel sluggish and find it difficult to go out for running with a full stomach. Or it may be the other way around and you feel like you can't really perform well when fasted. Maybe you're more prone to injury or need longer recovery when running rather than walking and it's not a good long-term strategy. Or maybe running works great for you and you prefer the shorter workout. Or perhaps you don't need to choose and resolve to exercising every day, but choose what to do and in what order depending on what works best that day. Anything to keep you on your diet and exercise regime in a healthy way is the better option rather than trying to micro-optimize at the cost of consistency.

Also use the true final arbiter, your personal results. Try each for at least two weeks (preferably longer) and observe what happens. Do you notice more weight loss with one approach over the other? Do you see a difference in body composition? Is one routine far more enjoyable for you? Then that is what works best for you subjectively.

References:

  1. Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 2014. Brad Jon Schoenfeld, Alan Albert Aragon, Colin D Wilborn, James W Krieger & Gul T Sonmez.
  2. Substrate metabolism when subjects are fed carbohydrate during exercise. The American Journal of Physiology, 1999. Horowitz JF, Mora-Rodriguez R, Byerley LO, Coyle EF.
  3. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scandinavian Journal of Medicine and Science in Sports, 2018. Aird TP, Davies RW, Carson BP.
  4. Determination of the exercise intensity that elicits maximal fat oxidation. Medicine and Science in Sports and Exercise, 2002. Achten J, Gleeson M, Jeukendrup AE.
  5. Maximal fat oxidation during exercise in trained men. International Journal of Sports Medicine, 2003. Achten J, Jeukendrup AE.
  6. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. The American Journal of Physiology, 1993. Romijn JA, Coyle EF, Sidossis LS, Gastaldelli A, Horowitz JF, Endert E, Wolfe RR.
  7. Fat metabolism in exercise. Advances in Experimental Medicine and Biology, 1998. Wolfe RR.

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