I was talking the other day to a new friend of mine about the effects of “starvation mode” during fasting. A common misconception is that not eating for x hours will result in the body reducing it’s metabolism and/or muscle breakdown. I suppose the rationale for this is that the body will reduce metabolism in order to sustain life for as long as possible. On top of that the body will burn amino acids from muscle as fuel through a process known as gluconeogensis. Due to these two reasons, individuals who believe in starvation mode believe that our metabolism will decrease, and we will get fat even while on a diet where we are below maintenance calories.
Research simply doesn’t support these concepts. This study was one of the few studies that I found that showed a decrease in metabolism as a result of fasting. The study concluded that a 72 hour fast would decrease your metabolism by ~8%. However, this isn’t a problem when IF’ing, as you only fast for 16 hours.
Let’s examine a few other studies to see if our body really does go into starvation mode while fasting. This study showed an 3.6% increase in metabolic rate from fasting for 48 hours. This study by Zauner et al, showed a 100% increase in serum norepinephrine (a hormone which stimulates our metabolism by various mechanisms) in an 84-hour fast.
In this study, they found that a 72 hour fast increased serum catecholamine levels as well as heart rate. Yet another study showed that after a 72-hour fast, there were increased circulating fatty acids in the blood stream. This means the body was utilizing fatty acids from fat (obviously) in order to fuel the body for exercise. I’d also mention in the study above, that cardiovascular performance (albeit at only 40%VO2 max) was maintained 72 hours into the fast.
Let’s turn our attention to muscle loss while fasting. Unfortunately, there isn’t any research on the subject of muscle retention or muscle loss while in a fasted state. So the next best thing to look at is muscle retention while in a negative energy balance.
Marks et al studied the effects of diet, cardiovascular training, and resistance training on 40 inactive women. The groups were control, diet only, diet + cycling, diet + resistance training, and diet + resistance training + cycling. The diet + resistance training + cycling group gained strength, reduced fat mass, and increased VO2 max. This was the only group to attain all three of the above goals. Interestingly enough, even those these women were only consuming 628 calories per day, they all maintained their fat free mass while losing fat. It is worth mentioning that the women were able to get these results training for only 30 minutes per session for 3 sessions per week.
There are a couple things to consider in this study. The women were all sedentary, so they probably had no prior resistance training experience. As you may know from some of my articles, novice trainees can gain muscle and lose fat simultaneously when beginning a weigh training program. It would be interesting to see a study on trained women or men on similar hypocaloric diets. I don’t recommend dieting in this fashion, but the research in this study was reasonably sound.
Another study examined the effects that GH has on muscle protein synthesis during a 40-hour fast. Since it is well substantiated that GH increases while fasting, they wanted to examine the if it was the spike in GH that was leading to protein conservation while fasting. They determined that GH through some form of mechanism is directly related to protein conservation.
Now, let’s put what we’ve learned from a few of these studies into complete lay-man’s terms. The majority of studies that I found showed that the body will increase our metabolism by increasing the release of various metabolic hormones during fasting. One study showed a 100% increase in norepinephrine in a 84-hour fast! This means that even after 84 hours fasting our metabolism is elevated above normal levels. So, the whole starvation mode myth is pretty much out the window.
Next up we looked at how low calorie diets effected our lean mass. The study I cited showed an increase in strength and VO2 max while losing fat on a hypocaloric diet. This goes to show you that you can maintain muscle mass while dieting down. That’s not to say you will
only lose fat and spare 100% of your lean mass. Resistance training needs to be added in order to keep as much muscle mass on as possible while trying to lose fat.
Finally we looked at how the increase in GH caused by fasting spares protein. So in a nut shell, starvation mode is not an issue when IF’ing. If you are only eating 3 meals a day, you are also not at any risk of reducing your metabolism. So if you are only eating 3 meals per day, don’t think that your metabolism will decrease and your muscles will melt off your body. We don’t work like that!