Fasting for 24 hours, once or twice per week, combined with resistance training is what has always worked for me. Or, at least it’s what has worked for me from when I first started fasting in 2006 until now. Yet, every now and then I veer off course and chase whatever shiny new object the nutrition and fitness world has to offer, and it has a lot of shiny objects to offer.
Luckily for me this doesn’t happen too often, but it does happen, and sometimes I get so lost that I have to go back to the basics to remind myself not only does fasting work, but I also have to remind myself why it works.
Fasting works. It works for weight loss and it works for fat loss.
And while it can be easy to doubt these facts in a world where everything is up for debate, we can look at the simple math behind fasting to give us the confidence we need to hold to our convictions.
Luckily for us, nearly 100 years of scientific study of the human body has shown us that we can use the amount of oxygen inspired and the amount of carbon dioxide expired to estimate how much fat and carbohydrate are being used as fuel by our bodies. And, we can use the amount of nitrogen lost in our urine to estimate how much protein has been lost.
[[ This understanding is the basis of most metabolic research, so we can feel safe to be confident in these measurements. ]]
Most of the time the amount of carbohydrate and fats you use as a fuel is directly related to the amount of carbohydrate and fats you eat.
[[ The higher percentage fat you eat, the higher percentage fat your burn, and the same goes for carbohydrates. ]]
However, what is most interesting to us is when you’re NOT eating you can use these same measurements to get a very clear picture of your body’s preferred fasting fuel source.
In a study published in 2017, measurements taken on 64 men and women during a 24-hour fast revealed an average fat oxidation of about 125 grams. And since no fat was consumed during those 24 hours, it’s safe to assume that most of those 125 grams came from their body fat.
Interestingly, due to the fact that what we call ‘body fat’ (adipose tissue) isn’t made up entirely of fat, this would mean 125 grams of pure fat loss would equate to about a 1/3 of a pound of body fat lost – and this is without any exercise and very little movement during the day.
Of course not everyone lost exactly 125 grams, but it was the average, and a good number for us to work with… We know for sure that when you fast, you use predominantly body fat as a fuel.
The amount will vary depending on your height, weight, age and activity level, but we can safely say you will lose between a quarter to a half pound of fat with every 24-hour fast. This is a lot of fat loss and would be hard to replicate in any other way that doesn’t include some form of fasting.
[[ Your WEIGHT LOSS can be over 5 pounds in a 24-hour fast, but most of that is your body losing excess water. ]]
The point is fasting works. It works for me, it will work for you. However… we do have to eventually eat, and eating is just as much a part of Eat Stop Eat as fasting.
Luckily, measurements were also taken while these same people were ‘eating to energy balance’. During this time the amount of fat used as a fuel closely matched the amount of fat that was eaten.
[[ This is a common finding in this type of research – the makeup of our diets, that is the ratio between carbohydrates and fats, tends to dictated the ratio of the carbohydrates and fats we use as a fuel source. ]]
This means as long as you are eating at roughly energy balance, you will not be gaining fat… you may gain back some WEIGHT after your fast, but that will be water weight plus the weight of the food in your stomach 😉
It sounds easy in theory, but ‘eating to energy balance’ does take some work; you do need some idea of the calorie content of the foods you are eating, and for some people, this is simply asking too much…
So what happens if you’re not exactly perfect?
Or, what happens if you overeat by say… DOUBLE the calories you need?
When the same subjects who fasted for 24 hours consumed double their calorie needs for 24 hours they ended up eating more fat than they were using as a fuel. This means that even though the amount of fat you use as fuel increases as you increase the amount of fat you eat, there are obviously limits 😉
In this particular study the subjects gained between 12 and 175 grams of fat per day, largely dependent on the amount of fat in their diets during those 24 hours of overfeeding.
[[ There were 4 overeating trials with varying ratios of carbohydrates and fats in the diet being studied. ]]
This means if you fast for 24 hours, then follow it up with 3 days of massive overeating, you will gain back the fat you lose, and then some.
[[ Fasting is great, but it’s not magic. 😉 ]]
However, on the bright side, mild overeating should not cause you to gain back all the fat you have lost.
Interestingly, the trick here does seem to be managing your total fat intake, as the people eating 20% protein, 20% carbohydrate and 60% fat had the most positive 24-hour fat balance, gaining almost 175 grams of fat in 24 hours (keep in mind they were overeating by almost 2,000 Calories in this 24-hour period), whereas when they ate 75% carbohydrates, 5% fat and 20% protein they gained the least amount of fat .
The bottom line is, if you fast for 24 hours, and do your best NOT to overeat, then you will stay ‘fat negative’ until the next fast. Repeat this once or twice a week and you will see nice, steady fat loss… your weight may fluctuate, but the fat loss will maintain as long as you pay attention to what you eat in between your fasts.
This is where I tend to falter, not the fasting, but the eating between the fasts.
And it’s usually because of protein.
After all these years I still suffer from protein guilt, that is, worrying whether or not I have eaten enough protein during the day. I also worry about losing muscle. I find it important to remind myself that we lose protein every single day, fasting or not.
The amount of protein we lose every single day is roughly 60 grams, which is why in most countries the recommended daily intake of protein is set at around 60 grams.
I also need to remind myself that these 60 grams we lose each day are not all muscle mass, as we lose a lot of this protein from the much more metabolically active gastrointestinal system.
This means I’m not losing 60 grams of muscle every time I fast, nowhere near.
In fact, I lose more protein when I’m eating than when I’m fasting – the only difference is when I’m eating, I’m also consuming protein. While a 24-hour fast may see you lose 50-70 grams of protein, a day of high protein eating may see you lose more than double that amount.
This is because protein acts similar to fat in that the more protein you eat, the more protein gets ‘used as a fuel.’
The difference being the amount of protein being consumed – During a high(er) protein diet, the amount of ‘protein in’ is generally a larger amount of the ‘protein out.’
This means that while we will lose some protein while fasting, a diet that consists of slightly higher than normal protein (70-90 grams) will make up for this loss, and anything higher than this (100+ grams per day) should allow for muscle growth, assuming you are resistance training.
[[ Also keep in mind that some research suggests that chronic resistance training actually slightly lowers the amount of protein you lose each day, so ‘60 grams’ may be a slightly higher average for people who resistance train. ]]
The bottom line is that we know you will lose fat when you fast. We also know we have the ability to gain this fat back when you are not fasting.
But when done properly, the process works. It is not magic, and you do have to eat like an adult on the days you aren’t fasting. And you do need to be active and be mindful of how much protein you eat, but again… the process works.
In this sense, the best you and I can do is to remember that fasting works, and then to do our best not to sabotage those results.
[[ NOTE: These studies were not examining adaptation to a high fat diet or anything of that nature, so while they do show less fat gained a low fat diet in 24 hours, I have no idea how that translates to chronic use of any of these diets ]]
*J Clin Endocrinol Metab, January 2017, 102(1):279-289