I am sure you have all heard of intermittent fasting (IM) and all of its wonderful benefits: weight loss, reduce inflammation, brain clarity and lower risk of diabetes. But not too long ago, you had all also heard of the wonderful benefits of small but frequent meals. Talk about contradictory health advice. To top it all off, both of these fashions were promoted initially based on what we thought our wild, uncivilized, hunter ancestors did. Now, I was not around back then, nor were you or any of the experts who promote one way over the other, so this is all based on speculation. Lets look what current science, based on actual evidence and clinical research has to say about intermittent fasting.
IM definition:
Simply put IM fasting means you eat for a specific amount of time, and besides zero calorie drinks like water and coffee, you do not eat the remainder of the time. The most common of IM is based on a 24 hour clock where you eat all your calories in a short, 6-8 hour window and fast the remainder of the time. Another less common version (and which I do not recommend at all) is to eat regularly most days of the week, but then restrict your calories 2 days a week. The idea is that during the fasting stage, your body runs out of glucose (carbs) and eventually turns into fat as the main source of its energy. Hypothetically this leads to weight loss, and more importantly leads to changes in the cellular level where your cells change from a pro-growth (think cancer) pro inflammatory state to a less growth and reduced inflammatory state. Let me be clear, most of this is theoretical or based on small, animal based studies. There is also some evidence that fasting leads to behavioral changes at the brain level, so after a short transition where you are getting used to not eating constantly and think you will die of starvation, trust me you will not, your brain eventually accepts the new you and actually becomes more clear. Some people use IM for weight loss, others use it to reduce inflammation and to improve their brain clarity. All subjective.
IM: real or fake:
The question is, does IM really make you lose weight and become healthier or is this the 2020s fashion and soon will die down just like so many other dietary fashions before it.
Scientific Evidence:
Unfortunately there are no long term clinical studies in humans. But this is typical of lifestyle changes, it is very hard to randomize bunch of subjects to long term lifestyle changes with strict instructions. One of the best studies up to now involved about 270 subjects: 1/3 in IM, 1/3 in calorie restricted but can eat anytime and 1/3 with no intervention. After about 9 months the IM group lost about 14 pounds and the calorie restricted group about 11 pounds, control had no significant weight loss. There is no real clinical significance between the 11 and 14 pounds and their difference could have been due to sampling chance. There was no significant difference between the IM and calorie restricted group in terms of biomarkers such as cholesterol and blood sugar. What was also important in this study is that both the IM and calorie restricted groups got dietary counseling, so they were eating an overall healthier diet.
So, what does this mean?
It means that the act of intermittent fasting in itself did not cause weight loss since both IM and calorie restricted diets lost similar weights. Rather, the weight loss comes simply because there is less time to eat and therefore one consumes less calories. It is also important to notice that both the IM and reduce calorie groups were eating healthier diets. So, the weight loss is not only due to less calories, but also to healthier calories. Shocking I know. Eat less, eat high quality food, you will lose weight.
My Take on IM:
I have had many patients who have tried IM. I have done it myself as an experiment so I know what I am preaching. As mentioned above, IM in itself is not going to make you lose weight. But if you find eating frequently a habit of yours, then IM is a great way to force yourself into a more structured eating. Be very aware, it only works if you eat healthy. Unhealthy diet does not make you healthier, does not matter if you eat it 24 hours a day or 10 hours a day. When I did my experiment I essentially ate my dinner early. 6pm dinner, then nothing till next morning around 8am, that is a 14 hour fast. Why 14 hours?
Because there is good evidence than anything past 12-14 hour fasting does not add any value. The problem with our modern lifestyle is that we continuously eat, including all the way to bedtime snacking while we watch, so really only time we are fasting is when we are sleeping, which for most people is 6-8 hours. I did lose a couple of pounds, but I was eating pretty clean to start with. What I did notice is that after a week of adjustment, I actually started feeling less hungry and did not think of food. Even at meal times I was not really starving, so I ate less. I felt better and more clear minded. Some of my patients who have done IM between 12-16 hours do report having lost weight. But more importantly, most of them report feeling better overall. Their brain is not constantly consumed with eating, and the feeling of hunger goes way. Most people report a mental clarity, which as I mentioned, I have to admit I experienced myself. One of the reasons for this, which I have included in pst blogs, is thought to be a reduction in insulin level.
Any calorie consumption stimulates the release of insulin into the blood stream. Insulin remains in the blood for about 2 hours and then subsides till next meal. So, during the IM fasting the blood insulin levels remain low for a prolonged time, whereas with frequent meals and snacking between, insulin never dips down. We now know that insulin is a significant hormone that plays a major role in metabolism and behavioral changes, all leading to weight gain. So, IM causes a reduction in insulin exposure, whereas frequent eating increases insulin exposure, leading to unhealthy outcomes.
Here is my final and simple recommendation: if over eating is a problem for you, or if you find yourself constantly snacking and hungry, or you want to try to lose weight, then there is no harm in giving moderate IM a try. 12-14 hours is plenty, 16 hours is the max I recommend. And since you have less time to eat, make sure you eat high quality food rich in healthy carbs, protein, fat and micronutrients. Make sure you give it at least one week, if you are used to snacking you may feel hungry at first, but this soon goes away as your brain adjusts to your new behaviour (you are not really calorie deficient, your brain is just used to frequent meals and is expecting it).
However, there are certain people that should never try IM by choice. It can actually be harmful. Here they are:
Kids and young adolescents. You should never restrict a growing child's food intake. But rather focus on giving them healthy food.
People on Insulin. If you are diabetic and taking insulin, you should not fast. Your. own body can regulate when to release insulin, so if you are fasting, you are not releasing insulin. But once you take exogenous insulin, then if you fast your blood sugar may actually drop to dangerous lows.
Pregnant women. See number 1. You got one in your belly.
Very elderly. Ok, this is optional Eat when you want, enjoy life. You have made it and deserve to do whatever you want, legally.
Athletes: this one is important and something I see often. You wake up one day and decide you want to totally and completely overhaul your life. You want to be a new you. Good for you, as your doctor, I am proud of you. So you start exercising regularly, and after searching the internet decide that IM is the new way trend to eat. So, you exercise, then you starve for a few hours, then you eat. Soon you make an appointment to see me because despite all your revolutionary changes, you actually feel like crap. Why? Because you are now starving your hungry muscles. It is highly recommended that you have a nutritious meal within 60-90 minutes of your exercise, otherwise your muscles do not have enough carbs and protein to repair and get stronger. So, your body actually enters into a starving state and you release stress hormones. You feel tired, do not gain fitness because your muscles can not repair in a timely fashion, and you may not even lose much weight. If your exercise is moderate (an hour of jogging or gym time or a group sport) then schedule your day so that even if you are following IM, one of your meals is within 60-90 minutes of your workout. For example, if you exercise in the morning, have a big breakfast, but then eat your dinner early, so you are fasting evening through the night. If you exercise after work, then have your dinner soon after and delay your breakfast. IF you are a moderate to advance athlete such as a marathoner or body builder, you need a lot of calories and need them frequently, IM is not for you. As a long distance runner, I try to follow a IM diet in my off season late fall through early spring when I am not running much, but once my training season kicks in high gear with a lot of miles running, I have a breakfast, morning snack, lunch, dinner and at peak training, even a bed time snack. But, its all healthy food.
Ok, that is all I have to say about IM. In honestly I. do not think IM is any revolutionary idea. Before the explosion of all the snacks in modern day, and late night TV watching because now we have a billion shows to watch, people ate three meals a day and that was about it. They may have had some fruits in between, but there was no packaged snacks. So everyone was fasting without being cool and savvy. Then came along a multi billion dollar snacking industry and the idea of frequent meals and snacks, and here we are. So IM is really just going back a few decades back and eat like our grandparents ate.
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