Intermittent fasting is a bit of a hot topic at the moment that seems to be popping up everywhere as the next amazing thing in nutrition. In reality it is a concept that has been around for a long time in various forms! The basic idea is quite obvious, periods of fasting alternating with periods of eating. It claims to help with weight loss, fat loss, improve your metabolism, improve digestion and even make you live longer! So today I am going to give you an overview of the different kinds of fasting, their pros and cons and some of the nutritional science behind them in hopes that you will learn a bit more about it and be able to make an informed decision as to if it is right for you. PS - if you aren't at all interested in the science and just want the verdict then feel free to skip to the end!
What is it?
The core principle of intermittent fasting is short periods of complete or partial fasting (anywhere from 0-100% reduction in calories) alternating with periods of normal, unrestricted eating. These periods can be anywhere from a couple of hours to several days and there are many variations and patterns out there. The main goal is to partially restrict the calories eaten in order to lose weight, as over a period of weeks to months your body will be in an overall calorie deficit, leading to weight loss. Its appeal is that it enables you to only "diet" for set periods rather than having to always restrict so many people feel that it still allows them to eat the things they love. There are many claims for secondary benefits such as improving your body's response to insulin (the hormone that regulates your blood sugar), improving appetite regulation, fat loss, lipid metabolism, muscle gain and even extending your life span! I will go in to these claims later, but first let's have a look at the most popular methods:
5:2: I am sure we have all heard of the 5:2 diet, it is made up of 5 days of normal eating and 2 days of eating 500-600 calories, usually the fast days are spread through the week. Other variations of this include alternate day fasting, where you fast every other day and end up with a 4:3 configuration.
Leangains: This method claims to be tailored specifically for strength and fat loss, it consists of eating for 8 hours and fasting for 16 hours with the eating occuring from around 12pm-8pm. Within this period 3 meals are eaten and the plan suggest high protein meals with more carbohydrate on gym days and more fat on rest days.
Time restricted feeding: This is basically the general form of the Leangains and Warrior diets where you restrict your eating to certain times in the day. There are many variations on the timing of this, from only eating in the morning to only eating in the evening.
The Warrior Diet: Eating very little throughout the day and then "feasting" for 4 hours in the evening, focusing on whole, paleo foods.
Eat-stop-eat: This diet consists of introducing a full 24 hour fast once a week.
Meal skipping: This is as simple as it sounds, skipping a meal or even two is considered fasting as your body is in the fed state for less hours of the day. The most common form of this is skipping breakfast or skipping dinner.
Is it good for you?
As with anything in the field of nutrition, there is no black and white, yes or no answer, only a series of pros and cons. One of the main ideas behind the concept is that animals in nature are not constantly in a fed state, hundreds of thousands of years ago humans were hunter gatherers and would regularly go for long periods without food. There were no 3 square meals a day and we were used to being in a fasted state. These days, with everyone eating a couple of meals and snacks a day, our bodies are almost always in the fed state where we are mostly storing energy and aren't so often in a true fasted state. The best way to understand the concept is to understand the scientific differences between the "fed" and "fasted" states.
Fasting: While fasting, our bodies are primed to use our stores for energy - our fat, carbohydrate and sometimes protein stores are broken down to provide energy for our body to burn. This means that during this state, we are likely to be burning fat as our carbohydrate stores are used up very rapidly and this is why people are suggesting intermittent fasting works for for weight loss and improving your metabolism, sounds great right?
Fed: From the minute we eat, our body starts working to digest and store the energy we have just eaten. Once digested, the broken down products of our food enter the blood to be redistributed as needed. Hormones such as insulin work to store carbohydrates in our muscles and liver and fatty acids are either used or stored in adipose (fat) tissue. While your food has cleared through your stomach in a couple of hours, the reality is that it takes around 6-8 hours for your body to properly get back to the fasted state as those fats remain in your blood for a long time after digestion.
Therefore, the aim of intermittent fasting is to allow for more time to be spent in this fasted state where our bodies are burning fuel and less time in a resting and digesting state. The claims say that this improves the metabolism and has all of these benefits on various other factors mentioned earlier. We must consider the point that while yes, our bodies were originally designed to fast for long periods, humans have been eating regularly for centuries and so are we still adapted to do that or have we evolved? It is completely true that you will not starve to death if you don't eat for a day and that many of us eat out of routine and boredom rather than true hunger. Fasting most definitely does allow you to be more in tune with your body as you will become more sensitive to what hunger feels like and likely will appreciate and taste your food more. It also will lead to weight loss if done properly as you will be in a calorie deficit. However normal weight loss through regular calorie restriction also improves the metabolism in similar ways to fasting as it improves the metabolic profile overall. Therefore, the big question is whether it is truly the fasting giving these benefits or whether it is just the effect of healthy weight loss.
The Scientific Evidence
Of the few studies done in humans, fasting patterns produced a similar amount of weight loss as a normal diet restriction - eating 1000 kcal a day as 2 day meals caused the same amount of weight loss as eating 1000 kcal a day as 4 meals. It is a similar story for changes in the metabolism, both methods of dieting produce similar improvements in things such as blood glucose and blood lipid control. With regards to appetite control, people who are intermittently fasting feel hungry and unsatisfied more than those following a normal weight loss diet. It is also common that people who are fasting then eat more in the feeding periods than they would usually which defeats the purpose of it. Lastly, the claims about it allowing you to live longer are unsupported in humans. Those studies were performed in rats and a more recent one done in monkeys showed that it had no effect on lifespan. Most of the studies on this subject are done in animals and so it is hard to know if these benefits are applicable to us.
You may be feeling utterly confused by this point, and unfortunately that is often the outcome in nutrition! The jist of it is, yes, intermittent fasting undoubtedly has benefits for weight loss and your metabolism but they are very similar to the benefits seen in normal calorie restricted diets. Therefore, the debate as to whether it is better completely depends on the person. If you are someone who finds it easier to do fasts because you can then eat normally at other times and feel unrestricted then by all means go for it and it will benefit you! Equally, if you find it hard switching and you get too hungry then there is absolutely nothing wrong with sticking to the classic 3 healthy meals and this will definitely benefit you too if you make the right choices. Nutrition and healthy eating is about doing what works for you and what you will be able to stick to the most. Many people find intermittent fasting is good to do for a short period to lose a bit of weight, but ultimately, changing your lifestyle to normal, healthy meals is the most sustainable and fulfilling option. In my opinion, fasting often ends up being just another way to diet for most people, when really we need to be saying no to diets and yes to eating in an intuitive, unrestricted but balanced manner. Anything that has strict rules in it is a restrictive diet and has no place in fostering a healthy relationship with food. I hope this has helped you understanding fasting a bit more, please get in touch if you have questions or would like a bit more detail - this is just the tip of the iceberg!
Metabolic impacts of altering meal frequency and timing – Does when we eat matter? (Hutchison & Heilbronn, 2016).
Metabolic effects of intermittent fasting (Antoni & Collins, 2017).