After hearing the hype, I just finished reading Dr. Jason Fung’s book The Complete Guide to Fasting. The book is great as a resource on the science behind fasting and its many benefits. But a fundamental motif of the book is Fung’s rejection of CICO – the “calories in, calories out” model of weight loss.
What is CICO? At its most basic level CICO says that to lose weight you must either eat less food or burn more energy. For CICO, obesity is fundamentally an energy balance disorder. Everyone has a “maintenance level” of calories. Let’s say it’s 2000 calories. If you eat more than 2000 calories a day you will gain weight. If you eat less than 2000 calories you will lose weight. If you eat exactly 2000 calories you will maintain your weight, hence the term “maintenance level”. Furthermore, as our bodies gain or lose mass the exact maintenance level will change itself, which makes sense: bigger bodies need more energy to “stay big” – smaller bodies need less energy to “stay small”. Seems intuitive right?
Fung thinks this model is wrong. He claims that obesity is not an energy balance issue, i.e. consuming too many calories while not burning them off, but rather, a hormonal issue i.e. elevated insulin levels and insulin resistance cause body fat to be gained.
Fung starts his criticism of CICO with “two incontrovertible facts”:
(1) Over the past 20 years, conventional weight loss advice has relied on CICO
(2) Over the past 20 years, obesity rates have exploded.
Fung’s conclusion? He draws a dilemma: either CICO is good advice but people don’t follow it, or people actually do follow CICO but it doesn’t work. I personally think the first horn is true: CICO is good advice but people don’t follow it properly. But Fung argues that people are in fact capable of implementing CICO but that CICO just doesn’t work. Which is weird, given the book is about how fasting can help with obesity. I’ll come back to that.
Fung’s main criticism of CICO is that when people try to follow it they inevitably fail and gain back all the weight. This is undeniable. But that doesn’t mean CICO is false or that CICO is not the best way to think about obesity.
Fung discusses the “Biggest Loser” diet where 400lbs people are put on 1500 calorie diets, exercise a ton, drop a bunch of weight, and then gain it all back. Fung uses this as evidence for something called “starvation mode” whereby if you eat less in CICO-fashion you will inevitably “damage your metabolism” and once you stop eating below maintenance you will gain it all back even faster because your metabolism is “damaged” from being in “starvation mode”. This post isn’t going to delve into the scientific details of why the Biggest Loser model doesn’t support the “starvation mode” hypothesis (see this reddit post for details if you’re interested). Instead I’m going to posit a much simpler explanation for why the Biggest Loser diet failed.
If you’re a contestant on Biggest Loser you obviously got there because you had poor life management skills – you couldn’t manage the basics of diet and exercise and so you allowed your weight to balloon up to 400-500 lbs or whatever. So you have no idea how to take care of yourself. Then you get on the Biggest Loser and you have pro trainers and dietitians doing all the hard work for you. They are motivating you. They are creating your meal plans. They are calculating the calories for you. They are creating your workout plan. They are pushing you in the gym. Not to mention the pressure of the TV cameras and the nature of social competition. So there are all these incentives lined up to get people to stick to a healthy-ish lifestyle for the duration of the show. And it works. They all lose a ton of weight.
But it’s extreme. It’s pushed to the max. Once the show is over they just go back to their lives and are expected to now do everything themselves. If you don’t know how to count calories properly, or you don’t have a healthy relationship with junk food, don’t have a good fitness plan in place, it’s going to be really difficult to maintain the healthy life style. It has absolutely nothing to do with “damaged metabolism” and everything with how they managed to gain all the weight in the first place: it’s really fucking hard to turn down a handful of french fries when you’re out at the bar. It’s hard to be healthy in the modern toxic food environment, where we are literally surrounded all day long with super-delicious, super-calorie-dense, sugary, fatty, and salty foods. Like really hard. Like our brains are not designed by evolution to be able to cope with that temptation in a way that makes it easy for us. Food scientists at major food processing companies have literally spent decades and hundreds of millions of dollars researching how to tinker with sugar, salt, and fat ratios in order to make food highly tasty and addictive. Modern food is engineered precisely to trick our brains into wanting more and more and more. That’s why the Biggest Loser candidates gained all the weight back: the same reason they and everyone else in the Western world gains it in the first place: they overate because it’s really easy to overeat in our modern food environment.
Okay, back to Fung. I want to now talk about the deep irony of Fung’s criticism of CICO. CICO is bad because it “restricts calories”. But what is fasting if not an even more extreme method of restricting calories? If you eat 500 calories less than your maintenance level you will lose 1 pound a week. But if you don’t eat anything for a few days you will obviously lose even more weight according to the exact same principles of thermodynamics. Mass can’t just be created out of no where. On a three day fast it is impossible to gain weight because there is no energy coming in from outside the system.
So fasting works as a method of weight loss because it is fundamentally a version of CICO. Where fasting differs from your typical “eat 500 calories less a day” version of CICO is that the process is different. For some, eating 500 calories less a day is difficult. First of all, most people absolutely suck at accurately tracking their calories. Like really suck. Most people who try CICO don’t even get a food scale and if you aren’t weighing your food precisely it’s really really hard to do CICO properly. So that right there makes CICO difficult to implement unless you’re a very conscientious person. Fasting is easier for some people because they don’t have to count calories on fasting days. They just eat nothing. And then because fasting itself helps develop a better attitude towards intuitive eating, hunger signals, and our relationship towards the modern food environment, fasting adherents have an easier time not overeating on nonfasting days. So joined together, the total energy balance for fasting and nonfasting days brings about a weekly net calorie deficit and thus you lose weight!
There are some more details here. Hormones, insulin, etc., do play a role in making it easier to stick with weekly calorie deficits. But they are not operating on a fundamentally different principle. CICO is still king. You can never escape CICO. You can only make it easier to stick with CICO. For some, eating a low-carb high fat diet and fasting makes it easier to stick with CICO. But that’s all well and good. But many body builders eat a ton of carbs while following CICO and still get shredded abs. Carbs don’t make you fat. You can absolutely shred your body fat while still eating carbs. Bodybuilders are proof of this. But bodybuilders intuitively understand CICO – hence why they go through “cutting” phases (calorie deficit) and “bulking” phases (calorie surplus). Bodybuilders use CICO to their advantage and make it work for them. But bodybuilders also have a better relationship with the toxic food environment than your average person – they are able to resist the temptation to eat a bunch of super palatable, super calorie dense foods all the same.
In conclusion, I highly recommend Fung’s book as an introduction to the benefits of fasting for fat loss and general health. But Fung, like many, creates a strawman of the CICO model and ultimately misunderstands how it works, thus creating the false impression that calories are irrelevant when thinking about fat loss. I’m sorry, but no: calories are still important and people ignore them at their peril. And if they do ignore them but have success anyway, that doesn’t prove CICO false, it just shows that CICO will work regardless of whether you are conscious of implementing it.
p.s. Just wanted to throw a link in here for women who are thinking about trying fasting but have read that fasting isn’t good for women. This article nicely addresses some of the worries women have about fasting especially intermittent fasting.