When it was published in 2013 I bought a copy of the bestseller The Fast Diet to see what it was all about. In case you don’t know, it was published as a result of the interest in the BBC Horizon documentary about Intermittent Fasting (IF), written by the programme presenter Dr Michael Mosley and journalist Mimi Spencer.
I believe that fasting is beneficial, but not necessarily advisable for everyone, so I wanted to read the book to discover new information and research, but also I was curious to see if it contained any words of caution. There are words of caution about fasting; a paragraph on page 124 warns those with Type 1 diabetes not to fast, those with an eating disorder, children, and those who are already very slim. And anyone with any medical condition should consult a doctor first.
If you bought a copy of my book, Eating Less, between 1998 and the first half of 2005, you’ve got an edition that contains a chapter on fasting once a week. As well as instructions on how to fast in a non-addictive way, I describe some good reasons not to fast. In later editions, I took out all mention of fasting, partly because people weren’t paying any attention to those reasons. Perhaps it’s time now to put them back in (if I could) but here’s how they appeared in those first editions of Eating Less:
- It’s not a good idea to fast if you are desperate to lose weight, or if you have a history of anorexia or bulimia.
- It’s not a good idea to fast if you have a tendency to overeat either before or after a fast.
- It’s not a good idea to fast if you go on a fast as a way to take control of your overeating.
- It’s not a good idea to fast if you are not in the best of health, if you’re coming down with an illness or recovering from one, or if you suffer from a condition such as diabetes or hypoglycemia.
- It’s not a good idea to fast if you don’t normally eat a high-quality diet at other times.
This has some similarity with Mosley and Spencer’s cautions, but also some differences. In particular, my caution not to fast if you don’t normally eat high-quality food would seem to contradict their advice to “eat what you like most of the time”. However, Mosley and Spencer say,
“You could pig out on your non-fast days…but you won’t do that. In all likelihood, you’ll remain gently, intuitively attentive to your calorie intake, almost without noticing. Similarly, you may find yourself naturally favouring healthier foods once your palate is modified by your occasional fasts. So yes, eat freely, forbid nothing, but trust your body to say ‘when’.”
So they seem to be saying that it’s fine to eat anything at all on non-fast days, but once you’ve started fasting you’ll end up eating healthy food anyway.
As you may know, I’m a great advocate of an if-it-ain’t-broke-don’t-fix-it approach to everything, so if IF works for you, that’s wonderful. But all too often people struggle with such advice – and blame themselves. They conclude, “for everybody else, fasting two days a week is not only fairly straightforward, but also sorts out all the rest of their crazy eating on the other five days… what’s wrong with me that I can’t even begin to do this?”
Maybe it’s not that fasting isn’t a good idea, but that there are other steps for you to take first. To return to my cautions:
- It’s not a good idea to fast if you are desperate to lose weight, or if you have a history of anorexia or bulimia. Note that both authors of The Fast Diet took on fasting entirely for health reasons. The health benefits of fasting – such as dipping into ketosis from time to time and the fascinating process of autophagy – are well established (1, 2). There’s also impressive research showing a beneficial impact on brain health (3). But Mosley and Spencer seem oblivious to the fact that many people will be motivated to fast primarily to improve their appearance, and this makes quite a difference, as you know from my books.
- It’s not a good idea to fast if you have a tendency to overeat either before or after a fast – and – It’s not a good idea to fast if you go on a fast as a way to take control of your overeating. It’s clear that neither of the authors have ever had an addictive relationship with food – what many people call ‘food issues’. The research they cite on the success of IF from the University of Chicago studied just 16 obese people over 10 weeks. (4) I’m sure you know of people who complied with various protocols for at least 10 weeks and then regained their weight in the longer term. They were able to ‘be good’ and ‘follow the rules’ for a while, but this simply doesn’t last for the majority. I’m not saying that fasting is a bad idea; I’m saying it might not provide a complete and permanent solution for everyone who generally overeats.
- It’s not a good idea to fast if you are not in the best of health, if you’re coming down with an illness or recovering from one, or if you suffer from a condition such as diabetes or hypoglycemia. I’m no expert on these health issues, but I’m not at all sure that fasting is good for those with Type 2 diabetes and especially hypoglycemia. This is why those with diabetes are exempt from fasting on religious occasions such as Ramadan.
- It’s not a good idea to fast if you don’t normally eat a high-quality diet at other times. This of course depends on what you call a high-quality diet, but as you know from previous newsletters, my idea is very low on the starchy carbohydrates such as grain-based foods and sugars. It’s important for your body to be very well nourished through eating the most nutrient-dense foods, so that it doesn’t go into ‘scarcity mode’ during a fast. In addition, fasting works much better in every way if your body has developed the ability to burn fat for energy, rather than only carbohydrate. For example, if you normally burn carbohydrate, you may struggle much more with hunger and low energy during a fast. (5)
I’ll add that if you exercise a great deal, if you regularly sleep badly, and/or if you are under quite a bit of stress, these also mean that fasting may not be right for you at the present time.
I suspect all this is sounding a bit negative, and the last thing I want to do is to dissuade you from fasting if it’s going to work for you. By all means give it a try. Notice and manage your addictive desire to eat and you can certainly find that it fits in very well with everything you’ve learned in my books and seminars.
The Fast Diet does advise against fasting for those with an eating disorder, and I agree with this. I’d take it further, though, because there are a great many people who have a tendency towards disordered eating who would do well to sort that out first, before considering a fast of any kind.
1. “Targeting insulin inhibition as a metabolic therapy in advanced cancer.” Fine EJ, Segal-Isaacson CJ et al (2012) Nutrition 28(10):1028-35
2. “The role of autophagy in aging” Bergamini E, Cavallini G, et al (2007) Annals of the New York Academy of Sciences 114:69-78
3. “Meal size and frequency affect neuronal plasticity and vulnerability to disease: cellular and molecular mechanisms.” Mattson MP, Duan W, Guo Z (2003) Journal of Neurochemistry 84(3):417-31
4. “Dietary and physical activity adaptations to alternate day modified fasting: implications for optimal weight loss.” Klempel MC, Bhutani S et al (2010) Nutrition Journal 9:35
5. “Effects of a high-protein ketogenic diet on hunger, appetite, and weight loss in obese men feeding ad libitum.” Johnstone AM, Horgan GW et al (2008) American Journal of Clinical Nutrition 87:44-55