Updated: Apr 26
By Dr. Shawna Darou, ND
There’s been lots of feedback from last week’s blog post about the Ketogenic diet, and for many of you it was followed by questions about intermittent fasting which is certainly another popular dietary trend.
Let’s start with a few definitions, because in many cases we’re actually using the wrong terminology:
Intermittent fasting – this refers to going without food for 24 hours or more, and is usually scheduled at regular intervals, ex. weekly or sometimes alternate day fasting.
Time restricted feeding – refers to consuming your day’s calories in a reduced time window though the day. This is usually what people are referring to when they say ‘intermittent fasting.’
What does the research say?
Most people are drawn to fasting or time-restricted feeding as a way to lose weight and reduce blood sugar issues, and there are certainly some good studies for weight loss, and also blood sugar metabolism improvement in men.
There is not however, much research that directly pertains to women and specifically what happens to our hormones with nutritional restrictions. Women’s bodies are MUCH more sensitive to metabolic distress, and perceived states of starvation can cause major havoc with ovulation, menstruation, sleep and fertility. If you are restricting your eating window in the day, the most important factor is not to let your daily caloric intake get too low. Calorie-restriction results in missed or irregular periods (hypothalamic amennorhea), suppressed thyroid hormone conversion (weight plateau, low energy, constipation and hair loss), and an activated stress response.
In more extreme fasting (alternate day fasting), one study found that after 3 weeks there was actually an unfavourable effect on glucose tolerance in women, and a lack of effect on insulin sensitivity. This is completely the opposite to findings with men in the same study, where it reduced diabetes risk, and this is one of the biggest reasons people start with intermittent fasting.
There have been very few human studies with intermittent fasting and time-restricted feeding specifically on women, but there have been several with rats and mice. I’ll just summarize to say that calorie restriction through fasting in female rodents created an absence of uterine activity, endocrine masculinization, a heightened stress response, and a reduction in sleep. These results were again different in male rodents who generally saw more favourable results.
What are generally safe and effective practices?
I’m going to base my recommendations on the work of Dr. Valter Longo, researcher and Director of the Longevity Institute at the University of Southern California, and author of “The Longevity Diet.” What I really like about his approach is that it is backed by research, is not extreme, and is a sustainable lifestyle change for those of you who are interested. Research shows that you can get excellent results from a moderate practice of time-restricted feeding. Here are the guidelines:
1. Eat for 12 hours, and then fast for a full 12 hours overnight and practice this consistently.
When most people of intermittent fasting, or more correctly ‘time-restricted feeding’, they assume a long fasting window of 16-20 hours or more per day. The downside, is that long periods of fasting increase the risk of gallstone formation in women, regardless of weight. Studies also show that these longer fasting windows can also increase cardiovascular risk factors.
On the other side, eating 15 hours or more in the day, meaning the over-night fasting period is less than 9 hours is associated with metabolic disorders (diabetes, insulin resistance, weight gain), and disturbed sleep.
Again, the ideal fasting window is simply 12 hours eating, 12 hours fasting, and this is the pattern practiced by most long-lived populations around the world. In practice, what this means is that if you eat your breakfast at 7:30am,you would finish your last meal before 7:30pm.
2. Avoid eating for 3 hours before bedtime.
To support your metabolism, and get the most out of your 12 hour fasting window, it’s best to stop eating 3 hours before bedtime. There are a few exceptions to this rule: pregnancy, extreme hypoglycemia, and there is always a caution with any type of restriction in women who have current or past eating disorders.
Again, in practice this means that if your bedtime is 10:30pm, you would finish eating by 7:30pm at the latest and avoid night-time snacking.
3. Eat breakfast.
As popular as is has become to skip breakfast, research shows that people who skip breakfast have higher rates of cancer, cardiovascular disease and overall mortality. If you don’t wake up feeling particularly hungry in the morning, just eat something small and light. The mechanism of these findings is not fully understood, but has come up enough times in research to pay attention: people who eat breakfast are generally healthier.
4. Think of this as a lifestyle change, not a diet.
Our hormonal systems are sensitive. As soon as you feel restricted or even think about restrictions, you’re more likely to overeat when you get a chance. Women’s bodies and hormones are especially sensitive to calorie restriction so less extreme is preferred. Work on the mindset of a lifestyle change rather than a diet to make your healthy habits sustainable for the long-run.
If your goal is to live a healthier life this gentle version of time-restricted feeding might be a practice that reduces overeating, supports healthy blood sugar levels, and promotes longevity. But remember, there is no supportive evidence that longer fasting periods are more effective for women, so keep it moderate with a 12 hour overnight fast.
A Few Cautions:
I am always cautious with anything extreme, and especially one that may restrict calories in the development or perpetuation of disordered eating. Sometimes a ‘healthy’ shift in eating habits can go too far. If watching the timing of your meals is too triggering, is leading to binges, food restriction or food obsessions then this way of eating is definitely not for you. Overall, I’m most in favour of long-term, sustainable, health-promoting nutrition practices. Yes, there are some potential benefits to this pattern of eating but not at the expense of your mental health. If you’re not sure if this applies to you, please ask.
Remember, I’m always happy to discuss the latest trends in nutrition with you, and to create a personalized nutrition plan that suits your life.