A Google search on “fasting” elicits over 36 million hits. Needless to say, it’s a hot topic.
Fasting simply means a voluntary abstinence from food and drink (not including water). It’s not new; it has been practiced by numerous cultures and religions throughout the world for millennia.
But studies, particularly on mice, have shown fasting may help weight loss and improve metabolic health markers, such as blood lipids (fat) and blood sugar regulation.
There are four main types of intermittent fasting:
- Alternate Day Fasting: alternate a day of fasting with a day of eating normally.
- Modified Fasting Regime: consume about 25% of energy needs on scheduled fasting days and eat as you would like on remaining days, example is 5:2 Diet.
- Time- Restricted Feeding: eat normally for a set period of time (e.g. 12 hours) and then consume no food for a set period of time (e.g. 12 hours)
- Religious Fasting: various fasting regimes undertaken for religious purposes such as Ramadan.
There have been very few human studies on Alternate Day or Modified Fasting Regimes. The few that do exist were very small and did not have control groups to compare the outcomes. However, these studies did show that the subjects lost weight and showed improvements in their blood glucose regulation and lipid profile, that is, they had a reduction in LDL cholesterol and total triglycerides. The downside is that both the alternate day fasting and modified fasting regime did not hold any benefits above a regular calorie reduction weight loss plan and many found the plans difficult to stick to. If you can’t stick to it, then long-term maintenance is impossible.
The evidence on time-restricted feeding is quite compelling. However, most research has been conducted on mice not humans. In one study the researchers gave slim mice a high fat diet and let them eat it all day long as much as they wanted. Those mice got fat. They took another group of mice and gave them a high fat diet but allowed them to eat it only during set hours of the day, so they were on a time-restricted feeding program. The mice stayed lean. Finally, they took another group of lean mice fed them a high fat diet and for 5 days restricted the time they were allowed to eat, but allowed them free access to food for the weekend. The result? Those mice stayed lean as well.
Interesting mice studies, but what about humans? A pilot study using human subjects gave the participants simple guidelines. They asked them to fast for 12 hours or more, ideally staring at 7pm; however, definitely no later than 8pm. They were not advised on what food to eat or how much, simply on when to eat it.
After one month the subjects had lost between 1.06- 1.53kg (2.3- 3.3 lbs). But what is even more noteworthy is the feedback they provided. Between 90-100% of participants said the fast:
As any frustrated dieter knows, it’s one thing to lose weight but keeping it off often proves the most challenging. Following a simple routine of ceasing eating after an early-ish dinner and not grazing, snacking or munching until breakfast time may be enough to see weight loss and overall benefits to health.
How Does It Work?
It’s hypothesized that nighttime fasting helps regulate our circadian rhythm. During daytime our bodies are hard-wired to eat, move and function. Testosterone is high, our blood pressure is up and melatonin is reduced. At nighttime, our body shifts into repair mode and uses stored energy for fuel. Intermittent fasting appears to be a circadian synchronizer and helps our metabolism stay in check.
It is also hypothesized that many functions of our gastro-intestinal tract exhibit robust circadian or sleep-wake rhythms and can even reduce the diversity of our microbiome (flora in our gut). Low-diversity in our microbiota is associated with increased levels of body fat, inflammation, triglycerides and insulin resistance, all of which are precursors to heart disease.
Of course, another factor into the mechanism of action as to why time-restricted feeding may prove to be beneficial is that if you don’t eat for 12-14 hours a day, you simply eat less. It can also reduce the amount of food consumed at night, which is ideal for weight maintenance (see http://nutritionsavvy.ca/5-slimming-summer-salads/). Finally, time-restricted feeding also appears to help regulate our appetite hormones, such as leptin, ghrelin and xenin which are integral to maintaining a healthy body weight.
The Bottom Line
Further research on humans needs to be undertaken before we can conclude definitively that intermittent fasting is as effective on humans as it is on mice. However, limiting your eating to a 12 hour period may result in health benefits including weight loss, improved triglyceride levels and blood sugar control and be something that you can do for life. As I often say “do not start a diet that you cannot maintain for the rest of your life”.
Please Note: Intermittent Fasting is NOT for everyone.
Who Should Not Fast:
- Children (under 18 years old)
- People who are underweight or have an eating disorder
- Type 1 diabetics and diabetics taking medication for their diabetes
- Pregnant women or breast feeding mothers
- People recovering from surgery
- Anyone suffering from an underlying medical condition should speak to their doctor first
- Those who are elderly and/or frail.
- Those who are not sure about whether it may affect their prescribed medications should to speak to their doctor first.
- People feeling unwell or have a fever
- Those taking Warfarin should consult their doctor first
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