Because it only dictates when to eat and not what to eat, intermittent fasting is more of a lifestyle than a diet. While this style of eating (or…not eating) is trendy at the moment, fasting is absolutely nothing new to humans. Humans have been fasting for thousands of years for religious, spiritual, or political reasons or simply as a result of limited food resources. Intermittent fasting, while it does come in many varieties, includes regular, less than 24-hour periods of fasting. It’s wild popularity has been recently bolstered by claims of fat loss and longevity. But what does the research say about these claims, and would intermittent fasting be a good option for you?
What kinds of intermittent fasting exist?
The most common form of intermittent fasting includes a daily pattern of an 8-hour eating window followed by a 16-hour fast. Since the vast majority of people already fast for 10+ hours overnight, an early dinner can be all it takes. If you eat breakfast at 9 a.m., lunch at 1 p.m., and dinner at 5 p.m., you’ll fast for ~16 hours.
6 Health Goals Intermittent Fasting May Support (evidence-based)
There’s no question that eating a healthy amount of food leads to all kinds of health benefits. If you currently consume more than enough calories, cutting down may lower your risk of diabetes, heart disease, and improve your health in general. Intermittent fasting is one useful eating style that can help you harness the benefits of eating an optimal amount.
1. Decreasing inflammation
One study found that fasting intermittently (in this case, for Ramadan) was associated with decreased markers of inflammation. These results were echoed by another study, though both studies have a fairly low number of participants (<100). Fasting for Ramadan, the ninth month of the Muslim lunar calendar, includes a daily fasting window from sunrise to sunset. Another study found that individuals who were overweight and asthmatic experienced decreased inflammation when fasting every other day.
2. Increasing longevity (in non-human animals)
Restricting calories, whether through fasting or other means, has long been associated with longer lives in worms, flies, and rodents (NIH overview). One 20 year study in monkeys suggests that caloric restriction increases lifespan and reduces rates of diabetes, cancer, cardiovascular disease, and brain atrophy. However, intermittent fasting specifically has little research behind it. There have been some positive results in non-human animals like rats and mice.
3. Preserving muscle mass
While there used to be more concern about losing muscle as a result of infrequent eating, this fear is relatively unsupported by research. For example, one study found that levels of muscle growth and atrophy regulators were the same between 3, 15, and 40 hours of fasting.
There is some evidence that fasting may actually preserve muscle mass better than general daily caloric restriction. One study found intermittent fasting and daily caloric restriction were equally effective in terms of losing weight and/or fat, but that intermittent fasting lead to more lean (muscle) mass retention.
4. Losing fat (especially visceral fat)
Intermittent fasting works for weight loss in both overweight and non-overweight individuals (study).
The research that compares intermittent fasting with daily caloric restriction shows mixed results. According to the largest literature review so far, there are no major differences between the two eating styles in terms of weight loss.
Intermittent fasting, however, may decrease appetite more effectively than other methods of calorie reduction (especially in individuals who are obese). This effect may be due to hormones, which largely control appetite. Non-obese individuals may not experience any reduction in appetite, as some studies show they do not experience the same hormonal changes in response to fasting.
5. Improving metabolic health
In prediabetic men, a promising study showed that even without weight loss, intermittent fasting can lead to impressive health benefits. These benefits included improved insulin sensitivity, decreased appetite in the evening, and lower blood pressure. Simplified, this means the men in the study experienced an improvement in their metabolic health.
Similarly to other eating strategies that involve eating less, intermittent fasting usually leads to decreased insulin resistance type 2 diabetes risk.
[To learn more about insulin and blood sugar read Diabetic or Not…Glycemic Load Matters]
6. Improving cardiovascular health
In one rat study, researchers found intermittent fasting (alternate-day) was associated with improvements in cardiovascular risk factors. Another rat study had similar results, concluding that alternate-day fasting protected the heart. The positive cardiovascular effects researchers observed in these studies may be a result of decreased inflammation.
In individuals who are obese, intermittent fasting (alternate-day fasting) for an 8-week period has also shown cardioprotective effects. The effects of eating every other day included lower blood pressure, cholesterol, body fat percentage, and triglycerides.
In individuals who are not obese, the cardioprotective effects of intermittent fasting have been less impressive. Though some evidence does suggest this eating style is effective for both groups.
Would it be a good choice for you?
Difference between men & women
Women are more likely to experience adverse effects when fasting. These negative consequences are likely a result of hormonal fluctuations that men don’t experience (article here). These effects may be reproductive in nature, like smaller ovaries (shown in rats), disrupted menstrual cycles, or early-onset menopause (shown anecdotally in women). Fasting may also worsen blood sugar control in women instead of improving it.
Difference between obese individuals and non-obese individuals
As cited in the studies above, intermittent fasting can be a useful tool for people of any weight. However, the results in obese individuals have been more powerfully positive.
Potential dangers and side effects
- Skipping meals and fasting is likely an unhealthy strategy for adolescents. However, one study found that fasting did not have any negative effect on body image, and actually reduced binge-eating and depression symptoms.
- Fasting can lead to short-term physical side effects like headaches, constipation, water retention, dizziness, general weakness, and bad breath.
- Fasting can also lead to emotional and mood-based side effects (think “hangry”).
How do you get started?
Shifting your eating style will likely be met with complaints from your system. As a result, making gradual changes will be far more comfortable. Once you’ve chosen which of the intermittent fasting styles you’d like to try, ease into it.
You can do this through modifying either the amount of food involved or the length of the fasting time window. For example, instead of eating absolutely zero calories, eat 50% or 25% of what you usually do. Or, instead of fasting for 24 hours right away, start with 16 and move your way up if/when it’s comfortable.
When it comes down to it, the best diet or eating style is the one that leaves you feeling healthy in the long term. If fasting leads you to binge, eat more unhealthy foods, or fixate on food, it isn’t the best eating strategy for you. If fasting decreases your performance at work or in the gym, or leads to crankiness or other side effects, stop.
But, if fasting leads you to cut out unhealthy nighttime snacks or allows you to eat less without any intense increases in appetite, go for it!
Whatever you end up choosing to do, keep in the mind that the quality and quantity of the food matter far more than when you eat it.