Get a holistic dietitian’s perspective on the latest research to help you decide if intermittent fasting is all it’s cracked up to be!
Note: This post was co-written with Keri Carpenter, Dietetic Intern, as part of a research partnership between Back To The Book Nutrition and post-graduate nutrition students from the University of Houston.
Intermittent fasting is definitely trending in the health world right now. But what exactly is intermittent fasting and should you incorporate it into your eating routine? Today we’ll review 5 types of intermittent fasting, as well as the benefits and drawbacks so you can decide for yourself!
WHAT IS INTERMITTENT FASTING?
Simply put, intermittent Fasting (IF) is a method of eating and fasting in designated cycles. IF is distinct from religious forms of fasting since it’s primarily done to achieve physical health outcomes rather than spiritual ones.
Proponents of intermittent fasting point out that it more closely mimics the eating patterns of our ancestors who didn’t always have ready access to food and were often forced to go without food for many hours or sometimes days at a time.
Medical research has shown a variety of health benefits to intermittent fasting, which has moved it more into the mainstream spotlight in recent years. Let’s take a closer look at what that research shows.
5 TYPES OF INTERMITTENT FASTING
Below are common intermittent fasting schedules. During all of these, liberal water intake is encouraged, even during the fasting period. Some also advocate for supplementing fasting periods with some form of electrolytes, whether sea salt, bone broth, or electrolyte supplements. Generally speaking, black coffee, unsweetened teas, and similar calorie-free beverages are acceptable during the fast. (6)
1. 14 Hour Fast (aka: 14/10 Method)
Fast for 14 hours, and eat during the remaining 10 hours. For example fasting from 8pm until 10am the following day. This time-restricted feeding schedule is usually followed daily, and is an easy method for beginners to implement.
2. 16 Hour Fast (aka: 16/8 Method)
Fast for 16 hours, and eat during the remaining 8 hours. For example fasting from 8pm until 12pm the following day. Like the 14 hour fast, this time-restricted feeding schedule is usually followed daily and is a good place for beginners to start.
3. 24 Hour Fast (aka: Eat-stop-Eat)
Eat normally on six days of the week, then fast for the remaining 24 hours.
4. 5:2 Fast
Eat normally for five days of week, then limit intake to just 500-600 calories on the remaining two days. Technically, this isn’t a true fast since it incorporates a period of very low calorie eating rather than going without food completely.
5. Alternate Day Fast (ADF)
Eat a normal, healthy diet one day, then limit intake to approximately 25% of daily calorie needs the next day (typically around 500 calories per day). Althought this every-other-day pattern has been studied as a form of fasting, some maintain it isn’t a true fast since it incorporates a period of very low calorie eating rather than going without food completely.
BENEFITS OF INTERMITTENT FASTING
Evidence of the health benefits of intermittent fasting is growing. It should be noted that various types of intermittent fasting have been studied, so more research is needed to determine which type of IF is best, and how these benefits compare to more traditional forms of dieting.
1. Helps with weight loss
Various methods of intermittent fasting help overweight/obese individuals lose significant amounts of weight. Current data indicate the amount of weight loss is fairly comparable to traditional calorie restriction diets, but some prefer IF because of the simplicity (eat or don’t eat, versus the sometimes cumbersome task of counting calories and/or macronutrients in other dieting methods). (1,4,10,11)
2. Increases insulin sensitivity and lowers insulin resistance
Improving insulin sensitivity helps with blood sugar control. Multiple studies of both ADF and 5:2 fasting show significant improvement in insulin sensitivity. (4)
3. Increases leptin sensitivity
Obese and/or highly inflamed individuals often have leptin resistance, essentially the brain not recognizing when enough food has been eaten, which leads to overeating and difficulty losing weight. A small human study showed 15 days of alternate day fasting significantly improved leptin sensitivity in males. (4)
4. Lowers inflammation
Multiple studies have shown decreases in C-reactive protein (CRP) and other markers of inflammation in individuals who intermittent fast. (1,4)
5. May improve sleep
Preliminary data show that daily time restricted feeding schedules (i.e., 14 and 16 hour fasts) are more effective than traditional calorie restriction at improving circadian rhythm, which may improve sleep. (4)
6. Improves asthma symptoms
A study of overweight adults with moderate asthma showed a decrease in symptoms during an 8-week regimen of alternate day fasting. (9)
7. Improves heart health
Studies indicate that intermittent fasting can reduce LDL cholesterol, triglycerides and blood pressure. (3,4,7-10)
8. Helps fight cancer
A study in 2016, concluded that nightly fasting of 13 hours was associated with lower risk for breast cancer than those who had a nightly fast less than 13 hours. Other studies indicate that intermittent fasting can also reduce side effect severity of chemotherapy treatments as well as has even slowed the progression of breast and skin cancers. (5,6).
9. Effects on mitochondrial activity
A Harvard study indicates fasting can impact our mitochondria to remain more “youthful”. And if our mitochondria are aging well, we are too! Mitochondrial dysfunction is associated with a variety of diseases and disorders, so there are hopes in research that future studies of mitochondrial activity can provide more evidence of success of fasting. (10)
10. Improving mood and depression
Study shows that fasting and calorie restriction regimen also improved mood for individuals of a group of overweight individuals. (4)
6 DRAWBACKS OF INTERMITTENT FASTING
Intermittent fasting isn’t without its down sides, especially for those who aren’t facing one of the specific conditions listed above.
1. May not be better than other forms of dieting
Several studies have compared IF (most of them using an ADF schedule) to daily calorie restriction and found many of their benefits to be fairly comparable. (4)
2. Overeating during the fasting period
If you overeat during feeding periods (i.e., “making up for” missed meals), it may compromise the benefits of the fast.
3. Low blood sugar
Blood sugar dips may occur as the body adjusts to your new fasting regimen. While this may have beneficial effects on insulin sensitivity and blood sugar regulation in the long run, it can lead to headaches, lightheadedness, and irritability in the short term.
4. Caffeine dependency
Be careful that the fasting regimen not create a new dependency on caffeine to curb your hunger!
The 24 hour, 5:2, and ADF methods of IF can be difficult for bowel regularity since total food intake as well as fiber intake varies significantly from day to day.
6. Difficulty exercising
If you’re following the 24 hour or 5:2 fasting methods, you may not be able to sustain heavy exercises on fasting days, though light to moderate exercise should be fine.
WHO SHOULD NOT INTERMITTENT FAST
Intermittent fasting may be beneficial for some, but the following groups should not intermittent fast.
Pregnant or breastfeeding women
Nutrient needs increase during pregnancy and breastfeeding, and most practitioners agree that restrictive dieting of any type should be discouraged during this time.
There is evidence that IF can improve insulin sensitivity, but should be medically monitored. If you are diabetic, discuss with your healthcare professional before fasting.
Those with a history of disordered eating
The psychological burden of IF may outweigh the benefits in individuals with a disordered relationship to food and their bodies.
Those with HPA-axis dysfunction
The extreme shifts between eating and not eating on an IF schedule may pose stress to the body and further disrupt cortisol rhythms in those with HPA-axis dysfunction. (1,2)
THE BOTTOM LINE
So, is intermittent fasting all it’s cracked up to be?
There are clearly benefits to intermittent fasting, both with regard to weight loss as well as other areas of health. But many of these benefits appear to be similar to those achieved by more traditional reduced calorie diets. More study is needed to tease out the differences between these two to determine whether one really is better than the other.
Until then, I view IF as a reasonable option for weight loss and other health benefits, especially when more intuitive methods of weight loss aren’t enough. I find the 14 hour and 16 hour methods especially helpful for those who struggle with late night eating or don’t like planning and counting their calories or food servings.
Want help deciding whether you should Intermittent Fast?
I’d be happy to help you determine whether IF is right you! Message me using the contact form on my Coaching Page.
Disclaimer: Information on this site is intended only for informational purposes and is not a substitute for medical advice. Always consult with a trusted healthcare provider before implementing significant dietary change. Read additional disclaimer info here.
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- Wan, Ruiqian, et al. (2010). Cardioprotective effect of intermittent fasting is associated with an elevation of adiponectin levels in rats. The Journal of nutritional biochemistry, 21(5): 413-7. Accessed: https://www.sciencedirect.com/science/article/pii/S095528630900031X
- Patterson, R.E., et al. (2015). Intermittent Fasting and Human Metabolic Health. J Acad Nutr Diet,115(8): 1203–1212. Accessed: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4516560/
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- Marinac, C.R., et al. (2016). Prolonged Nightly Fasting and Breast Cancer Prognosis. JAMA Oncol. 2(8):1049-1055. Accessed: https://jamanetwork.com/journals/jamaoncology/fullarticle/2506710
- Varady, K.A., et al. (2009). Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults. The American Journal of Clinical Nutrition, 90(5): 1138–1143. Accessed: https://academic.oup.com/ajcn/article/90/5/1138/4598070
- Varady, K.A., et al. (2013). Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial. Nutrition Journal 12(1): 1. Accessed: https://nutritionj.biomedcentral.com/articles/10.1186/1475-2891-12-146
- Johnson, J.B., (2007). Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma. Free Radic Biol Med.42(5):665-74. Accessed: https://www.ncbi.nlm.nih.gov/pubmed/17291990
- Weir, Heather J. et al. (2017). Dietary Restriction and AMPK Increase Lifespan via Mitochondrial Network and Peroxisome Remodeling. Cell Metabolism 26(6):884-896. Accessed: http://www.cell.com/cell-metabolism/fulltext/S1550-4131(17)30612-5
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