Intermittent Fasting – Everything you need to know!
Intermittent Fasting and everything you need to know. My first experience with intermittent fasting came about when I hit a serious plateau prepping for a fitness competition. I was already lifting over an hour per day, doing 20-30 minutes of fasted cardio in the mornings, eating impeccably clean, drinking a gallon a day, sleeping 7 hours per night, and taking all my supplements. My calories were all the way down to 1,500kcal, so I didn’t feel like I could safely go any lower without bingeing or completely crashing.
In a desperate attempt to budge the scale, I tried intermittent fasting. And guess what, it worked! The scale started trending down with no other changes to my training or macros. Naturally, this led me to research why intermittent fasting might be better for fat loss, whether muscle loss was a concern, and how healthy it was long term.
What Exactly Is Intermittent Fasting?
Intermittent fasting is simply eating within set “feeding periods” with prolonged “fasting periods” in between. For example, I would fast from 9:00pm until 1:00pm the next day, basically a 16-hour fasting period with an 8-hour feeding period in which I would fit all of my regular meals. This is known as the 16:8 Method, a type of time-restricted fasting also called the “Lean Gains” protocol.
Other popular methods of intermittent fasting are “Eat-Stop-Eat” and “The 5:2 Diet.” Eat-Stop-Eat requires fasting for a full 24 hours one or two days of the week, then eating normally the other days. The 5:2 Diet involves restricting calories to 500-600 calories on two nonconsecutive days of the week, while eating normally the other 5 days.
Another method, called Alternate-Day Fasting, is similar to the 5:2 diet in that calories are restricted by 75% every other day, eating according to hunger the other days. This method has been highly effective in obese populations.
The 16:8 Method is the most popular and the only method I’ve really tried, for reasons I will address later on.
What Are the Benefits of Intermittent Fasting?
- Appetite Suppression
The first thing I noticed with intermittent fasting was appetite suppression. Intermittent fasting reduces appetite1 through several mechanisms in the body. Fasting promotes the burning of ketones from fatty acids and amino acids instead of glucose. Ketones have been extensively studied to suppress appetite.2 Even though most people will not get into full ketosis until they’ve fasted for 48 hours or longer, ketones increase in the blood even with shorter fasts.
Fasting also increases levels of the hormone norepinephrine (aka noradrenaline), which suppresses appetite.3 Contrary to popular belief, fasting for 24 hours or less does not significantly increase ghrelin or cortisol levels (two hormones that increase appetite), in men or women.4-6 Fasting for longer than 24 hours, however, may result in elevated cortisol levels and increased physiological stress, both of which can induce hunger.7
Another factor that may reduce appetite during intermittent fasting is having bigger, more satisfying meals. For example, eating three 600-calorie meals rather than six 300-calorie meals.
Not having food as an option may also help some people take their minds off eating. In my experience, the more I focus on food, the more I want to eat!
- Enhanced Fat Burning
Obviously, my biggest motivation for trying intermittent fasting was to force more fat loss. Intermittent fasting definitely boosts fat loss for multiple reasons.
For one, fasting allows insulin levels to fall, stimulating fat burning.8 Every time we eat food (carbs, protein, even fats!9), insulin levels increase, temporarily inhibiting lipolysis (fat burning).10 It makes sense then, that fat burning happens when we fast between meals.
Secondly, a 24-hour fast increases Human Growth Hormone (HGH) by over 300% in men, and over 200% in women.11 HGH is one of the few hormones that promotes both fat burning and muscle growth! Over a six-month study, subjects administered HGH increased muscle mass by an average of 5.7 pounds and decreased body fat by an average of 4.8 pounds more than the control group.12
Boosting HGH naturally with fasting is much safer than using artificial HGH injections, which have been associated with a number of potential side effects.13,14
But that’s not all! Due to increased norepinephrine (noradrenaline), a 24-hour fast can boost metabolic rate as much as 14%.15 In other words, someone who normally burns 2,000kcal per day burns an extra 282kcal. This phenomenon has been observed in both men and women, and continues until day 2-3 of fasting. Around day 3-4 of fasting, metabolic rate starts to fall (a survival mechanism).
Finally, reducing meal frequency and the number of daily meals could mean fewer calories consumed. Fewer calories in plus increased calories out generally equals greater fat loss, which is what we see with intermittent fasting.
- Maintaining Muscle Mass
Now for the burning question: can you gain muscle while intermittent fasting? Yes, but it’s no better (or worse) for muscle-building than six meals per day, given comparable macronutrient ratios.16,17
Although fasting exhibited higher HGH levels, they also measured lower testosterone levels, which could explain why there was no noticeable difference in muscle growth (both HGH and testosterone boost muscle growth).16 That said, the fasting athletes did shed more body fat over 8 weeks than those on the standard plan!
Intermittent fasting isn’t better for building muscle, but it may be superior for preserving muscle while dieting, compared to continual caloric restriction.18 It’s hard to say for sure, since the available studies didn’t control macronutrient intake.
Protein starts getting broken down for energy around day 3-4 of fasting, indicated by increased urea-nitrogen.15 So rest assured, it takes several days before you start “burning muscle.” Muscle loss is simply not a concern if you’re following the 16:8 method and getting adequate protein.
- Type 2 Diabetes Prevention
The different forms of intermittent fasting have varied success in preventing type 2 diabetes.
In a 2016 study, pre-diabetic men following a time-restricted fasting protocol (6-hour daily feeding window) experienced increased insulin sensitivity and blood sugar control, decreased blood pressure, improved pancreatic function, and reduced appetite compared to men following a standard 12-hour feeding schedule.19
Time-restricted fasting also appears to improve blood sugar control in healthy men, and to a lesser extent, women.20 In type 2 diabetics, time-restricted fasting improves blood sugar control (a little), but not insulin resistance.21
When it comes to alternate-day fasting, a 2014 review stated the method is “no more effective” than standard dieting for lowering insulin levels, improving insulin resistance, or reducing visceral belly fat (a risk factor for diabetes).22 A 2012 study actually showed worse blood sugar control in healthy women after three weeks of alternate-day fasting (fasting every other day).23
Finally, the 5:2 Method of intermittent fasting has been shown to improve insulin levels, inflammation and visceral fat, but not so much blood glucose levels.20,24
More research needs to be done, especially in women, before intermittent fasting can be recommended for the prevention or treatment of type 2 diabetes. All things considered, time-restricted fasting (like the 16:8 method) shows the most promise.
- Improving Heart Health
The different modes of intermittent fasting also have unique benefits for heart health.
Time-restricted fasting (like the 16:8 method) promotes beneficial changes in blood triglycerides, LDL and HDL cholesterol levels, as well as overall inflammation levels.16,20,24,25
Alternate-day fasting has been shown to improve inflammatory markers, blood triglycerides, and HDL cholesterol, but can increase unhealthy LDL cholesterol.20 Overall though, the method seems to reduce heart disease risk.
The 5:2 method greatly improves inflammatory markers, with mixed results in lowering triglycerides or cholesterol levels.20,24
Intermittent fasting has some notable benefits for heart health. Being the easiest to follow and having the most positive results, the 16:8 method seems best for improving heart health.
- Cancer and Disease Prevention
Could fasting be protective against cancer and disease? Studies definitely support the notion. Various intermittent fasting protocols have been shown to improve markers of oxidative stress and inflammation.24-27
Oxidative stress is a key driver of chronic diseases such as cancer, heart disease, diabetes, stroke, rheumatoid arthritis and other types of chronic inflammation, as well as aging and degenerative diseases like Alzheimer’s disease, Parkinson’s disease.28
Chronic inflammation is also a major culprit in numerous disease states and health conditions including cancer, heart disease, diabetes, insulin resistance, thyroid disease, obesity, Cushing’s disease, Celiac disease, Crohn’s disease, ulcerative colitis, other types of inflammatory bowel disease, Alzheimer’s, rheumatoid arthritis, allergies, and asthma.29
Besides improving markers of oxidative stress and chronic inflammation, intermittent fasting promotes positive changes in gene expression that help our cells cope with stress, not to mention, resist disease and cancer.30,31
Similar benefits have been found with continuous calorie restriction, but intermittent fasting may be more practical long-term.24
Intermittent fasting also facilitates autophagy, a key cellular repair and waste removal mechanism that prevents cancer, disease, and aging.32-34 Over time, defective proteins build up inside your cells and limit functionality. When you fast, your cells literally “eat up” these broken proteins, allowing the cells to operate efficiently.
- Improved Brain Health
Many of the benefits associated with intermittent fasting also foster brain health, such as reduced oxidative stress, reduced inflammation, reduced blood sugar levels and insulin resistance, and enhanced autophagy.
With Alzheimer’s, cellular waste and debris clog neuron connections and cut off blood supply to brain cells. Intermittent fasting promotes autophagy in the brain, helping to clean up the neuronal “mess” and restore brain functioning.30
Intermittent fasting also boosts brain-derived neurotrophic factor (BDNF), a factor that’s low in populations suffering from depression, Alzheimer’s, Parkinson’s and Huntington’s diseases.35
Furthermore, the 16:8 method was shown to increase neurogenesis, the growth of new brain cells, in mice.36 Higher rates of neurogenesis have been linked to improved brain performance, memory, focus, and mood.
The increase in norepinephrine that comes with intermittent fasting also increases alertness, memory, and focus (but can also cause restlessness and anxiety).
- Longevity and Anti-Aging
Due to the ability to clean up cellular debris and reduce oxidative stress, intermittent fasting is a very promising anti-aging approach.
Scientists have identified seven factors that cause aging: genome damage, epigenetic factors, telomere shortening, unfolded protein response, mitochondrial dysfunction, cellular senescence and stem cell exhaustion. Intermittent fasting has been shown to diminish at least three of these!20,24,30-34
In rodents, alternate day fasting extends lifespan up to 30%, but more studied are needed in humans.37 Again, similar benefits have been discovered with continuous calorie restriction, that is, consistently eating below your calculated energy requirement.
- It’s How Our Ancestors Ate
Finally, intermittent fasting was how our ancestors ate. Even modern hunter-gatherers don’t have immediate access to food like we do; they typically spend the morning acquiring food, and eat much later in the day.38 So yes, our bodies are perfectly capable of adapting to an intermittent fasting lifestyle – it’s what we were designed to do.
What Are the Negatives of Intermittent Fasting?
- Women May Not Tolerate Fasting Well
Females present some negative responses to intermittent fasting, such as impaired blood sugar control, reduced fertility and sex hormones, and menstrual abnormalities. These negative effects are mainly seen with the Eat-Stop-Eat38 and alternate-day fasting protocols.23 This is why I recommend women stick to the 16:8 method, or even just 12 hours of fasting if they have a history or menstrual irregularities or hormone issues. Research tells us that women are more susceptible to hormone imbalance from energy restriction and stress (fasting is a form of stress) than men are.38,39
In addition, pregnant and breastfeeding women have increased energy demands which can be hard to meet with intermittent fasting, even following the 16:8 method. I’ve personally found I do best with six small meals while pregnant and breastfeeding, otherwise I experience low energy and dips in my supply.
- Intermittent Fasting “Can” Exacerbate Hormone Imbalance
Men with clinically low testosterone may not want to try intermittent fasting because it has been shown to lower testosterone (even if only slightly).16
Those with adrenal insufficiency may not well suited to intermittent fasting either. Fasting causes a stress response that an impaired adrenal system may not be equipped to handle properly, potentially exacerbating high cortisol levels and hormone imbalance.
Those with thyroid issues like hyperthyroidism or Grave’s disease typically have trouble maintaining weight and should avoid any kind of fasting. People with hypothyroidism or Hashimoto’s should also be cautious with intermittent fasting, since fasting can exacerbate poor blood sugar control and hormone swings. On the other hand, others have reported great results limiting calorie intake to 8 daylight hours, to help regulate normal circadian rhythm.40
- It’s Not for Everyone
Individuals struggling with eating disorders should not try intermittent fasting, as it can promote overly restrictive behaviors or binge-eating during the feeding window.
Intermittent fasting is not recommended for type 1 diabetics either, since going without food for 16 hours puts them at risk for severe hypoglycemia. That said, some have reported success manipulating their basal insulin via pump or injections. Obviously, this tactic should be closely guided by a medical professional.
People who take medications for blood pressure or heart disease, as well as those in need of regular food intake for medications, should consult their doctor before trying intermittent fasting.
Some experience digestive issues from fasting and feasting, while others find it difficult to enjoy the social aspects of eating. Still others have headaches or difficulty sleeping. It’s not for everyone, that’s for sure!
What Type of Intermittent Fasting is Best?
The type of intermittent fasting that is best for both men and women, preservation of muscle mass along with fat burning, diabetes prevention, and heart health is the 16:8 method. The 5:2 method seems best at reducing inflammation, while alternate day fasting is the most studied for longevity. This is not to say the 16:8 method can’t provide similar benefits!
How Do I Get Started?
Now that you know everything there is to know about intermittent fasting, you may be wondering how to actually apply it.
When should I fast and when should I eat?
If you’re doing the 16:8 method of intermittent fasting (recommended), you will be sleeping through about half of your 16-hour fast. The remainder of your fast will either fall in the hours leading up to bedtime or in the morning/early afternoon hours after you wake. Plan your 8-hour feeding window during a time that works best for your training and social schedule.
For example, if you train at 7am each day, perhaps a 9am-5pm feeding window works best for you, or you can train in the middle of your feeding window, as I do. If eating dinner with the family is a priority, try fitting your meals in between 1pm-9pm.
Intermittent fasting is extremely flexible, so you don’t have to fast the same timeframe each day, so long as you’re fasting for 16 hours in a 24-hour period (maybe less if you’re a woman).
Doesn’t fasting training cause muscle loss?
Contrary to popular belief, training in the fasted state does not cause muscle loss.41,42 It does, however, reap beneficial performance adaptations!43 That said, I do recommend getting a post-workout meal with at least 25 grams of quality protein within 3 hours post-workout to enable muscle repair and growth.44-46
But what about meal frequency? Isn’t there a limit to how much protein my body can digest in a single meal?
I am so glad you asked – because yes there is! A recent study estimated that the amount of protein your body will use from a single meal is roughly 0.4 gram per kilogram (0.2 grams per pound) of bodyweight.47 Knowing that we need about 1.6 grams per kilogram (0.8 grams per pound) daily to maximize muscle growth, four meals would seem ideal to fully max out lean gains (4 meals x 0.4g/kg/meal = 1.6g/kg).
However, this study has some gaps:48 For one, strength training increases the amount of protein your body can utilize at a single meal beyond 0.4 grams per kilogram. Two, a mixed meal has a greater ability to reduce protein breakdown compared to the whey protein used in this study. Third, fasting increases the anabolic response to a meal, increasing the amount of protein utilized at a single meal beyond 0.4 grams per kilogram. All things considered, I would still aim for three to four meals during your 8-hour feeding window, if you goal is to maximize muscle growth.
Do I still need to track macros or calories?
Yes, you should keep tabs on your nutrition. You still need adequate calories and macronutrients, as well as nutrient-dense whole foods for the majority of your diet. Fasting doesn’t change what you need to eat, just when you need to eat it!
I hope you’ve found this article informative and easy enough to understand. If you have questions, feel free to leave a comment below or email me at firstname.lastname@example.org.
Sarah Wilkins, Nutritionist, B.S. Dietetics
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