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Intermittent Fasting: A Primer

If you haven’t had a client ask about it yet, you will soon. Intermittent fasting has hit the mainstream, and a lot of people are taking notice.

Proponents claim that intermittent fasting causes more rapid weight loss than other approaches; that it makes dieting easier; and that it improves blood glucose control and blood lipids. Does the current body of evidence support these claims? Let’s find out.

What Is Intermittent Fasting?

Fasting is a pretty simple concept: For a period of time, you don’t eat. And with intermittent fasting, you break up your
fasting periods with eating periods.

While the popularity of this type of fasting is soaring, it is nothing new. Humans have fasted for most of our history—during sleep, amid times of famine or food scarcity, or for religious or spiritual reasons. And modern-day researchers studied the effects of intermittent fasting as far back as 1946 (Carlson &
Hoelzel 1946).

What is new is that clinical research on the potential benefits of occasional fasting is beginning to catch up to the practice. While the majority of this research has been done on animals, human trials are on the rise.

How Do People Practice Intermittent Fasting?

There are many approaches to intermittent fasting. The most popular and most studied seem to be alternate-day fasts and extended morning fasts (skipping breakfast, for example). Let’s look at some of the best-liked techniques:

Alternate-Day Fasting

(36-Hour Fast, 12-Hour Feed)
With this plan, you simply eat every other day. So on Monday, you’d eat within a 12-hour window, such as from 8:00 am to 8:00 pm. Then you’d fast overnight on Monday, as well as all day and overnight on Tuesday. You’d eat again from 8:00 am to 8:00 pm on Wednesday. And so on. Alternate-day fasters are encouraged to make good eating choices, but they’re allowed to eat what they want on non fasting days.

Meal Skipping

(Random Fast and Feed Cycles)
Some intermittent-fasting proponents believe we should behave in the same way our evolutionary ancestors did: As they evolved, they got their food and exercise randomly; therefore, so should we. This brand of intermittent fasting includes eating unprocessed “evolution-friendly” food (think Paleo-diet type), randomly cycling daily caloric intake, and randomly skipping a breakfast or dinner meal once or twice a week. The rules are very flexible (it is random, after all).

Eat, Stop, Eat

(24-Hour Fast, One or Two Times per Week)
On this plan, you fast for 24 hours once or twice per week, eating sensibly (consuming higher amounts of protein, minimizing processed foods, etc.) the rest of the week. It’s flexible: You can choose whichever 24 hours you want, fasting from breakfast to breakfast, or dinner to dinner, for example.


(16-Hour Fast, 8-Hour Feed)
This brand of fasting is based on an 8-hour feeding period followed by a 16-hour fast. However, it also layers a few other food rules on top. The diet should

  • have high protein content;
  • cycle carbohydrates;
  • include fasted training; and
  • use nutrient timing (eating the bulk of your calories after exercise).

On this plan, you fast from dinner until lunch the following day, skipping breakfast and morning snacks. If you plan to exercise, you do so just before lunch, taking in 10 grams of BCAAs (branched-chain amino acids) during training. After training, you eat two or three meals during the rest of the day, with your biggest meal coming right after exercise. The fast begins again every evening, lasts until the following lunchtime, and repeats every day.

Warrior Diet

(20-Hour Fast, 4-Hour Feed)
On this plan, you fast—or eat very lightly—for 20 hours each day, working out during your fasted state. Then, you eat your daily food intake within the 4-hour feeding window. After that, the fasting cycle begins again. Generally, most people place their 4-hour feeding window at the end of the day, as it’s more convenient for family dinners and after-work training sessions. However, the 20 hours of fasting and 4 hours of feeding can occur at any time.

As you think about these different intermittent-fasting variations, don’t focus too much on the differences between them. Instead, take a second and ask yourself what’s similar about the programs. You’ll find they’re all variations on a single theme.

Where Is the Evidence?

While evidence of the benefits of fasting is mounting, the research is far from conclusive, and more studies are needed. No one knows for sure if intermittent fasting is truly any more beneficial than a traditional type of diet.

For now, research on intermittent fasting suggests it may reduce blood lipids (Varady et al. 2009), blood pressure (Mattson & Wan 2005), and markers of inflammation and oxidative stress (Johnson et al. 2007). Intermittent fasting has even been shown to decrease lymphoma occurrence in rodents, but not yet in humans (Descamps et al. 2005).

The research also seems to indicate an increase in fatty-acid oxidation (Heilbronn et al. 2005a), growth hormone release (Ho et al. 1988) and metabolic rate (Zauner et al. 2000; Mansell, Fellows & Macdonald 1990). This suggests a potential physiological advantage to fat loss, or to maintenance of lower body fat.

A recent review compared traditional calorie restriction with intermittent calorie restriction. The review, albeit limited by the number of intermittent calorie restriction trials involving humans, concluded that while weight loss and fat loss were equal between the two approaches, intermittent calorie restriction was superior for maintenance of lean mass (Varady 2011.).

One note of caution: The majority of the intermittent calorie restriction trials used bioelectrical impedance analysis to measure body composition, while the majority of the traditional calorie restriction trials used dual X-ray absorptiometry or magnetic resonance imaging. Bioelectrical impedance analysis is a less accurate method of assessing body composition, so results should be viewed cautiously until replicated with better methodology.

Beyond affecting body composition, intermittent fasting may also improve appetite control and attenuate hunger during a fat loss diet (Wadden et al. 1987). In animals, the method has been shown to improve blood sugar control (Anson et al. 2003), but this result has not been seen consistently in humans. However, in humans a reduction in diabetes risk has been found, along with improvements in other markers of diabetes, such as insulin sensitivity (Heilbronn et al. 2005b; Halberg et al. 2005).

Intermittent-fasting trials in humans have also found increases in HDL-C (the “good” cholesterol), along with decreases in triglycerides, leading to improved blood lipid profiles (Heilbronn et al. 2005a). However, it can’t be ruled out that these improvements were simply from trial subjects’ loss of weight and body fat. In rodent models, intermittent-fasting trials have consistently shown improvements in cardiovascular health (Mattson & Wan 2005).

Overall, these improvements are intriguing, but certainly not conclusive. Anecdotally, many people have reported tremendous results from practicing different forms of intermittent fasting. However, many others, especially women, have reported complications such as amenorrhea (absence of periods), sleeplessness and dysregulated appetite.

Do Results Differ Between Men and Women?

Men and women may not receive the same benefits from intermittent fasting. The research seems to indicate a fair number of differences in how men and women respond.

Where women see a rise in HDL-C from fasting, men do not. And where men see a drop in triglycerides, women do not. No one knows why the changes are sex-specific (Heilbronn et al. 2005a).

With insulin sensitivity, men see an improvement, whereas women usually do not. In fact, glucose tolerance for women has worsened during intermitent-fasting trials (Heilbronn et al. 2005). That said, the trials have been of short duration, so more studies are needed to determine whether this is consistently true across all populations.

Overall, the majority of research on intermittent fasting has been done on males, so it is unclear if the potential benefits will apply to women, especially lean women or perimenopausal women.

So, Should I Train Before or After Eating?

A question that invariably comes up when discussing intermittent fasting is whether clients should train in a fasted or fed state. Because there are strong advocates on both sides of the fence, trainers are bound to be asked, “What is the best way to train?” Let’s dig in.

A recent study looked at moderately trained subjects during 2 hours of moderate-intensity cycling. This study found that ingesting carbohydrates before training suppressed fat oxidation and other markers of lipid metabolism, in both men and women, compared with training in a fasted state (Wallis et al. 2006).

This adds to previous research comparing more-trained cyclists in fasted vs. fed conditions. In these subjects, too, consuming carbohydrates prior to training (at a low intensity) significantly inhibited fat oxidation compared with cycling in the fasted state (Coyle et al. 1997). These trials seem to indicate that eating before exercise will inhibit how much fat you burn during training, which could be a problem for clients looking to lose body fat.

However, three trials give credence to the idea that eating before training not only improves training performance, but does not decrease use of fat as fuel. When moderately trained cyclists completed a 2-hour cycling bout at a moderate pace and ingested a high-glycemic carbohydrate solution several times during the session, no difference in fatty-acid oxidation was seen between them and the fasted control group (Horowitz et al. 1999).

Interestingly, lipolysis (fat mobilization) decreased significantly, but this did not impact fat oxidation. This suggests that lipolysis is not a rate-limiting step to fat oxidation (Horowitz et al. 1999).

Other research has found similar outcomes, with trained cyclists who consumed carbohydrates prior to training seeing improvements in performance without concurrent decreases in fat oxidation (Febbraio et al. 2000).

Finally, Paoli et al. (2011) looked at the effects of fed vs. fasted exercise in trained subjects who completed moderate-paced running on a treadmill in the morning. When subjects were fed prior to the run, fatty-acid oxidation rates were higher 12 and 24 hours after the exercise bout, but not during it. In addition, the fed condition resulted in higher oxygen consumption during the run, as well as 12 and 24 hours after it, indicating greater calorie expenditure. This demonstrates that while eating before training may blunt fatty-acid oxidation during the exercise bout, oxidation may increase over the rest of the day, and overall caloric expenditure may also rise.

In the end, it is shortsighted to view how much fat you oxidize during training as the biggest key to weight loss. What is truly important is net fat balance over a course of days, rather than over a number of hours around your training session (Schoenfeld 2011). While training fed vs. training fasted is an interesting question, the reality is that substrate utilization is governed by a multitude of factors and is in constant flux.

The most important role of exercise in improving body composition is how it influences 24-hour fat balance rather than how much fat is oxidized during and immediately after the exercise bout (Hansen, Shriver & Schoeller 2005). Training fasted vs. training fed for most individuals will therefore come down to personal preference and goals, as neither appears to be the superior physiological choice across the board.


In the end, intermittent fasting appears to be one potential helpful dietary approach, among many. It is helpful for some, but is not a panacea for weight loss and health improvements for all. The best approach still comes down to building consistently healthy habits, eating nourishing foods, and finding what works for you.

1 Comment

  1. Avatar Tyler Stierhoff on February 18, 2020 at 11:02 pm

    Personal preference is exactly the answer! I’ve trained fasted and fed and really didn’t notice a huge difference either way.

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