Intermittent Fasting 101: A complete guide to trying IF

Posted by David Brummert on

  • Time-restricted feeding, or Intermittent Fasting (IF) as it’s known, has many benefits for health, body composition, and inflammation.
  • One of the main benefits is that you don’t have to change what you’re currently eating in order to get started.
  • The types of foods you eat, and the timing of when you eat them are important factors when doing IF.
  • There are a number of populations that can benefit from using IF, including athletes.
  • IF could be combined with other dietary strategies like Keto or carb cycling to potentially improve results further.

Hello friends! By now you’ve probably heard about “intermittent fasting” (IF) from a website, a friend, a coworker, or someone else that just HAS to let you know they’re doing it. Or maybe you’re the person doing it already, and telling all your friends how much you love it. Either way, I’m here to shed some light and bust some of the myths surrounding IF and cut to the heart of why you should or shouldn’t give it a try.

What is Intermittent Fasting anyway?

Basically, it just means condensing your meals for the day into a smaller window of time. For example, you’d eat all of your meals between 10 am and 6 pm. But there are a number of different ways to do it. Technically speaking, eating in a set window each day is referred to as, “time-restricted feeding” but let's be honest – intermittent fasting sounds a lot cooler.

Why should I purposely not eat for 16 hours?

There are many clear benefits, and a few possible ones: When you eat less frequently, your body won’t have as many spikes in blood sugar and as a result, when you do eat, your body will be able to control your blood sugar levels better. That’s obviously really important for people with diabetes, but also for people that are overweight. IF also reduces the amount of inflammation in your body. There’s a cycle where chronic, low-grade inflammation in the body can cause fat cells to grow, and then the fat cells themselves release substances that cause even more inflammation.  IF actually makes your body produce more anti-inflammatory substances, like adiponectin, and less proinflammatory substances like Interleukin-6 and Tumor Necrosis Factor alpha (TNF-a).

Like many variations of fasting, IF reduces body fat, mainly by causing a decrease in the number of calories you eat during the day. Another of the really important benefits is that IF improves cell clean-up, known as autophagy. Think of autophagy as the body’s way to declutter itself: anything that isn’t working properly gets cleaned out so that the healthy cells can do their work normally. This is essential in helping to prevent abnormal cell growth that can lead to diseases like cancer. IF can also improve your cholesterol and triglycerides, helping to reduce your risk of cardiovascular disease.

There may also be a psychological benefit, similar to the marshmallow test. It may actually help you resist other urges for food or ‘instant gratification’ type things.


  • Improves insulin sensitivity
  • Decreases inflammation
  • Improves body composition
  • Can help lose weight
  • Increases HDL
  • Decreases LDL
  • Improves triglycerides
  • Improves brain health
  • May have positive effects on longevity, cancer, and Alzheimer's disease

    Who should try it?

    People interested in fat loss or body composition, or people trying to improve their overall health and reduce their risk of diabetes, heart disease, etc. It’s also a good tool for people that have had a hard time adhering to other diets, as the concept and application are pretty simple.

    Ok so how do I start doing IF?

    Probably the most common and feasible is a daily 16 hour fast and an 8 hour ‘feeding window’. You can also do 18/6 (6 hour feeding window) or 20/4 (4 hour feeding window) or something called Alternate Day Fasting (ADF), where you alternate between eating your normal calories one day and then anywhere from 0 - 25% of your normal calories the next (So 2000 calories and then 500 and then 2000, etc). An actual 100% fast every other day is uncommon and could cause some loss in muscle mass over time. 16 hours seems to fit our normal work schedules pretty nicely and has all of the benefits of fasting without getting to the point of muscle loss. People that have done IF before may see benefits from longer fasts as their bodies adjust to the routine. In the end, pick a window that works with your schedule and lifestyle.

    Why is it better than a regular diet?

    Depending on the diet, IF may reduce the amount of muscle loss and avoid the decrease in metabolism that is often seen with longer-term calorie restriction (like eating 1200 calories every day for 3-4 months). Nothing in particular says it is ‘best’ compared to any other – it’s just another tool to try, and it happens to be a really effective one.

    Does it matter what I eat?

    Of course! It works for weight loss because you can create a calorie deficit, and some people like IF better than regular dieting because it lets them be less strict with their meals.  Especially if you’re just starting off, eating 2 larger meals that are ‘less healthy’ and tastier, while still keeping a calorie deficit and losing weight and fat is a nice trade-off. Since some people have a hard time eating all their calories in ‘healthy’ foods in 4-8 hours it could potentially allow you to choose more “cheat”-type foods. But as always, your body responds to what you put in it. Vitamins, minerals, fiber, sufficient protein, and hydration are all important factors. And there are many pros vs anti-inflammatory benefits to eating ‘clean’ versus eating Chinese food and pizza.

    Do I have to count calories? Because I hate counting calories

    Maybe. If you’re just starting out, then probably not. And if your main goal is just the health benefits and not body fat or weight loss, then it’s also probably not necessary. As you move closer toward your goal, or if you’re already somewhat lean, then it would help make sure you’re getting dialed in and hitting your macros. It’s like starting in the middle of Kansas and being told to walk to a street address in New York. At first, you can just generally walk in the right direction and you’ll be getting closer, but the closer you get, the more important it is for you to know the exact correct route to take in order to make sure you actually get to where you’re going.

    Can I drink coffee, tea, or vegetable juice?

    Coffee and tea, (sweetened with Stevia ONLY) for sure – these may even enhance some of the fat-burning effects of IF (as long as there is no sugar or cream in them – I know, I know… nobody likes to have their coffee messed with, but it’s worth it. Some would argue that heavy cream is ok too because it doesn’t have carbs and won’t elicit much, if any insulin response.) Vegetable juice – it depends on the type of IF you’re doing. During Alternate Day Fasting (ADF) it’s probably advisable for the beneficial nutrients and some calories. For a 16 hr fast/8 hr feed, it’s probably a no go if you’re outside of your feeding window. BCAA’s are also a no because they also cause an insulin response and won’t have enough of a muscle-building response to justify it.

    Should I work out while I’m fasting?

    Yes, for sure. Depending on the person, WHEN you work out could shift, but working out actually helps prevent muscle loss during fasting. If your main goal is to lose body fat, working out at the end of your fast and right before your feeding window makes a lot of sense: You burn calories when your body is already primed to be using fat as a main source of fuel, and you sensitize your muscles to the protein and nutrients you’re about to feed it. However, this is a terrible idea if you’re an explosive athlete, or if your workout is going to be very high intensity. For these people, workouts in the middle of the feeding window makes more sense, as the availability of carbs is vital for their performance.

    Is this right for athletes/people trying to build muscle?

    Maybe. It depends on WHY you’re doing it. If the goal (as an athlete) is to improve body composition or lose weight for power:weight ratios, then it could be a possible strategy. If the goal is explosive performance this probably isn’t going to HELP that. For building muscle, it's not clear that it's actively worse than a regular diet, if calories are the same, and may actually be beneficial in terms of body composition during bulking season (it’s always bulking season).

    What if I get light-headed?

    Be careful! There’s an adjustment period, for sure. It’s somewhat normal to feel a little light-headed as your body adjusts to the longer periods without food. I tried to start IF when I was working on Olympic lifting, and that didn’t go well at all. I got tired very quickly, didn’t feel strong or explosive, and often felt light-headed.  I quickly switched to more strength-based, lower-intensity workouts, and I adapted pretty quickly. Not only did I maintain my strength, I actually built strength and decreased my body fat at the same time. Don’t be silly - if you have blood sugar issues, check with your doc. If you feel the overwhelming need for food or if you get nauseous or dizzy – then eat or drink something!

    Will I lose muscle?

    Probably not if you’re at maintenance calories, or even in a slight deficit. And it also depends on the type of fasting. ADF is probably worse than 16/8 for muscle loss. And long-term calorie restriction is worse than both for losing muscle mass.

    Should I do fasted cardio, too?

    This is a little complicated. One analysis showed that there wasn’t much difference in fat loss between fasted cardio and non-fasted cardio, but that study wasn’t looking at IF, just an 8 or 12 hour fast. Doing cardio at the end of your fasting window will make the fat loss more pronounced if you’re obese or overweight, and it may also help prevent muscle loss. Remember, any type of exercise will help your body slow down the rate of muscle breakdown if you’re just starting out. However, fasted cardio may impair strength gains if that’s a goal. That’s because you’re basically telling your muscle cells to be ‘endurance’ cells instead of ‘strength/power’ cells. As your body adapts to the routine of fasted cardio, the muscle fibers trade contractile proteins (which make the cell contract with more force) for mitochondria (which make more fuel to run the cell for more time), which makes them each a little smaller. Some people will say ‘but mitochondria burn fat!’ which is true. That’s why there aren’t many obese marathon runners. As you get leaner though, and if you want more muscle mass (and let's be honest, almost everyone wants nice-looking muscles), longer duration or fasted cardio is probably not ideal for that.

    Do I have to do it all the time?

    I think everything can be done in moderation. I do IF 5 or 6 days a week, but allow flexibility on weekends just for personal mental ‘resets’ but if you don’t need those, don’t take them. If you do need them, take them! I like the 80/20 rule. Be perfect with your food 80% of the time and eat what you like the other 20%. Live a happy life first. And after a while, you’ll probably need to adjust your eating schedule or feeding window anyway, like with anything. Some people prefer cyclic dieting where you restrict for 2 weeks and eat maintenance calories for 2 weeks. That seems to go against the conventional wisdom to preserve muscle mass, but studies have shown you can use it and lose weight and fat while maintaining muscle mass. There’s something for everyone!

    Does it matter what my first meal after the fast is? Does it matter what my last meal before the fast is?

    Phenomenal question! I don’t know! But maybe/probably! If you start the feeding window with fats and proteins, you’re likely to keep burning fat a little longer and maybe see even more insulin sensitivity versus if you start with carbs and proteins. But that’s just a guess. And if you end the fast the same way, you may see the same thing. However, you can take advantage of the improved insulin sensitivity to help you sleep.  If you have a moderate amount of carbs an hour or two before bed your blood sugar will spike, insulin will be released, and then it’ll drop and you’ll become more tired. If you work out at the end of the feeding window (with weights or cardio) and don’t eat afterward, you may teach your body to build more mitochondria, although potentially at the expense of muscle mass. Maybe there’s something to that idea for endurance athletes. Who knows! It’s a fascinating question, though.

    Is there anyone that should not try IF?

    People with uncontrolled blood sugar issues or that have a history of eating disorders are probably not ideal candidates for IF. Explosive/Power athletes may see their performance suffer depending on the timing of their workouts/competitions and their feeding window.

    Is there a way to make IF more effective?

    You could conceivably combine this with the ketogenic diet (eating most of your calories from fat and less than 5% from carbs) or carb cycling/flexible dieting (where you adjust either calories or carbohydrates each day to match the amount and type of workouts you’re doing) to see an additional benefit, but those are maybe progressions to try once you’ve become acclimated to Intermittent Fasting.

    And remember: Your worth as a person isn’t tied to your weight, body fat percentage, or muscle mass. Just be kind.