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Fasting vs. Eating Breakfast: Everything You Need To Know


Intermittent fasting (IF) is a popular health and weight loss tactic that works well with the ketogenic diet. Benefits include easy weight loss, balanced blood sugar, mental clarity, and reduced inflammation.

But eating breakfast is touted by many nutrition experts as the most important thing you can do for your health.

So, should you fast or eat breakfast?

There’s plenty of science to back up both. In fact, choosing the best morning routine may depend less on the science and more on your body, physiology, stress levels, goals, and a host of other factors that make you uniquely you.

So then, to fast or not to fast? Let this article be your guide.

Benefits of Intermittent Fasting

Intermittent fasting is the regular practice of daily, semi-daily, or even more occasional fasts. A 12-hour fast is generally considered the IF minimum, and anything beyond 24 hours falls into the “long-term fast” category.

When you fast, you activate a spectrum of stress-response pathways — like NrF2 and AMPK. These pathways help you clean up cellular junk, burn fat, and think more clearly. Some of these benefits don’t manifest until after the fast, when you refeed.

Stress-response pathways are called stress-response pathways because they’re triggered in response to a stressor.

Fasting, like exercise, is a type of stress. In the right amounts, it can be “good stress” from which you bounce back better, stronger, and healthier. But too much fasting, like too much exercise, can be more harm than good.

To get the most out of intermittent fasting, you should be:

  • Fat-adapted (able to burn fat as fuel)
  • Getting enough sleep
  • Avoiding excessive exercise
  • Not overly stressed
  • Not pregnant or nursing

So, if you’re overly stressed, you can skip to the breakfast eating portion of this article, because fasting isn’t for you.

Listed below are some of the researched benefits of intermittent fasting. Note, however, that these benefits do not apply to everyone, and they may not apply to you. You’ll learn more about who should and shouldn’t fast later.

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#1: Weight Loss and Fat Loss

Weight loss is probably the number one reason people fast. In support of this point: a recent meta analysis found that skipping breakfast was associated with lower energy intakes and less weight gain[*].

More convincing data comes from an 8-week randomized controlled trial on 34 resistance-trained men. One group did a 16-hour daily fast, the other did not. At the end of the trial, the fasted men had less fat mass (yet equivalent muscle mass) than the control group[*].

Why does fasting trigger fat loss? For one, it turns out that periodic nutrient deprivation activates an enzyme called AMPK. In turn, AMPK boosts both lipolysis (fat breakdown) and fatty acid oxidation (fat burning)[*].

Another reason relates to glucose metabolism. Fasting increases insulin sensitivity, which helps keep blood sugar and insulin levels low[*]. Less insulin, less fat storage.

One final reason? If you eat during a shorter window of time, you’ll probably fewer less calories, which can result in fat loss. Fewer calories, more weight loss.

#2: Cognitive Function

Many folks report better brain function and mental clarity during a fast. There are several possible explanations for this effect.

In humans, intermittent fasting has been shown to increase orexin-A — a brain chemical linked to alertness[*]. Fasting can also activate the sympathetic nervous system and increase adrenaline levels[*]. Adrenaline wakes up your brain.

Another potential brain-booster involves ketosis. When you fast, your blood sugar and insulin levels stay low — and this signals your body to start burning fat and producing ketones.

Ketones then cross the blood-brain barrier to power your brain with clean, efficient energy.

In fact, ketones have been shown to boost mental performance in both mice and men[*]. And excess blood glucose, believe it or not, has been shown to impair cognition[*].

#3: Autophagy

Fasting is a bit like cleaning house. Decluttering, steam-cleaning, dusting: whatever analogy you like best.

Technically, this cellular cleanup is known as autophagy. When a cell undergoes autophagy, its lysosome (the cleanup organelle) digests old, unused, or damaged parts. When you eat again, autophagy switches off and your body restores itself with new parts.

Neuronal autophagy (or brain cell cleanup) is a commonly-cited fasting benefit, and may reduce the risk of neurodegenerative diseases like Alzheimer’s and Parkinson’s[*].

One study on male and female cell cultures shows that female neurons resist autophagy during starvation. While starvation is a little different than intermittent fasting, it does suggest females may not get the cell cleaning benefits of fasting that men experience[*].

As a general rule: the longer the fast, the more autophagy. Unfortunately, there isn’t a good lab test for autophagy yet, so figuring out the optimal fasting protocol for this purpose remains a guessing game.

#4: Exercise Enhancement

Your body burns more fatty acids during fasted exercise than during a fed session, which means you may lose more weight with regular fasted gym sessions.

In fact, one trial suggests you could burn up to 45% more fat[*].

If, on the other hand, you eat carbs before yoga class — you’ll simply burn blood sugar as you flow through your vinyasas. Your body fat will remain untouched.

Fasted exercise may also help with muscle building goals. A 24-hour fast, in fact, has been shown to boost human growth hormone (HGH) levels by 1300% in women and 2000% in men![*]. Growth hormone, of course, helps you grow muscle.

Some of these adaptations, however, appear to be gender-specific. For example, women have better skeletal muscle adaptations when they exercise after eating[*].

And another study on overweight women showed no effect of intermittent fasting (combined with interval training) on body composition or fat burning capacity[*].

Who Should Try Intermittent Fasting

Intermittent fasting can work for many different types of people, but the research suggests it’s best for those who are overweight or have metabolic issues. The science also suggests fasting may be harder on women’s bodies than men’s.

Obese people have the most fat to burn, and the most fat to lose by restricting their eating windows. Lean folks, on the other hand, run the risk of becoming underweight and losing muscle mass if they cut calories too much.

Men tend to respond better to IF than women — an effect likely explained by differences in hormonal and stress responses.

Those with metabolic issues — like type 2 diabetics — can benefit from IF because periods of nutrient deprivation increase insulin sensitivity and fatty acid oxidation[*].

But lean people needn’t avoid fasting entirely. Recall that, in resistance trained men, 16 hours of daily fasting reduced fat without reducing muscle mass[*]. To be clear, the fasted men ate the same number of calories as the non-fasting control group.

The takeaway is simple. If you want to preserve lean mass on IF, eat the same amount of calories you usually would in your compressed feeding window.

Lastly, fasting may protect against chemotherapy toxicity, with most of this evidence coming from animal studies[*]. In humans, the data is more limited, but a handful of case studies suggest that IF mitigates chemo-related side effects[*].

Benefits of Eating First Thing in the Morning

Intermittent fasting often means skipping breakfast and eating your first meal around lunchtime. There are, however, potential issues with this strategy.

The main issue involves your circadian rhythm, or your 24-hour wake / sleep cycle. The circadian rhythm, by the way, regulates a huge chunk of your genome — genes that govern metabolism, cognition, DNA repair, and more.

So what controls the circadian rhythm? The obvious trigger is light. The less obvious trigger is food.

It’s true. When you eat food, you stimulate secondary clocks scattered throughout your muscle and organ tissues.

In the morning, food is your “get up and go” signal. Because of this, you could argue that breakfast is the most important meal of the day.

Eating breakfast doesn’t only affect wakefulness, but also affects sleep. For instance, one study showed that a high protein breakfast lead to elevated levels of melatonin — the sleep hormone — later at night[*]. In other words, eating breakfast may help you conk out later on.

When it comes to a healthy breakfast, protein is the key. Protein contains the amino acid tryptophan, a precursor to both serotonin (happy chemical) and melatonin (sleepy chemical).

Simply put: morning protein stimulates serotonin in the AM, which sets up melatonin in the PM.

Below are other potential benefits of eating breakfast. (Note: some apply only to non-fat-adapted folks).

  • Better exercise performance: skipping breakfast can hamper endurance throughout the day[*]
  • Improved memory: according to one paper: “healthy adults show a small but robust advantage for memory (particularly delayed recall) for consuming breakfast”[*]
  • Better nutrient intake: in ninth grade students eating breakfast[*]
  • Lower cortisol and blood pressure: female breakfast skippers had a heightened stress response[*]

Eating breakfast is especially important for certain groups of people. Keep reading.

Who Should Eat Breakfast (And Avoid Fasting)


Fasting isn’t suited for everyone. Since it’s a stressor, IF has the potential to do harm.

First of all, it’s important to address fasting-related gender differences. Most of the IF benefits studies are on men.

Fewer studies have been conducted on female populations, and the studies that have been done raise warning flags. According to this research, women are more likely than men to suffer negative effects from a fast.

Here are some examples:

  • In 11 overweight women, a two day fast provoked a significant stress response, marked by sympathetic nervous system activation[*]. Men appear to have the opposite response.
  • Women have higher levels of ghrelin (the hunger hormone), which could make extended fasts more difficult[*]
  • When female rats were fasted, their fertility suffered[*]
  • Men have enhanced musculo-skeletal adaptations in a fasted vs. a fed state. The opposite is true for women[*].

The bottom line? Women should be more cautious with fasting than men, and may do better on shorter, 12-14 hour fasts.

With that in mind, here are some groups that probably shouldn’t skip breakfast.

#1: If You’re Pregnant or Nursing

Being pregnant means eating for two people: the mother and the growing fetus. Because of this, any protocol that increases the chance of caloric restriction is not recommended. Nutrients shouldn’t be limited during pregnancy.

After the baby is born, moms should still be careful with fasting. Nursing women have higher-than-normal caloric requirements, whether or not they choose to breastfeed.

#2: If You’re Trying to Conceive

Conception is an energy-expensive process for women — so fasting is probably not the best idea for couples trying to conceive.

One group of scientists found that intermittent fasting affects the hypothalamus (a brain center crucial to reproduction), reducing fertility in both male and female rats[*].

#3: If You’re Underweight

About 1.7% of the US population is underweight, meaning they have a BMI under 18.5. That’s 1% of men and 2.4% of women[*].

Being underweight, unfortunately, is linked to a variety of conditions — cardiovascular disease, sarcopenia (muscle wasting), neurological issues, and decreased bone density — to name a few[*][*][*].

To gain weight, underweight folks should be eating breakfast, lunch, dinner, and maybe a few calorie-dense snacks in between. Fasting is not advisable for weight gain.

#4: If You’re Stressed or a Competitive Athlete

You can only take so much stress. If you’re overly-stressed from a job, relationship, or health issue — fasting may act as a “bad stress” rather than a “good stress,” causing more harm than good.

A similar principle applies to sleep. If you aren’t sleeping enough, you’re less resilient to other stressors. And so if you have a bad night of sleep, you probably shouldn’t skip your morning meal.

Finally, competitive athletes have much higher nutrient requirements than the rest of the population. Fasting may help with training-related adaptations, but it’s unlikely to help with gameday performance.

Keto and Fasting vs. Breakfast

After a few weeks on a ketogenic diet, your body adapts to using fat for energy. This means you can hum along on body fat, and don’t need a steady stream of carbs to power your day.

Being fat-adapted is a prerequisite for intermittent fasting. Because of this, keto-adapted folks will likely find it easier to skip breakfast.

But you shouldn’t fast unless you can do so comfortably.

Comfortably means you can fast for 12, 16, even 24 hours without:

  • Excessive hunger
  • Headaches
  • Fatigue
  • Irritability
  • Feeling overly-stressed

Again, women run a higher risk of these side effects than men.

Keto or not, the four aforementioned groups — pregnant or nursing women, couples trying to conceive, underweight people, and very stressed folks — probably shouldn’t fast much longer than 12 hours.

But there’s nothing shameful about a 12-hour overnight fast. It allows for circadian rhythm enhancement without overdoing it.

The Takeaway: Fast or Eat Breakfast?

To fast or not to fast? Well, that all depends.

Intermittent fasting has many benefits, but most of these benefits have been researched on male populations. Due to biological differences, eating breakfast — which has many benefits as well — may be more important for women than for men.

If you’re unsure about fasting, consider starting with a 12-hour overnight fast. Simply eat your last meal of the day at 7 PM, and eat your next meal — breakfast — at 7 AM the next day. Make it a whole food, protein-rich meal and you’ll be priming your circadian rhythm to fire on all cylinders.

Most importantly: experiment, listen to your body, and find out what works best for you. That’s how you level up your health.


6 thoughts on “Fasting vs. Eating Breakfast: Everything You Need To Know

  1. I do not eat breakfast and I been eating my last meal at 5 each day. But I’m stuck in a rut it seems. I drop about 27 lbs then there no lost in a month or so. I do the IF and have been doing the 16 hour and still no loss I don’t want to keep up with the counter that they say use cause I monitor everything I eat and in small portions so was just wondering what may be the cause of a stall. Or crash so to speak where I’m not even losing any weight.

    1. There are debates about whether black coffee breaks a fast or not. If you’re not in a strict diet, black coffee is fine since it has no calories (just don’t add cream!). As per the keto base, it may actually help you improve your fast.

  2. So, if I understand correctly, it might be better to eat breakfast and have next meal within 6-8 hours and then fast until breakfast again? I am on keto diet starting 6 month.

    1. You can do a 12-hour overnight fast (time it so you could eat breakfast). But it’s always best to listen to what your body is telling you. Note that not everyone reacts the same to a certain diet and eating habit.

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