Early Fasting Reduces Brain Function, Burns Up More Muscle Than Fat & Ruins Circadian Rhythm

Point 1: Don’t start intentionally fasting until the last third of your day, if at all

Our cells continuously produce protein; they never stop. Protein is not stored in the body, so when protein from our last meal is no longer available, the body is forced to break down muscle to provide amino acids to more vital organs. Our continual ongoing protein production slows down while we sleep, which is why fasting in the evening results in more fat loss than muscle loss, whereas morning fasting can eliminate twice as much muscle as fat (as shown by Lowe and coworkers in a 2020 UCSF study published in the Journal of the American Medical Association). Likewise, starting an evening fast too early leaves too many waking hours with a rapid muscle loss before going to sleep; a 2024 meta-analysis in the journal Diabetes and Metabolic Syndrome showed that starting your fast at 1 in the afternoon burns up 3 pounds of muscle and only half that amount of body fat over for 8 weeks, which is just as bad as morning fasting. This is in contrast to starting an evening fast a few hours later (around 4 pm), which ends up burning only one-half pound of muscle while burning 3 pounds of fat, meaning that late fasting burns up 6 times as much fat as muscle. It is not therefore how many hours you are fasting, but rather how many of those hours overlap with your sleep that matters most for both sustainable muscle-driven weight loss and to maintain your natural circadian rhythm.

Point 2: Deep sleep accelerates gene expression, which precedes protein production

Eating half your Calories in the last third of your waking day before sleep significantly reduces sleep depth for your brain and the circadian rhythm for your liver, digestive, endocrine, immune, and other systems. This reduces the clearance of metabolic waste that builds up in the brain while awake, and reduces the acceleration of DNA gene expression throughout the body that occurs during our deepest sleep in the fasted state.  This is why late fasting (or simply eating lightly in the evening) is referred to as “circadian nutrition” or “chrono nutrition” by the medical research community. Eating healthy food after waking in the morning triggers your body to fully shift into energy and protein production mode, whereas eating lightly in the evening enables your body to shift back to the restorative mode of DNA gene expression and systemically clear out metabolic waste accumulation in the brain. This raises the question as to why most people who intermittently fast choose to skip breakfast when their goal is ostensibly to increase health. The answer is that adrenaline associated with waking up reduces hunger perception, whereas later in the day we are more likely to perceive not only hunger but also cravings at the same time as social eating while relaxing with others or while watching a screen. In other words, late eating combines the consumption of food with relaxation and social activity as opposed to having a focus on providing our body what it needs. Replacing breakfast with a large dinner responds to the whims of perception instead of thoughtfully nourishing the body and mind within a natural circadian rhythm using the relative amounts of food our cells need when they need it.

Point 3: Morning fasting expands a blunted hunger perception into blunted mental function

The brain can use ketones as fuel but always has a glucose requirement. This is why mental function declines when the combination of blood ketone and blood sugar levels are low. We experience this as a reduced cognitive capacity, including logical executive function, slower learning, slower reaction time, and a mood shift that includes a higher stress response and increased aggressiveness in social interactions. This can cross over into depression symptoms when driven to the point of chronically low nutrient levels associated with rapid weight loss, overtraining, or both. Since skipping breakfast feels easiest due to a blunted hunger perception and contributes to blunted mental function, we can think of early or morning fasting as being “mindless” in the literal sense in that our blunted perception leads to a further blunting of overall mental function.

Point 4: Eating only once or twice per day increases cardiovascular disease and mortality

Most people who skip one or more meals skip breakfast because that is when they notice hunger the least and are focused on getting on with their day even though it abandons their body into disease and blunted mental function. In 2023, research published in the Journal of the Academy of Nutrition and Dietetics tracking 24,000 adults for 15 years showed that those eating twice per day (compared to 3 times per day) had both a higher cardiovascular disease risk and higher overall mortality by roughly 10%.  Those who averaged only 1 meal per day had a 30% higher overall mortality and 80% higher cardiovascular mortality, nearly doubling their heart disease risk. This research showed that skipping lunch or dinner hurt people the least, whereas skipping breakfast was 3 times worse. It also showed that having meals less than 5 hours apart when eating 3 meals per day tended to increase mortality by 20%, likely because of an unbalanced nutrient flow through the day. These results are difficult to explain if ignoring that muscle loss can exceed fat loss when someone’s deepest fast (after not having eaten for more than a dozen hours) occurs while awake as opposed to overlapping that deepest part of a fast with the circadian sleep cycle where it increases sleep’s benefits to metabolic health. Similarly, ignoring the health value of muscle makes it equally difficult to explain the obesity paradox, which is that overweight individuals tend to have a higher risk of cardiovascular disease but also survive longer with the disease if they have more muscle as part of being overweight. An incredible example of this fact is the doubling of survival after a heart attack in the half of patients who are physically stronger on a leg extension machine than the other half of patients. This is not to say that we should all do strengthening exercises for health, but rather that we should properly nourish our body with both quality sleep and quality food timed with our natural circadian rhythm in order to burn up more fat than the lean muscle that our metabolic health depends on.

Point 5: Muscle has by far the highest metabolic capacity of any organ system in our control 

Functional muscle mass improves health so much because it eats up Calories of both sugars and fats, which reduces metabolic disease risks including obesity, diabetes, and heart disease. Longevity, health span, and quality-of-life measures are increased equally with cardiovascular health and hand-grip strength as a surrogate indication of overall muscle and strength capacity. Our digestion, ventilation, and circulation are the supply systems meeting the demands of our entire body, with muscle being the main demand that we control with our lifestyle choices. Those choices must include how many hours we nourish our muscles compared to how many hours we break them down. That is how we can shift the hours ratio towards both achieving and sustaining our health and fitness goals.

Point 6: Sensible eating was well understood by our grandparents and thousands of years ago

Eating nothing when you wake ignores the blunted physiological and psychological function and mortality associated with the starved state we are erroneously referring to as fasting. Logic tells us that after not eating for a while the body would need to eat itself, and yet many people simply assume they only need to eat when they notice hunger, at which point they further assume the body wants and therefore should get double the amount it needs, with the extra being dessert and ongoing snacks into the evening. The belief that a lack of hunger after not eating for a dozen hours indicates that the body does not need food incorrectly assumes we are always in touch with our nutrient needs independent of adrenaline and other hormonal factors. But this does not mean breakfast needs to be the largest meal of the day or that it even needs to be as big as your other typical meals. To reverse the detrimental outcome of skipping breakfast, you need only eat 5 to 10% of your total daily calories in protein and slow-digesting healthy carbohydrates to protect your body from eating up muscle until you can get to a regular meal a few hours later. I call this a “Protect Lean Tissue” or “PLT” snack since it amounts to a 1-minute effort of eating some yogurt (since dairy has an equal amount of protein and carbohydrate), or oats as the slow carb with milk as the protein, or a hard-boiled egg followed by a piece of fruit on your way out the door. You could have another PLT snack halfway through your morning so that you never end up really eating a full meal at any one time and can stay on course with your busy schedule. But if you want to push through for a half dozen hours until lunch, then you would need double the amount of PLT snack, meaning roughly 20% of your daily Calories. The goal is to protect your metabolic health, as opposed to overreacting with a massive breakfast that overloads your body. Healthy moderation and using food as medicine were concepts described by Hippocrates over 2,000 years ago and practiced by our grandparents without their even needing to think about it very much. And these concepts hold true today just as much as they did then.

Point 7: Once you reengage thoughtful common sense you will notice hunger after waking

Skipping breakfast is marketed as a scientifically sophisticated approach to meal timing by ignoring the obvious and somewhat horrifying evidence against it. Extending a fast for an entire day or more can reduce the rate of muscle loss because of a shift into ketosis, but muscle loss is never eliminated because of the continual ongoing cellular need for amino acids. And if you are physically active, extended fasts are catastrophic because active muscle robs the glucose made for the brain by the liver during ketosis. These concepts apply directly to undereating and skipping meals due to low hunger from the use of Ozempic-type weight loss medications. We must maintain a circadian rhythm to our flow of nourishment regardless of what keeps us from noticing hunger. In one of my Stanford classes, only a quarter of the students raised their hands when I asked who regularly noticed hunger and ate something after waking. That day's lecture covered this topic in much greater depth than I am covering it in this essay. The next week, when I asked who had started to notice hunger more each morning after waking after hearing the lecture on the topic, more than half of the students raised their hands. It seems we have a much greater perception of what our body needs when our thinking mind is aware of what those needs are. Likewise, a false belief that eating when we wake is bad (meaning that continuing to fast into the starved state through the morning is good) likely blunts morning hunger perception more than is already occurring from the adrenaline response to waking and stress.

Point 8: The bottom line is that thoughtful common sense provides a clear path

The combination of early fasting and late gorging leverage each other into disease by combining muscle loss the first half of the day with a faster lipid accumulation later in the day due to the earlier muscle loss combined with Caloric overload all at once later on. While that eating pattern might sound great to anyone way more interested in dinner than in breakfast, failing to provide anything after waking and oversupplying Calories into an overflow later is the worst combination for metabolic health. The loss of muscle in the first half of the day increases how much body fat accumulates from eating more than the body can handle in the second half, even when under-eating overall. And to make things worse, this early fasting with late eating deranges the sleep depth that clears out the sludge build-up leading to dementia and blunts the accelerated DNA gene expression that happens throughout your body that is the first metabolic step towards protein production and cellular vitality. We can, therefore, use functional muscle together with cardiovascular health as a gauge of our supply-and-demand Calorie-burning metabolic flow critical for health, fitness, and longevity.