The Unpleasant Nuance of Intermittent Fasting - Vibrance Nutrition and Fitness

Why Intermittent Fasting Works for Some People… and Totally Backfires for Others

Intermittent fasting (IF) has been marketed as the Swiss Army knife of metabolic health: it has been touted as the go-to method to stabilize blood sugar, reverse prediabetes, melt fat, improve energy, clear brain fog, and prevent cancer.

And while IF can be an effective tool for certain metabolisms, it’s not the magic bullet it’s sometimes made out to be. In fact, for many of the people I work with—especially those already juggling stress, hormone shifts, family schedules, and blood sugar instability—IF can have potential drawbacks and challenges that can worsen future health outcomes for some. Here’s why:

The Metabolic Reason Intermittent Fasting Isn’t “One-Size-Fits-All”

The research that made IF explode in popularity came largely from studies showing improvements in:

Fasting glucose
Insulin sensitivity
Circadian alignment
Cellular autophagy (mostly in animals with wildly different feeding schedules than humans)

But here’s the nuance: these benefits occur most reliably in metabolically stable people, not in those with rollercoaster blood sugar, cortisol dysregulation, or chronic stress.

If your foundation is wobbly, “just don’t eat for 16 hours” is like putting a new roof on a house without repairing the foundation. It looks nice at first, but eventually something caves in.

Many people are not metabolically healthy enough to get through IF without negative consequence. If there is underlying blood sugar issues like reactive hypoglycemia (which is unlikely to show up on a standard blood test) or blood sugar instability, they may suffer from headaches, fatigue, vertigo and irritability when fasting.
Underlying endocrine or hormone imbalances (commonly labeled as adrenal fatigue or cortisol dysregulation) are also negatively impacted by longer fasting periods, as the body perceives the lack of food as a stressor, driving up stress hormones as blood sugars fall below the threshold your brain is comfortable with.

And when your brain is convinced you’re starving, it does what any good survival-oriented organ would do: it pushes you to eat big, fast, and usually at night—when willpower is at its lowest and snack foods are at their most seductive.

The All-Day Faster Who Couldn’t Lose Weight

One client, who I will refer to as Josh, came to me proudly sharing that he had been eating one meal per day for over a year, averaging 18 hour fasting windows with a 24 hour fast once a week. Yet he was still 50 pounds above goal weight – and stuck. No matter what he tried, the scale wouldn’t budge. Increased exercise did not move the scale in a meaningful way, and ultimately led to repeated injuries. When he came to me, he was limited in movement due to injury and looking for a way to fine tune his method to continue to get the results he initially saw. Additionally frustrating were episodic evening binges of pizza or take-out after a tough work day. While Josh attributed this to stress and lack of willpower, I saw a different picture.

But here’s what was actually happening:

All-day fasting kept his stress hormones elevated.
Cortisol stayed high, which drives up glucose and encourages fat storage—particularly around the midsection.
His metabolic flexibility slowed way down.
He wasn’t efficiently burning fat; he was merely under-fueled all day, resulting in metabolic slowdown. Stress hormones and low glucose eventually led to repeatedly over-fueling in one sitting.
Josh was eating the largest meal when his circadian rhythm was least able to handle it.
Glucose tolerance is highest in the morning and lowest at night. One huge evening meal made a bad situation even worse.
His protein intake was chronically low.
One meal simply wasn’t enough time to get the protein and micronutrients his body needed, hence repeated exercise injuries. The result?
- stalled fat loss
- significant muscle loss as part of overall weight loss
- slower metabolic rate

Josh wasn’t “doing IF wrong.” If fact, he had demonstrated pretty impressive willpower for a long period of time! But ultimately, he was doing the wrong style of IF for his physiology.

Cortisol: The Reason Why Stress + Fasting = Blood Sugar Chaos

When your body perceives fasting as stress, cortisol rises. Cortisol’s job is to elevate blood sugar so your brain doesn’t run out of fuel.

This is why some people do IF and feel:

Wired
Shaky
Craving sugar
Distracted
Irritable

What people call ‘hangry’ is actually often hypoglycemia symptoms!

If cortisol stays elevated long enough, it triggers something else: rebound overeating, usually in the evening. This is what was happening to Josh. This pattern is tightly linked to weight gain and disrupted sleep.

Why Late-Eating IF Backfires—Even Though Early IF Can Help

Research on time-restricted feeding shows clear benefits for early eating windows (e.g., 7am–3pm).
Unfortunately, with work schedules and family/social obligations, this eating window is hardest to follow in real life!

Most people end up:

Fasting all day
Eat from 5–10pm
Go to bed full
Wake up groggy
Repeat

This goes against:

circadian rhythm
insulin sensitivity patterns
digestive capacity
sleep cycles

While research generally shows that total caloric intake trumps timed eating, when it comes to weight loss, clinically I have found that individuals under chronic stress have more challenges managing weight when the largest meal is at night. Ironically, restricting intake more during the day to ‘save’ those calories for dinner often makes it worse! The human metabolism is not the calories in/calories out calculator we were taught. Multiple hormones play a massive role in how efficiently – or inefficiently – we use the calories we ingest.

Ultimately, fasting all day and then making indiscriminate and less healthy choices at dinner which can exacerbate weight issues, disordered eating habits, and nutrient deficiencies.

Muscle Loss: The Hidden Downside No One Talks About

For those of us over 45, muscle mass is non-negotiable.
Not optional.
Not “nice to have.”
Muscle is the number-one predictor of metabolic health and functional aging. Without sufficient (or even a surplus) of muscle mass, the risk of diabetes and osteoporosis is much, much higher.

Using IF without adequate protein intake is an express lane to muscle loss. I saw this over and over again in clients committed to IF who would take quarterly biometric scans.

Here’s why:

After about 12 hours without food, muscle amino acids increasingly become the raw material for glucose production.
Women especially (and my clientele are about 85% female) already face anabolic resistance with age, meaning we need more protein, more often – not less!
Cramming 100–150g of protein into a 6–8 hour eating window is physiologically challenging for anyone, regardless of gender.

A health strategy that costs you muscle mass is not a health strategy at all. It’s a slip and slide to insulin resistance, rebound weight gain, and rapid aging.

The Diabetic Client Who Spiked Higher from Fasted Exercise

Another client—a woman with diabetes—believed fasted workouts would flatten her morning glucose.
Instead, Katrina saw dramatic post-exercise spikes.
Not tiny blips… big, unmistakable spikes.

Why?
When you exercise without fuel on board, the body:

Releases adrenaline to access stored glucose
Raises cortisol to keep energy available
Signals the liver to dump glucose into the bloodstream

For someone with impaired insulin signaling, this is a perfect recipe for elevated glucose after exercise. While it is normal for anyone doing high intensity training to get a glucose spike from the effort, hers was especially exaggerated.

Once Katrina added a protein-forward breakfast before training, paired with a modest amount of carbohydrates, those spikes came down significantly and more quickly returned to normal.

Her body didn’t need to dump extra glucose into the system because she had already given it a steady supply (protein + carbs makes those carbs absorb more slowly).

While many people have been led to believe that fasted exercise burns more fat, when the intensity of the exercise is high, glucose is a non-negotiable to make it happen. The liver has your back in this regard, and will release glucose into the bloodstream to ensure you can complete the task on hand. That glucose will need to be used up before fat becomes a preferred fuel source. A little ahead of time allowed less to be released for the activity, resulting in a quicker and more effective fat-burning system overall.

Who IF Helps… and Who It Doesn’t

Intermittent fasting may work well for people who are:

Male, or post-menopausal female
Sleeping well
Not under chronic stress
Able to eat earlier in the day
Active but not overtrained
Capable of meeting their protein and nutrient needs in a compressed window

Intermittent fasting often backfires for those who are:

Perimenopausal or in child-bearing years
Prone to reactive hypoglycemia
Stressed or sleep deprived
Dealing with cortisol dysregulation
Exercising fasted
Eating one large nightly meal
Struggling with binge eating tendencies
Unable to get enough protein during a short eating window
Managing diabetes without medical supervision

IF can be a useful tool, but it is only one tool. Like any tool, it works brilliantly in the right hands and terribly in the wrong situation.
I find those who want to implement IF have the least negative consequences if they:

Limit the fasting window to 10-14 hours
Consistently and doggedly ensure they get adequate protein, determined by muscle mass if possible.
Engage in low-intensity exercise over high intensity, demanding activities ( ie – choose yoga, leisurely swimming or walking, and moderate weight training over Crossfit, running, and endurance cycling)
Have no history of chronic restrictive dieting or eating disorders.

While there is promising research on IF reducing insulin and blood sugar levels, this is only one tool to accomplish this. There are many tools available, so before you jump on the IF bandwagon, you may want to critically analyze whether or not this is ideal in your situation, for your body, with your personal and medical history.

Furthermore, individuals with medical conditions such as diabetes may require medical supervision before attempting IF, especially if medications are part of one’s treatment plan.

Before hopping on the IF bandwagon, it’s worth asking yourself: