The Hidden Red Flags of Intermittent Fasting: What Every Client Should Know

By Prishita Vora, Health Coach

Intermittent fasting (IF) has undeniably become a popular tool in the wellness space—promoted for weight loss, metabolic benefits, and even longevity. But as with any eating strategy, IF is not a one-size-fits-all solution, and there are important “red flags” that both you and your clients should watch out for. Below, I’ll walk through what to monitor, when to pause the approach, and how to integrate fasting more safely if it’s appropriate.

1. Feeling poorly (headaches, dizziness, low energy)

One of the earliest signs that a fasting protocol may not be aligning with an individual’s physiology is: fatigue, headaches, dizziness, irritability or brain fog. These symptoms often emerge when the eating window is too restrictive, hydration is inadequate, or caloric intake drops too steeply.

For example, research from the Harvard Health Publishing notes that people practicing forms of IF may experience such side-effects and that long-term safety remains uncertain. (Harvard Health)One more recent review flagged fatigue, mood disturbances and sleep disruption as common among IF participants. (npjournal.org)

What to watch for:

• Persistent headache or light-headedness after starting a fasting window.

• Unusual levels of tiredness, especially outside training days.

• Craving or overeating during the feeding window due to extreme hunger.

Coach tip:If a client reports these symptoms, consider loosening the eating window (e.g., from 8 hours to 10-12 hours), ensure adequate hydration (aiming for ~30-35 ml per kg body weight), include electrolytes if needed, and monitor recovery and sleep quality.

2. Eating behavior goes off track

Another red flag: when a fasting structure inadvertently triggers disordered eating patterns. Restricting when you eat can, for some people, lead to an all-or-nothing mindset, binge-restrict cycles, or an unhealthy relationship with food.

A commentary from the University of Illinois at Chicago pointed out that while IF does not inherently cause eating disorders in healthy volunteers, people with a history of eating-disordered behavior are at increased risk and should approach with caution. (ahs.uic.edu)

What to watch for:

• Client becomes overly anxious about “breaking the fast” or eating outside the window.

• Shaming self or feeling out of control after the eating window closes.

• Using fasting to compensate for over-eating or emotional eating rather than addressing root causes.

Coach tip:Always assess eating history and relationship with food before introducing fasting. Emphasize flexibility, attunement to hunger/fullness cues, and that the goal is sustainable behavior, not just time frame. If any signs of disordered behavior arise, pause the fasting protocol and refer to a specialist if needed.

3. Lack of clarity on long-term effects & potential risks

The hype around intermittent fasting often focuses on the benefits, but less attention is paid to the gaps in long-term human research and emerging signals of potential risk—especially when protocols become extreme.

For instance:

• A 2013 review observed that many benefits of fasting were derived from animal studies and cautioned about “fad behavior”. (PMC)

• More recently, a 2024 commentary flagged preliminary data suggesting people who restricted eating to less than 8 hours per day had a higher risk of cardiovascular death—but stressed that the data are mixed and not yet definitive. (McGill University)

• A 2023 review noted side-effects like dehydration, constipation, hypoglycemia and sleep disturbance when fasting was applied without individualization. (npjournal.org)

What to watch for:

• A client jumping into very aggressive fasting (e.g., 18-hour fasts) without monitoring.

• Ignoring underlying health conditions (e.g., diabetes, cardiovascular issues, older age, underweight).

• Neglecting nutrient quality and overall caloric adequacy.

Coach tip:Always tailor fasting to the individual. Use a conservative approach initially (e.g., 12-14 hour overnight fast) rather than immediately launching into aggressive regimes. Continually monitor biomarkers, health history, and response. Emphasize that timing is only one component of nutrition—what and how much we eat remain foundational.

4. When fasting becomes unsustainable or leads to rebound overeating

One practical red flag: the client finds the fasting approach unsustainable, and it results in rebound overeating, energy crashes or mood disruption. If the plan is too rigid, it may undermine long-term adherence.

Harvard Health is clear: the most studied forms of IF show that appetite tends to increase and metabolism may slow—just as with any significant calorie restriction. (Harvard Health)

What to watch for:

• Client is overeating in the feeding window because of extreme hunger.

• They feel forced into “reward-eating” after the fast.

• They have frequent errors or feel life/social circumstances are being disrupted to maintain the eating window.

Coach tip:Re-frame fasting as a tool, not a rule. Encourage flexible windows that allow for social meals, celebrations and personal context. If the client is slipping into overeating or feels deprived, shift to a gentler protocol or consider regular eating with mindful portion control instead.

5. Inappropriate for certain populations unless supervised

While IF may have benefits for many people, certain groups are at higher risk and require close supervision—or should avoid fasting altogether. This includes:

• Those with eating-disorder history

• Underweight or malnourished individuals

• Adolescents (still growing)

• Pregnant or breastfeeding women

• Older adults with low muscle mass or osteoporosis risk

• People on medications that require food (e.g., for diabetes, blood pressure)

Harvard Health explicitly states that older adults may lose too much weight and risk muscle, bone and immune health when fasting without oversight. (Harvard Health)Also, studies highlight safety concerns in those on medications or with complex health status. (The Emily Program)

Coach tip:Conduct a full health screen before recommending fasting. If a client falls into one of the higher-risk categories, either modify the protocol (gentler window, more feeding days) or prioritize other evidence-based nutrition strategies instead.

Final Thoughts & Practical Implementation

As your coach and nutrition partner, my goal is to help you apply fasting (or any strategy) wisely, not merely because it’s trendy. Here’s how we’ll implement it safely when appropriate:

• Start slow: e.g., a 12-14 hour overnight fast (say finishing eating at 8 pm and breakfast at 10 am) for 1–2 weeks.

• Monitor symptoms: track energy levels, mood, sleep, hunger, digestion and recovery from training (if relevant).

• Prioritize nutrient-dense feeding windows: ensure adequate protein, healthy fats, fiber, vitamins and minerals.

• Hydrate: include water, herbal teas, electrolyte balance—especially if fasting period is long.

• Adjust based on lifestyle: training days, social commitments, travel—flexibility is key.

• Reassess continuously: if any red flags appear (e.g., excessive fatigue, bingeing, mood disturbance), adjust the approach.

In short: fasting can be a helpful tool—but not a magic bullet. Its success depends more on how it’s integrated into a client’s overall lifestyle, nutrition quality, training load and health status than on the window alone. Let’s use it strategically and safely, tailored to you.

References

1. “4 intermittent fasting side effects to watch out for.” Harvard Health Publishing. (Harvard Health)

2. Collier R. “Intermittent fasting: the science of going without.” CMAJ. 2013. (PMC)

3. Nye K, et al. “Intermittent Fasting: Exploring Approaches, Benefits, and Risks.” npj Journal. 2024. (npjournal.org)

4. “Intermittent Fasting and the Perils of Fast Facts.” McGill University OSS. 2024. (McGill University)

5. “Myths about intermittent fasting, debunked.” University of Illinois Chicago. 2024. (ahs.uic.edu)

6. “Is Intermittent Fasting Dangerous?” Emily Program. 2024. (The Emily Program)