Who Should NOT Fast: Clear Contraindications and Cautions

Upton Sinclair was one of fasting's most enthusiastic advocates. He called it "Nature's own remedy" and credited it with reversing years of chronic illness at a time when mainstream medicine largely dismissed the practice. Yet even Sinclair acknowledged plainly that fasting is not appropriate for everyone — and that attempting it without understanding the contraindications could cause harm.

Over a century after Sinclair published The Fasting Cure in 1911, modern medicine has added clinical precision to his observations. The core cautions he identified remain valid. The principles he wrote about are now confirmed by endocrinology, gastroenterology, and psychiatry.

The Direct Answer

Most healthy adults can fast safely when they start gradually and drink enough water. However, certain groups — including pregnant women, people with specific medical conditions, those with a history of eating disorders, and anyone with significant anxiety about fasting — should approach it with caution or direct medical guidance. The biggest risks are not usually from fasting itself but from fasting in the wrong medical context, without proper preparation, or while breaking the fast incorrectly.

What Sinclair Identified in 1911

Sinclair surveyed 277 fasting cases from his readers and was honest about what he found. Of 109 people reporting outcomes, 17 reported no benefit — and several cases of harm occurred not during the fast, but immediately after it, when people broke their fast too aggressively.

His specific cautions were few but clear.

Tuberculosis and significant wasting. Sinclair's one major exception was tuberculosis patients, who had often already lost significant weight and muscle mass. He wrote that fasting was inadvisable when a person had already lost weight and struggled to regain it — a principle that still applies to any condition involving serious nutritional depletion or wasting.

Mental composure. Sinclair's most striking observation was that "the first danger of fasting is fear." He described cases where psychological terror during a fast produced genuine physical harm — not from the absence of food, but from the stress response itself. He recommended that anxious people never fast alone, and always have a calm, experienced companion. He compared earthquake survivors who died during what should have been survivable hunger timelines to healthy fasters who were fully comfortable over the same period — and attributed the difference entirely to mental state.

Not drinking enough water. Sinclair identified dehydration as the most common cause of fasting failures. He gave one instruction more than any other: drink large amounts of water throughout the fast. People who fasted without adequate hydration became sick and often blamed the fasting rather than the dehydration.

Breaking the fast incorrectly. This was not a reason to avoid fasting but to approach the refeeding phase with as much care as the fast itself. Several of Sinclair's cases of harm occurred when long fasts were broken with heavy meals. He was explicit: "You must reintroduce food slowly."

What Modern Medicine Adds

Current clinical guidance expands Sinclair's list with conditions that were poorly understood in 1911.

Groups Who Should Not Fast Without Medical Supervision

Pregnant women. The developing baby has non-negotiable nutritional needs throughout pregnancy. Fasting significantly alters nutrient availability and hormonal signalling in ways that carry risk to foetal development. This is a clear contraindication.

Breastfeeding mothers. The infant is entirely dependent on the mother's nutritional status. Caloric restriction during breastfeeding can reduce milk supply and deplete key micronutrients that the baby requires.

People with an active or recent eating disorder history. Fasting can trigger or intensify disordered eating patterns in people with a history of anorexia, bulimia, or orthorexia. The restriction framework of fasting is incompatible with recovery from restrictive eating disorders. This is one of the clearest modern contraindications.

Type 1 diabetes on insulin. Blood sugar dynamics in Type 1 diabetes are complex and highly sensitive to food timing. Fasting without medical supervision carries a real risk of dangerous hypoglycaemia. Not an absolute prohibition for all, but requires close physician involvement.

Severe underweight (BMI under 18.5). Fasting draws on stored body fat and muscle for fuel. When reserves are already critically low — particularly if there is any history of restrictive eating — the body has no safe buffer. Professional supervision is required before any fasting protocol.

Groups Who Should Proceed With Caution

Type 2 diabetes on glucose-lowering medication. Fasting can improve insulin sensitivity quickly, which is beneficial but can also cause blood sugar to drop faster than medication accounts for. People on metformin, insulin, or other blood sugar-lowering drugs need close monitoring and should work with their prescriber before starting.

High blood pressure medication. Blood pressure often falls during extended fasting. If you are already on antihypertensive medication, this can cause pressure to go too low. Medication dosing may need adjustment — particularly for longer fasting windows.

Kidney disease. Fasting affects fluid balance, electrolytes, and metabolic load on the kidneys. People with existing kidney disease should have their planned fasting window reviewed by a nephrologist before starting.

Children and adolescents. Growing bodies have distinct nutritional requirements. Caloric restriction or extended fasting during development can interfere with normal growth and is not appropriate.

Elderly individuals with frailty. Older adults who are already frail or at significant risk of muscle loss need careful consideration. The protein needs of ageing muscle are higher, and very long or very frequent fasts can accelerate muscle loss. Shorter windows with protein prioritised at breaking the fast are a safer approach.

Chronic anxiety or high psychological stress. Fasting is a mild stressor that increases cortisol temporarily. In people with already-elevated cortisol from chronic anxiety or burnout, this can push the stress response into territory that worsens the underlying state rather than improving it. Starting with shorter windows of 12–13 hours and building slowly is the appropriate approach.

The Common Thread

Looking at Sinclair's 1911 observations alongside modern clinical guidance, a pattern emerges. Fasting is most likely to cause harm when:

1. The body has no reserves to draw on — wasting disease, significant underweight, malnutrition
2. A medication or medical condition creates dangerous interactions with blood sugar, fluid balance, or electrolyte levels
3. The person has a psychological relationship with food that makes restriction harmful rather than neutral
4. The fast is broken too aggressively or without adequate hydration

In nearly every case, the risk is not the fast itself. It is the context around it.

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Frequently Asked Questions

Can people with heart conditions do intermittent fasting?

Most people with stable, well-managed heart conditions can do intermittent fasting, but fasting affects blood pressure, electrolytes, and in some cases heart rhythm — factors that require physician review first. People with severe or unstable cardiac conditions, or those on cardiac medications, should get explicit medical clearance before starting.

Is it safe to fast with hypothyroidism?

Mild to moderate hypothyroidism is generally not a contraindication to shorter intermittent fasting windows. However, most thyroid medications (including levothyroxine) should be taken on an empty stomach, and the timing relative to the eating window matters. Very long fasts can temporarily suppress T3. Women with thyroid conditions should start at 12–13 hours and monitor energy, mood, and hair loss.

Can elderly people fast safely?

Healthy, non-frail older adults can generally fast safely. Shorter windows (12–14 hours) are typically more appropriate than very long or frequent fasts. The key risks are muscle loss and electrolyte imbalance. Protein should be prioritised when the eating window opens, and resistance exercise should be maintained.

Why is fear dangerous during fasting?

Sinclair observed in 1911 that psychological terror could produce real physical symptoms during a fast — and modern endocrinology explains the mechanism. Extreme fear activates the cortisol and adrenaline response, which affects heart rate, blood pressure, and blood sugar. Starting with shorter, gradual windows and understanding exactly what to expect during a fast reduces this risk significantly.

What are signs that fasting is not right for someone at a given time?

Signs that suggest fasting may not be appropriate include: persistent dizziness or fainting, heart palpitations, extreme cold intolerance, worsening anxiety, hair loss that accelerates over weeks, loss of menstrual period in women, or blood sugar readings outside safe ranges. These are signals to stop, reassess, and consult a healthcare provider before continuing.

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Sinclair, U. (1911). The Fasting Cure. Mitchell Kennerley.

This article draws on historical research from 1911 and is for informational purposes only — not medical advice. Always consult a qualified healthcare professional before making any dietary changes.