What are the downsides of intermittent fasting?
For most healthy people, intermittent fasting produces similar weight loss to ordinary dietary advice. For vulnerable groups, such as people with eating disorders or serious heart problems, there are indications of specific risks.
Hunger, fatigue and headaches are the most commonly reported complaints with intermittent fasting, especially in the first few weeks. They are mentioned in several overviews of the literature, but precise figures on how often they occur are not available: the underlying studies are too varied and too small to measure this reliably.
That you lose a notable amount of weight from it is less certain than the popularity of IF might suggest. An analysis combining 22 studies with nearly 2,000 participants shows that intermittent fasting leads to an average of only 0.33% more weight loss than ordinary dietary advice. That is too small to matter in practice. On quality of life, IF also scores no differently than standard dietary guidance.
People with an eating disorder, a mood disorder, or a vulnerability to psychosis or mania should proceed with caution. Smaller studies in those groups more often observed a worsening of symptoms: greater preoccupation with food, relapse into disordered eating, increased anxiety, and in rare cases a psychotic or manic episode. In metabolically healthy people with no psychiatric history, modest improvements in low mood and stress have actually been found, but these are based on small groups of participants.
For older adults with heart problems, heart specialists warn that widespread use of IF cannot yet be considered responsible. Possible benefits for blood pressure and blood sugar are weighed against possible risks in people whose hearts and blood vessels are already vulnerable. Concrete risk figures are not available, but the authors advise caution. In addition, there is no data on what happens over the longer term (more than one year): most studies last no more than 12 months.
In laboratory settings it has been shown that prolonged and severe fasting can activate the cells' self-cleaning process so strongly that cells begin to break themselves down too far. Whether this is a relevant risk for people in practice has not been studied. From a psychological and social perspective, long-term feasibility is a separate sticking point: maintaining a strict eating schedule regularly clashes in practice with social situations and everyday obligations.
This answer draws mainly on a Cochrane analysis of 22 studies and on several other overviews of the literature. The researchers themselves are critical of the quality: most analyses were rated as having 'low' or 'very low' certainty, due to risk of bias, conflicting outcomes and small groups of participants. One analysis of side effects included only 60 participants and was labelled 'extremely uncertain'. The evidence on psychiatric risks, long-term effects and cellular damage is qualitative and based on small or non-human studies. Where the evidence is strongest (weight loss and quality of life), it points to no meaningful advantage over ordinary dietary advice.
Does a lithium deficiency increase the risk of dementia?
Research shows that lithium use is associated with lower rates of dementia, and in people with early memory problems there are initial indications that it may slow cognitive decline. Whether this also applies to advanced dementia remains unclear.
Is eating seaweed good for your health?
Eating seaweed regularly or taking it as a supplement has shown measurable benefits in people for blood pressure, blood sugar control, body fat and LDL cholesterol. The effects are modest but consistent, and strongest in people who already have an elevated risk.
Can people soon grow new teeth?
Stem cell research has produced tangible results for repairing damaged tooth tissue, such as in children treated with stem cells from baby teeth. Growing a completely new tooth in people has not yet been achieved.
What does intermittent fasting do to your insulin sensitivity?
Intermittent fasting improves insulin sensitivity in most people, but watch out for possible muscle loss and hormonal effects, especially if you combine it with intensive exercise.
Is intermittent fasting good for longevity?
Intermittent fasting improves health markers such as weight and insulin sensitivity, but whether it also makes you live longer has not yet been proven in humans. It is worth considering, but discuss it with your doctor first if you belong to a risk group.