A doctor has explained how the body can respond after a ‘prolonged fast’.
Approaches like intermittent fasting have become popular for both health and weight-management goals, but medical professionals continue to stress that fasting of any kind should be handled carefully and with the right precautions.
According to Cleveland Clinic, some research suggests intermittent fasting — where a person spends longer periods without eating before returning to normal meals — can support weight loss and may help improve some markers linked to heart and metabolic health. Even so, experts say it is not suitable for everyone and can sometimes bring unwanted side effects, while the long-term evidence is still mixed.
Although shorter fasting plans are often the focus of discussion, going without food for 36 hours is a much longer stretch and may have a different effect on the body. Johns Hopkins Medicine notes that longer fasts such as 24-, 36-, 48- and 72-hour fasting periods are not necessarily better for you and may be dangerous for some people.
“Prolonged fasting can have a significant impact on our bodies. After 36 hours, the body may have used much of its available glucose, meaning it will then rely on stored fat for energy,” Dr Donald Grant, GP and Senior Clinical Advisor at The Independent Pharmacy, says.
That shift happens because, after many hours without food, the body starts using up stored glycogen and increasingly turns to fat stores for fuel. During this time, some people may also produce more ketones as an alternative energy source.
He warns that blood sugar levels may also fall during that period, which can lead to headaches, weakness, tiredness, irritability, and trouble concentrating.
Those symptoms are consistent with warnings from major health experts. Harvard Health says intermittent fasting can bring side effects including headaches, lethargy, crankiness and constipation, particularly as the fasting window gets longer or if someone is not well prepared.
Dr Grant also notes that many fasting-related symptoms are connected to dehydration, making it especially important to keep fluid intake up.
Dehydration can make symptoms worse and may leave people feeling dizzy, lightheaded or nauseous. This is one reason clinicians often warn against confusing fasting with simply going without both food and fluids, especially during longer fasts.
Dr Grant says those ‘with pre-existing medical conditions such as diabetes, people with a history of eating disorders or pregnant and breastfeeding women,’ should not attempt these type of fasts without advice first.
People taking regular medication should also be cautious, as meal timing can affect how some drugs work and how well they are tolerated. For people with diabetes in particular, fasting can raise the risk of hypoglycaemia, dehydration and, in some cases, dangerous complications if medicines are not adjusted properly.
“Extended fasting places stress on the body, so anyone with a long-term health condition, those taking regular medication, or women who are pregnant or breastfeeding should always seek medical advice before attempting this type of fast,” he says.
“People who are fasting should stop immediately if they experience confusion, extreme dehydration, or if they become severely weak, as these can be signs that the body is not coping well,” Dr Grant adds, before warning that ‘breaking the fast safely’ is also vital.
Health experts say people ending a fast should avoid jumping straight into a large, heavy meal. However tempting foods like burgers and chips may seem after a long period without eating, they may not be the easiest option for the body to handle.
To avoid symptoms such as nausea, vomiting and digestive discomfort Dr Grant advises that when breaking their fast, people should opt for a ‘balanced meal and continuing to prioritise hydration.’
Many dietitians recommend starting with a moderate portion that includes protein, fibre and easily tolerated foods, rather than overeating immediately. Eating too quickly after a long fast can leave people with bloating, stomach pain or digestive upset.
While fasting continues to attract interest, specialists generally say longer is not always better. For many people, structured eating plans with shorter fasting windows are more practical, easier to sustain and less likely to trigger side effects than a 36-hour fast.
If anyone is unsure whether fasting is appropriate for them, they should speak to a doctor before trying it.