Intermittent fasting has gained a massive following over the last few years, and it’s easy to see why so many people swear by it.
Plenty of studies back up the hype, linking time-restricted eating to steady weight loss, better blood sugar control, and lower risks of long-term health problems. Skipping breakfast or shrinking your daily eating window sounds straightforward on paper, but jumping straight into it isn’t always smooth sailing.
From raging brain fog and sudden energy crashes to mood swings that make you snap at everyone in the house, adjusting to long stretches without food can hit your body like a ton of bricks. If you’re thinking about giving it a go, knowing the common pitfalls and side effects before you clear your plate will save you from an absolute nightmare of a first week.
Hunger tends to hit hardest early on.
Cutting your eating window down, or going long stretches without food, understandably makes you hungrier. One study involving people restricting themselves to a couple of low-calorie days a week found they reported higher hunger scores than people on a steady, continuous calorie-restricted diet.
The good news is that this tends to ease off. Research following people through fasting periods of several days to a few weeks found hunger was mostly an issue in the first few days, settling down as the body adjusted to a new rhythm.
Headaches tend to hit during the first few days.
Headaches are a fairly common complaint when starting out with any kind of fasting, typically showing up early before tapering off. A review of studies covering intermittent fasting found several participants reporting mild headaches during their fasting windows.
These “fasting headaches” tend to sit at the front of the head and stay mild to moderate in intensity. People who already get headaches regularly seem more prone to them during fasting too, and researchers suspect low blood sugar and caffeine withdrawal both play a part.
Digestive upset is a common side effect, at least at the beginning.
Indigestion, nausea, bloating, and diarrhoea can all crop up when you change how and when you’re eating. Cutting back on food volume can throw off digestion, and the broader change in diet that often comes with fasting can unsettle things further for some people. Leaning on fibre-rich, nutrient-dense foods during eating windows tends to help settle this down.
Irritability and mood swings are inevitable for many.
Low blood sugar during fasting periods can leave people feeling irritable, anxious, or struggling to concentrate. One study of women found participants noticeably more irritable during an 18-hour fast compared with a non-fasting day. Interestingly, the same participants also reported a stronger sense of achievement and self-control by the end of the fast than they had at the start, suggesting the mood dip isn’t necessarily the whole story.
Fatigue and low energy are common.
Feeling tired and a bit flat is a common early complaint, usually down to lower blood sugar leaving you feeling weak, plus any knock-on effect on sleep that then carries into the next day. It’s one of the more expected side effects when you’re asking your body to run on less fuel than usual. That said, some research has found the opposite over time, with fatigue actually easing the longer someone sticks with a fasting routine, once the body’s had a chance to adjust.
Bad breath can and often does crop up.
This is an unglamorous but unfortunately common side effect. Fasting pushes your body toward burning fat for fuel, and acetone, a by-product of that process, builds up in the blood and breath as a result. Dehydration, another common fasting side effect, doesn’t help either, since a dry mouth is a well-known contributor to bad breath.
Disrupted sleep can be intense.
Trouble falling or staying asleep is often cited as one of the more common fasting side effects of intermittent fasting. One study tracking people through fasts lasting several days to a few weeks found 15% reported sleep disturbances, making it the most frequently reported issue in that particular group.
Early fasting days can involve the body releasing more salt and water through urine than usual, which can tip into mild dehydration and lower salt levels, both of which can affect sleep. That said, the evidence isn’t entirely consistent. A separate study following people on an alternate-day fasting and low-carb combination for six months found no real effect on sleep quality, duration, or insomnia severity.
Dehydration is an important one to watch out for.
In the early days of fasting, the body naturally releases more water and salt through urine, a process sometimes called natural diuresis. If those fluids and electrolytes aren’t replaced, dehydration can follow.
It’s also easy to simply forget to drink enough while adjusting to a new eating pattern, particularly in the first week or two. Keeping water intake up throughout the day and checking urine colour, aiming for a pale straw shade rather than anything dark, is a simple way to stay on top of it.
Malnutrition is a risk if intermittent fasting isn’t done correctly.
Most people manage to meet their calorie and nutrient needs comfortably on a well-planned fasting routine. The risk comes with very long fasting stretches, or restricting calories to an extreme degree without replenishing nutrients afterwards.
Getting this wrong over an extended period can tip into malnutrition alongside other health complications. Eating a varied, nutrient-dense diet during eating windows matters just as much as the fasting itself, and working with a healthcare professional experienced in fasting can help make sure calorie and nutrient targets are actually being met.
Who should think twice about fasting?
Intermittent fasting isn’t right for everyone, and certain groups face a higher risk of complications. This generally includes anyone pregnant or breastfeeding, children and teenagers, older adults dealing with frailty, anyone with a weakened immune system, anyone with a current or past eating disorder, people living with dementia, and anyone with a history of traumatic brain injury or post-concussive syndrome.
This isn’t an exhaustive list, and there are exceptions, fasting has been used clinically to help manage epilepsy in children, for instance. Anyone with an existing health condition, or currently taking medication, should talk it through with a GP or other healthcare professional before starting, since individual risk varies considerably from person to person.
When to call it quits
If side effects like extreme hunger, nausea, irritability, headaches, fatigue, or faintness stick around rather than easing off after the first week or so, that’s usually a sign the approach isn’t working for your particular body. There’s no obligation to push through a fasting routine that’s making day-to-day life miserable.
It’s also important to remember that fasting is just one route to better health, not the only one. A balanced diet, solid sleep, regular movement and decent stress management all move the needle too, and matter just as much, if not more, for overall wellbeing.
