Most of the side effects people notice when they start fasting — headaches, cramps, brain fog, low energy, reduced libido — come from two things: the sodium and fluid your body sheds when insulin drops, and the time it takes to switch from running on glucose to running on fat. Both usually settle within days to a few weeks. Neither means fasting is dangerous, but some of them can be reduced, and fasting isn’t appropriate for everyone.
What counts as fasting?
Worth defining, because these three terms get used interchangeably and they aren’t the same thing. The differences matter biochemically.
- Fasting — no food at all. No MCT oil, no ketone supplements, nothing containing carbohydrate, fat or protein. Water, black coffee and black tea are generally accepted.
- Time-restricted eating (TRE) — food is restricted to a set window, but calories aren’t deliberately reduced. Eating within an 8-hour window and fasting the other 16 is the common example.
- Intermittent fasting — similar to TRE, but usually restricts calories as well.
The distinction matters because a lot of research described as “fasting” is actually one of the other two, and the physiological effects differ.
Who shouldn’t fast?
This comes first rather than last, because it’s the part most articles leave out.
Speak to your doctor before fasting if you:
- Take insulin or sulfonylureas for diabetes. This is the most important one. Fasting while on these medications carries a real risk of hypoglycaemia, and doses generally need adjusting rather than the fasting simply being added on top⁵. Some diabetes medications carry much lower risk than others, but that’s a conversation with your prescriber, not something to work out yourself.
- Are pregnant or breastfeeding
- Are underweight, or have been losing weight without intending to
- Are a child or adolescent
- Take blood pressure or heart medications, which can interact with the fluid and electrolyte shifts described below
- Have kidney disease or a history of electrolyte problems
- Are postmenopausal: Gallstone risk rises with age and is higher in women, and fasting adds to that by reducing gallbladder contraction, allowing bile to sit and concentrate. Choline requirements also rise after menopause, because the enzyme that produces phosphatidylcholine internally (PEMT) is estrogen-dependent — and phosphatidylcholine is what keeps cholesterol dissolved in bile. Fasting is not off the table, but shorter fasting windows, adequate choline intake, and avoiding rapid weight loss all matter more in this group.
A note on eating disorders. If you have a history of disordered eating, or you notice fasting becoming something you feel anxious or guilty about breaking, this isn’t the right approach for you. Fasting has been associated with eating disorder behaviours and psychopathology in adolescents and young adults⁶, and prospective research has found that fasting increases the risk of later binge eating and bulimic pathology⁷. That’s not a reason nobody should fast. It is a reason to be honest with yourself about your relationship with food before starting, and to stop if the pattern shifts.
Why do I get headaches when fasting?
Usually sodium and fluid, particularly in the first few days.
Insulin tells the kidneys to hold on to sodium. When you fast or cut carbohydrates sharply, insulin falls, and the kidneys switch from retaining sodium to excreting it — a well-documented effect known as the natriuresis of fasting¹. Sodium loss is greatest in roughly the first four days and stops promptly once carbohydrate is reintroduced². Water follows the sodium out, which is also why the earliest weight loss on any low-carbohydrate approach is mostly fluid.
Adding some salt back is the usual answer, and broth or salted water is a common way to do it. If you have high blood pressure, heart failure or kidney disease, check with your doctor first rather than increasing salt on your own.
Less commonly, changes in blood glucose appear to affect pain sensitivity in some people, producing a fasting headache independent of hydration.
Why do I get cramps when fasting or eating keto?
Same mechanism. Sodium isn’t the only thing lost — potassium excretion rises alongside it, and magnesium is often marginal to begin with². Those three minerals are what muscle and nerve function depend on.
Electrolyte supplements are the practical answer, with one catch worth knowing: most contain glucose, which will break a fast and counts as carbohydrate on a ketogenic diet. Sugar-free versions containing sodium, potassium, magnesium and calcium avoid that, or take them during your eating window.
Why do I feel like I have the flu when I start fasting or keto?
“Keto flu” is a real and well-described phenomenon, and it isn’t an infection. It reflects the combination of electrolyte loss described above and the metabolic switch from using glucose as the primary fuel to using fat and ketones².
Commonly reported symptoms include:
- Stomach pain, diarrhoea or constipation
- Dizziness and nausea
- Muscle soreness and cramping
- Irritability
- Brain fog and poor concentration
- Sugar cravings
It typically appears in the first few days and settles within a week or two. If it lasts longer, worsens, or you develop a fever, see your doctor — that isn’t keto flu.
Fluid, adequate sodium, and not cutting carbohydrate to zero overnight all reduce it. People who are already metabolically flexible tend to experience less of it.
Why does my brain feel foggy?
Your brain runs primarily on glucose when glucose is what’s available. When you stop eating, the body works through circulating glucose, then liver and muscle glycogen, before shifting toward fat and ketones as the main fuel. The foggy period is the gap between those two states.
Most people find this resolves as they become fat-adapted. How long that takes varies, and insulin resistance tends to make it slower. It’s worth setting a realistic expectation here: some people report noticeably better mental clarity once adapted, others simply return to feeling normal. Both are ordinary outcomes.
Why does fasting reduce my libido?
When fuel is limited, the body prioritises. The brain and heart come first, followed by organs like the kidneys, liver and immune system. Reproduction sits at the bottom of that list, because from a physiological standpoint it can wait.
So reduced libido during prolonged fasting or sustained calorie restriction is an expected consequence rather than a malfunction, and it affects both sexes.
If it’s a problem, that’s useful information rather than something to push through. Shortening the fasting window, eating more within it, or fasting less often are all reasonable adjustments. A persistent drop in libido alongside fatigue, poor sleep or cycle changes suggests the approach is more restrictive than your body currently has capacity for.
What about fasting and my thyroid?
Covered separately in Is Fasting Bad For Your Thyroid?
Does exercising while fasted work better?
Exercise depletes glycogen faster, so training in a fasted state does accelerate the shift toward fat as fuel.
Fasting also raises growth hormone substantially⁴, and growth hormone helps preserve lean mass while fat is being mobilised — which is the combination most people are after.
The practical caveat is that fasted training is harder, particularly before you’re fat-adapted, and combining a new fasting protocol with a new training load is how people end up exhausted and abandoning both. Adding one at a time works better.
What should I eat when I break a fast?
Protein and fat tend to be easier than a large carbohydrate load.
The reason connects to the sodium mechanism above: carbohydrate raises insulin, insulin drives sodium retention, and fluid follows. That produces the post-fast puffiness and apparent weight gain people find discouraging. Research on refeeding after fasting shows carbohydrate causes rapid sodium and water retention, while protein acts more slowly².
This isn’t an argument that carbohydrate is bad. It’s about how you re-enter. Something like eggs, fish, avocado, chicken or a salad with both protein and fat is generally more comfortable than a large carbohydrate-heavy meal, and easing in gradually helps more than the specific food choice.
How long does it take to become fat-adapted?
It varies enough that any specific number would be misleading. Insulin resistance tends to slow it. Most people report the initial symptoms settling within one to a few weeks.
Worth saying plainly: if a strict approach doesn’t suit your life, a less strict one still has value. Eating fewer refined carbohydrates, eating within a slightly narrower window, or simply moving more all have benefit independent of whether you ever reach ketosis. The version you can sustain outperforms the stricter version you abandon.
What is fasting actually good for?
Being accurate here matters, because this is where fasting content tends to overreach.
The strongest evidence is in type 2 diabetes and metabolic health, where intermittent fasting improves glycaemic control and supports weight loss, with recent reviews arguing it’s effective enough to warrant a place in clinical guidance⁹. Evidence for type 1 and gestational diabetes is not sufficient to recommend it⁹.
There is reasonable evidence for weight loss, though largely comparable to conventional calorie restriction rather than superior to it, and for improvements in insulin sensitivity independent of weight change⁴.
For autoimmune disease, cardiovascular disease and mitochondrial function, the mechanistic rationale is plausible and early research is interesting, but the human evidence is considerably thinner than the popular claims suggest. Treat those as promising rather than established.
The short version
Most early fasting side effects are explained by sodium and fluid loss, and by the time it takes to switch fuel sources. Both are manageable and both usually pass.
What matters more than the protocol is whether it suits you. Fasting isn’t a moral test and there’s no virtue in pushing through symptoms that are telling you something. If it’s making you feel worse rather than better, adjust it or stop — and if you’re on medication, have that conversation with your doctor before you start rather than after.
Frequently asked questions
What is the difference between fasting, time-restricted eating and intermittent fasting?
Fasting means no food at all. Time-restricted eating limits food to a set window without deliberately cutting calories. Intermittent fasting is similar but usually restricts calories as well.
Why do I get a headache when I fast?
Mainly sodium and fluid loss. When insulin falls, the kidneys switch from retaining sodium to excreting it, and water follows. Replacing some salt usually helps — check with your doctor first if you have high blood pressure or kidney disease.
Is keto flu real?
Yes, though it isn’t an infection. It reflects electrolyte loss combined with the metabolic switch from glucose to fat as the main fuel. It usually appears in the first few days and settles within a week or two.
Can fasting lower your sex drive?
Yes. When fuel is limited the body prioritises vital organs, and reproductive function sits low on that list. Libido usually recovers when intake increases or the fasting window shortens.
Is it safe to fast if I have diabetes?
Not without medical advice. Fasting while taking insulin or sulfonylureas carries a real risk of hypoglycaemia, and medication doses typically need adjusting. Speak to your prescriber before starting.
Should I break a fast with carbohydrates?
Protein and fat are usually more comfortable. Carbohydrate raises insulin, which drives sodium and fluid retention and produces the post-fast puffiness many people notice.
References
- Spark, R. F. et al. (1975). Renin, aldosterone and glucagon in the natriuresis of fasting. New England Journal of Medicine, 292, 1335–1340.
- Symptoms during initiation of a ketogenic diet: a scoping review of occurrence rates, mechanisms and relief strategies. Frontiers in Nutrition (2025). doi:10.3389/fnut.2025.1538266
- Fasting-induced natriuresis and SGLT: a new hypothesis for an old enigma. Frontiers in Endocrinology (2020).
- Insulin resistance reduction, intermittent fasting, and human growth hormone: secondary analysis of a randomized trial. npj Metabolic Health and Disease (2024).
- Strategies for management of intermittent fasting in patients with diabetes. Cleveland Clinic Journal of Medicine (2017), 84(5), 357. doi:10.3949/ccjm.84a.16118
- Ganson, K. T., Cuccolo, K., Hallward, L. & Nagata, J. M. (2022). Intermittent fasting: describing engagement and associations with eating disorder behaviors and psychopathology among Canadian adolescents and young adults. Eating Behaviors, 47, 101681.
- Stice, E., Davis, K., Miller, N. P. & Marti, C. N. (2008). Fasting increases risk for onset of binge eating and bulimic pathology: a 5-year prospective study. Journal of Abnormal Psychology, 117(4), 941–946.
- Ketosis, salt, and water: novel mechanistic insights into diet and mineralocorticoid metabolism. Journal of Clinical Endocrinology & Metabolism (2023). doi:10.1210/clinem/dgad106
- Intermittent fasting to treat diabetes: time to update clinical practice guidelines. Lancet Diabetes & Endocrinology (2026).
