Carbohydrate isn’t something the body must get from food the way it must get vitamins or protein, so a true deficiency is almost unheard of in healthy people. Cutting carbs hard still produces real signs, mostly in the first days and weeks, and they come largely from the body shedding water and salt rather than from a lack of sugar. The bigger risks build slowly and quietly, from what a very low-carb diet leaves out and what it adds, so feeling fine isn’t the whole picture.
Why Your Body Can Run Without Eating Carbs
The official lists of nutrients humans must eat include water, protein building blocks, two fats, vitamins and minerals, but no carbohydrate.1The American Journal of Clinical Nutrition. Dietary carbohydrate: a reasonable nutrient requirement? In the general population the body can make all the carbohydrate it needs, with no signs of deficiency when none is eaten.2Nutrition Research Reviews. Application of nutrient essentiality criteria to dietary carbohydrates The liver’s stored sugar runs out after roughly 30 hours without food, and from then on it builds new glucose from other materials.3PubMed Central. Regulation of glucose metabolism from a liver-centric perspective A few rare inherited metabolic disorders do make dietary carbohydrate necessary, and people with them need it in food.4Nutrition Research Reviews. Application of nutrient essentiality criteria to dietary carbohydrates
As fat becomes the main fuel, the liver turns some of it into ketones. Once blood ketones rise above normal, the brain takes them up and burns them for energy.5PubMed Central. The Implication of Physiological Ketosis on The Cognitive Brain: A Narrative Review On a very low-carb diet, blood ketones can reach about 1.0 to 1.5 mmol/L within two weeks while fasting blood sugar stays normal.6PubMed Central. Very‐low‐carbohydrate diet enhances human T‐cell immunity through immunometabolic reprogramming That is a mild, controlled level, nothing like the very high levels of diabetic ketoacidosis, a dangerous emergency seen mainly in type 1 diabetes when insulin is missing. A healthy person eating low-carb does not slide into it from diet alone. People taking insulin or certain diabetes drugs should talk to their doctor before cutting carbs.
How much carbohydrate a person actually needs isn’t precisely known, and it depends on how much fat and protein they eat.7EFSA Journal. Scientific Opinion on Dietary Reference Values for carbohydrates and dietary fibre “Not essential” also means the body can survive without dietary carbs, not that it thrives on none. Carb-containing foods bring fibre, vitamins and minerals that matter in their own right, so the real question is what replaces the carbs and what gets lost with them.
Early Signs After Cutting Carbs
People who sharply cut carbs often report a cluster of symptoms nicknamed “keto flu.”8PubMed Central. Consumer Reports of “Keto Flu” Associated With the Ketogenic Diet Compared with their starting point, adults on carb-restricted diets had small rises in headache, constipation, diarrhoea, bad breath, muscle cramps, muscle weakness and light-headedness.9PubMed. Effects of differing levels of carbohydrate restriction on mood achievement of nutritional ketosis, and symptoms of carbohydrate withdrawal in healthy adults: A randomized clinical trial The same people had less bloating and fewer cravings for sugar and starch.10PubMed. Effects of differing levels of carbohydrate restriction on mood achievement of nutritional ketosis, and symptoms of carbohydrate withdrawal in healthy adults: A randomized clinical trial Symptoms were strongest on the strictest, ketogenic level of restriction and mildest on moderate restriction.11PubMed. Effects of differing levels of carbohydrate restriction on mood achievement of nutritional ketosis, and symptoms of carbohydrate withdrawal in healthy adults: A randomized clinical trial
Much of this comes from salt and water loss. Insulin normally tells the kidneys to hold on to sodium. When carbs and insulin drop, the kidneys release extra sodium, potassium and water.12PubMed Central. Symptoms during initiation of a ketogenic diet: a scoping review of occurrence rates, mechanisms and relief strategies That means most of the fast weight loss in the first days is water, not fat.13PubMed Central. Symptoms during initiation of a ketogenic diet: a scoping review of occurrence rates, mechanisms and relief strategies This salt loss is heaviest in days 1 to 4 and settles down after about 14 days.14PubMed Central. Symptoms during initiation of a ketogenic diet: a scoping review of occurrence rates, mechanisms and relief strategies The mild drop in fluid volume can cause dizziness, light-headedness on standing, fainting, fatigue, constipation and headache.15PubMed Central. Symptoms during initiation of a ketogenic diet: a scoping review of occurrence rates, mechanisms and relief strategies
Days 1 to 4 are often the hardest, with headache and tiredness. Standing up quickly may bring a head rush. Getting up slowly, sitting a moment on the edge of the bed, and drinking fluid with some salt in the morning are reasonable ways to blunt it. Constipation can appear as fibre and fluid drop. Most people feel better by the second week as the salt loss settles.
Some signs are not keto flu. Fainting, chest pain or a racing, irregular heartbeat need medical attention, not home remedies. Severe vomiting or confusion is not a normal adjustment symptom. People on blood pressure or diabetes medicines may feel stronger dizziness and should call their doctor.
Mood is less clear. Carbohydrate helps raise brain serotonin, a chemical tied to mood, by letting more tryptophan reach the brain.16PubMed Central. Role of Dietary Carbohydrates in Cognitive Function: A Review But even a small amount of protein in a meal blocks this effect, so it rarely happens with normal mixed meals.17PubMed. Carbohydrate ingestion, blood glucose and mood Any low-mood link from cutting carbs is therefore uncertain rather than proven.
How to Ease Keto Flu and How Long It Lasts
The remedies people use most focus on drinking enough and replacing electrolytes.18PubMed Central. Consumer Reports of “Keto Flu” Associated With the Ketogenic Diet Drink water steadily and salt your food or have broth, unless a doctor has limited your salt. Eat potassium-rich low-carb foods like leafy greens and avocado, and keep up fibre from vegetables and nuts to ease constipation. Go easy on hard exercise in the first week.
The salt loss stops promptly when carbohydrate is eaten again.19PubMed Central. Symptoms during initiation of a ketogenic diet: a scoping review of occurrence rates, mechanisms and relief strategies So adding some carbs back is a fast fix if symptoms are too much, and they can ease within a day or so. A moderate step back, such as adding a portion of fruit, beans or oats, is a way to find a level you can tolerate.
If symptoms last well past two weeks, the cause may be something else. Ongoing dizziness on blood pressure drugs may mean the dose needs review, and a doctor can check blood salts if cramps or weakness persist.
Vitamins and Minerals You May Be Missing
Cutting out fruit, grains and starchy vegetables removes much of the diet’s fibre, vitamins and minerals.20PubMed Central. Ketogenic Diets and Chronic Disease: Weighing the Benefits Against the Risks Intakes of thiamin, folic acid (folate), magnesium, calcium, iron and iodine fall by 10% to 70% on carb-restricted diets.21PubMed. Impacts of carbohydrate-restricted diets on micronutrient intakes and status: A systematic review Low-carb diets are often low in vitamin A, vitamin E, vitamin B6 and potassium too.22PubMed Central. Ketogenic Diets and Chronic Disease: Weighing the Benefits Against the Risks In one carb-restricted diet, moderate iodine deficiency rose from 15% to 73% of people over six months.23PubMed. Impacts of carbohydrate-restricted diets on micronutrient intakes and status: A systematic review Even a strict keto diet built from nutrient-rich foods can fall short on vitamin K and most water-soluble vitamins.24PubMed Central. Ketogenic Diets and Chronic Disease: Weighing the Benefits Against the Risks Without a multivitamin, people on low-carb diets risk outright deficiencies.25PubMed Central. Ketogenic Diets and Chronic Disease: Weighing the Benefits Against the Risks
Fruit, whole grains, beans and starchy vegetables are major sources of folate, magnesium, potassium and fibre. Fortified foods such as breakfast cereals and breads supply added folate, thiamin and iron for many people, and these go when grains go. Dairy, if cut back to save carbs, takes calcium and iodine with it.
Low iron can bring tiredness and breathlessness. Low magnesium and potassium can add to cramps. Low thiamin can cause fatigue and, if severe, nerve problems. Many shortfalls cause no symptoms for months, so feeling fine isn’t proof of getting enough.
To fill the gaps, lean on leafy greens, nuts, seeds, eggs, fish and dairy. Ask about a multivitamin with iodine, folate and thiamin, and use iodised salt if you eat little dairy or seafood. A doctor can check iron and vitamin levels after a few months.
Cholesterol Changes to Watch
LDL cholesterol, listed on blood test reports as LDL-C, is the “bad” cholesterol that builds up in arteries over time.26PubMed Central. Ketogenic Diets and Low‐Density Lipoprotein Cholesterol in Adults With Normal Weight: An Emerging Clinical Challenge In short trials of healthy normal-weight adults on keto diets, average LDL rose by 18 to 70 mg/dL.27PubMed Central. Ketogenic Diets and Low‐Density Lipoprotein Cholesterol in Adults With Normal Weight: An Emerging Clinical Challenge In young normal-weight women eating a keto diet rich in saturated fat, LDL rose in every participant within four weeks.28PubMed Central. A Ketogenic Low-Carbohydrate High-Fat Diet Increases LDL Cholesterol in Healthy, Young, Normal-Weight Women: A Randomized Controlled Feeding Trial Among people eating low-carb, high-fat diets, severely high cholesterol was more common than on a standard diet, about 7 in 100 versus about 6 in 100.29PubMed Central. Association of a Low-Carbohydrate High-Fat Diet With Plasma Lipid Levels and Cardiovascular Risk The idea that this rise is harmless in otherwise healthy people has no supporting evidence.30PubMed Central. Ketogenic Diets and Low‐Density Lipoprotein Cholesterol in Adults With Normal Weight: An Emerging Clinical Challenge LDL raises heart disease risk steadily the higher it is and the longer it stays up.31PubMed Central. Ketogenic Diets and Low‐Density Lipoprotein Cholesterol in Adults With Normal Weight: An Emerging Clinical Challenge
The largest jumps have been reported in people already at normal weight, and weight loss in heavier people can offset part of the rise. Rises vary widely, so personal testing matters more than averages.
Have a lipid test before starting and again about two to three months in. A standard panel reports total cholesterol, LDL, HDL and triglycerides, and the LDL number is often calculated rather than measured directly. Non-HDL cholesterol is total cholesterol minus HDL and counts all the artery-clogging particles together. ApoB counts the number of these particles and can give a fuller picture when LDL looks misleading, though not every clinic offers it. Ask for LDL, non-HDL and ApoB if available. Swapping butter and fatty meat for olive oil, nuts and fish may soften the LDL rise.
Kidney Stones, Gout and Bones
Six weeks on a low-carb, high-protein diet puts a heavy acid load on the kidneys and raises stone-forming risk.32PubMed. Effect of low-carbohydrate high-protein diets on acid-base balance, stone-forming propensity, and calcium metabolism On that diet, the urine’s tendency to form uric acid stones more than doubled.33PubMed. Effect of low-carbohydrate high-protein diets on acid-base balance, stone-forming propensity, and calcium metabolism Meat, fish and eggs add acid to the body, and the kidneys must clear it into the urine. That load can lower urine citrate, which normally helps stop stones forming, and can raise calcium in the urine.
People eating the most low-carb, animal-heavy diets also had higher odds of high uric acid, which can lead to gout.34PubMed Central. Associations of low-carbohydrate diets patterns with the risk of hyperuricemia: a national representative cross-sectional study in Korea Low-carb diets built on plant foods showed no such link.35PubMed Central. Associations of low-carbohydrate diets patterns with the risk of hyperuricemia: a national representative cross-sectional study in Korea Fruits and vegetables likely help protect against stones,36PubMed Central. The Relationship between Modern Fad Diets and Kidney Stone Disease: A Systematic Review of Literature which fits with plant foods making urine less acidic.
To lower the risk, drink plenty of water so urine stays pale, and don’t let protein climb too high, since low-carb isn’t the same as high-meat. Include plant foods such as greens, nuts and seeds. Anyone with past stones or gout should check with a doctor first.
In adults, keto and low-carb diets have not shown clear bone loss in studies so far.37PubMed Central. Effects of the ketogenic diet on bone health: A systematic review Over a year, bone density fell slightly on both low-carb and low-fat weight-loss diets alike, likely from the weight loss itself.38PubMed. Long-term effects of a very-low-carbohydrate weight-loss diet and an isocaloric low-fat diet on bone health in obese adults People who skipped calcium supplements on keto lost some bone mineral,39PubMed Central. Effects of the ketogenic diet on bone health: A systematic review and long-term data in adults are thin.40PubMed Central. Ketogenic diet and β-hydroxybutyrate in osteoporosis: current progress and controversy
Extra Risks in Pregnancy and Childhood
Women eating restricted-carb diets got less than half the folic acid of other women, partly because fortified grains are cut out.41PubMed. Low carbohydrate diets may increase risk of neural tube defects They were somewhat more likely to have a baby with a neural tube defect, a birth defect of the spine or brain.42PubMed. Low carbohydrate diets may increase risk of neural tube defects In an earlier study, mothers of affected babies were twice as likely to have eaten very low-carb diets around conception.43PubMed. Women’s periconceptional lowered carbohydrate intake and NTD-affected pregnancy risk in the era of prefortification with folic acid Anyone who could become pregnant should take a folic acid supplement if eating low-carb.
The neural tube closes very early in pregnancy, often before a woman knows she is pregnant, so folic acid needs to be in the body before conception. If you’re planning a pregnancy, start folic acid before trying and tell your doctor or midwife about your diet early.
Children on medical keto diets for epilepsy can grow more slowly in height.44PubMed Central. Long-term impact of the ketogenic diet on growth and resting energy expenditure in children with intractable epilepsy In one group, height growth slowed in 12 of 15 children over 15 months.45PubMed Central. Long-term impact of the ketogenic diet on growth and resting energy expenditure in children with intractable epilepsy In another, only about 9% showed slowed growth over two years.46PubMed Central. A Prospective Study on Changes in Nutritional Status and Growth Following Two Years of Ketogenic Diet (KD) Therapy in Children with Refractory Epilepsy Children should only follow keto under medical supervision with growth checks.47PubMed Central. A Prospective Study on Changes in Nutritional Status and Growth Following Two Years of Ketogenic Diet (KD) Therapy in Children with Refractory Epilepsy Teenagers with type 1 diabetes on low-carb diets fell short on iron, calcium, thiamin and folate.48PubMed Central. The Impact of a Low-Carbohydrate Diet on Micronutrient Intake and Status in Adolescents with Type 1 Diabetes
Hormones and Exercise Performance
The stress hormone cortisol tends to rise in the early weeks of a keto diet, and it usually settles by around 6 to 8 weeks.49PubMed Central. Ketogenic Diet and Thyroid Function: A Delicate Metabolic Balancing Act Cortisol helps the body raise blood sugar, so an early rise fits with adjusting to little carbohydrate and is not a sign of lasting harm.
Active thyroid hormone T3 fell on a keto diet in healthy people, while T4 rose.50PubMed Central. Could the ketogenic diet induce a shift in thyroid function and support a metabolic advantage in healthy participants? A pilot randomized-controlled-crossover trial Thyroid blood tests taken on keto may therefore look different, so it’s worth telling your doctor about your diet.
Low-carb diets may help keep strength but give no consistent benefit for endurance or sprint performance.51PubMed Central. A Systematic Review of the Effects of Low-Carbohydrate Diet on Athletic Physical Performance Parameters Elite race walkers on a low-carb diet for three and a half weeks performed worse in real races.52PubMed Central. Does a low-carbohydrate diet impede endurance sports performance? Debate Consensus Over four weeks, though, regular exercisers doing interval training showed no drop in endurance.53PubMed Central. Effects of a 4-Week Very Low-Carbohydrate Diet on High-Intensity Interval Training Responses