Many people report keto-flu symptoms during the first days to weeks.1 Across randomized trials, keto raised low-density lipoprotein cholesterol by about 8 mg/dL on average, and kidney stones are documented in some groups.2-4 Anyone taking insulin or a sulfonylurea needs medication adjustment with a practitioner before starting.5,8 A baseline assessment and planned follow-up give you a safer way to evaluate how your body responds.
Keto Flu in the First Weeks
Keto flu describes a group of temporary symptoms reported during the first few weeks of a ketogenic diet. A 2020 analysis reviewed 448 online forum posts from 300 users. Reported symptoms included headache, fatigue, nausea, dizziness, brain fog, digestive discomfort, decreased energy, faintness, and heartbeat changes.1
Symptom reports peaked during the first week and declined after four weeks. Eight users reported that their symptoms resolved between days 3 and 30, with a median of 4.5 days.1 That timeline comes from a very small self-report group. It cannot predict how long your symptoms will last.
The study did not establish how many people develop keto flu, what causes it, or which remedies work. Do not assume every new symptom comes from ketosis. Tell your physician when the symptoms began, whether they are improving, and which medications you take.
Note the date you started the keto meal plan so you can match symptoms to it. You can also review how the keto diet produces ketosis before deciding whether the plan fits your goals.
Tracking LDL Cholesterol on Keto
Keto can raise low-density lipoprotein cholesterol, usually shortened to LDL cholesterol, although the size of the change varies. Two 2026 meta-analyses of randomized controlled trials found average LDL increases of 8.49 mg/dL and 8.22 mg/dL compared with control diets.2,3
The same analyses found lower triglycerides and higher high-density lipoprotein cholesterol. However, the certainty of the LDL finding was rated low in one meta-analysis. Neither analysis established whether keto-related LDL increases lead to heart attacks or strokes because long-term cardiovascular outcome data were unavailable.2,3
Small trials in lean, young adults found larger responses. In one four-week trial, all 17 women who completed the study had higher LDL cholesterol, with an average treatment effect of 1.82 mmol/L.6 A three-week trial involving 30 young adults found a 44% LDL increase compared with controls. Individual changes ranged from 5% to 107%.7
These results show why a pooled average cannot predict your response. Get a baseline lipid panel before starting, then have your physician repeat it after you have been on the diet long enough to assess a trend. Your wider cardiovascular profile also matters when evaluating heart disease risk in men.
Kidney Stone Prevention on Keto
Kidney stones are a documented complication in therapeutic ketogenic-diet studies. A meta-analysis of 36 studies involving 2,795 people found a pooled incidence of 5.9% over an average follow-up of 3.7 years.4
Most participants were children using ketogenic diets to manage epilepsy. The adult estimate was 7.9%, but its 95% confidence interval ranged from 2.8% to 20.1%. That wide range prevents the estimate from being applied directly to a healthy adult following keto for weight management. Uric acid stones were the most common type, followed by calcium-based stones.4
Tell your physician about previous kidney stones or kidney disease before starting. Your medical history determines whether kidney monitoring should be part of the plan.
Managing Digestion on Keto
Constipation, nausea, vomiting, and diarrhea have all been reported during ketogenic diets. A 2026 review found wide ranges across mixed study populations. Adult constipation ranged from 1% to 68%, while diarrhea ranged from 2% to 23%.8
Those ranges combine different populations and study designs, including therapeutic diets used for epilepsy. They do not provide one reliable rate for a healthy adult starting keto.
Track when digestive symptoms begin and whether they persist. The fiber and net-carb figures by food can help you identify fiber-containing vegetables that fit the diet. A physician or dietitian can then assess your food intake against your symptoms and health history.
A Medication Review Before Starting Keto
Very-low-carbohydrate eating can lower blood glucose quickly enough to require medication changes. The American Diabetes Association consensus report states that practitioner involvement is necessary at the start to reduce insulin and other medications that can cause low blood sugar.5
A 2026 review also identifies insulin and sulfonylureas as medications that may need to be reduced or discontinued when keto lowers blood glucose.8 Your prescriber sets those changes. Do not adjust prescription medication on your own.
If you use insulin, a sulfonylurea, or another glucose-lowering medication, arrange a medication review before changing your diet. A fasting insulin test may add context when your physician is assessing insulin resistance, but it does not replace glucose monitoring or medication management.
Who Should Check With a Physician First
Certain medical conditions and medications require clinical review before you start keto. A 2026 narrative review lists rare absolute contraindications involving selected disorders of pyruvate carboxylase activity, carnitine transport or use, fatty acid oxidation, and porphyria.8
The same review lists several relative contraindications:
- Acute pancreatitis
- Advanced liver or kidney disease
- Familial hypercholesterolemia
- Conditions that may worsen with keto-related metabolic changes
- Concurrent propofol use
Additional caution applies if you have diabetes treated with glucose-lowering medication, treated hypertension, gallbladder disease, electrolyte disturbances, or a cardiac arrhythmia. Pregnancy, lactation, low body weight, intense physical activity, major psychosocial stress, and postoperative recovery also warrant physician involvement.8
The American Diabetes Association report found insufficient evidence to recommend very-low-carbohydrate plans for people with chronic kidney disease or disordered eating patterns, as well as women who are pregnant.5 These findings do not establish that keto is unsafe for healthy adults in general. They show why your medical history and medications need to guide the decision.
The Opt Take on Keto Side Effects
Your physician should interpret keto side effects against your symptoms, medications, baseline labs, and goals. One LDL result provides limited context without a baseline. A lipid trend, glucose markers, symptoms, and medication changes create a more useful pattern.
Opt Health starts with that pattern. A physician reads your labs, builds a personalized plan, and adjusts it as your numbers and goals change. If you are evaluating keto for men, your lipid, insulin, and hormone results can help determine whether the diet fits your broader health plan.
Get started: Begin with Opt Health, where a physician reads your labs, builds your plan, and adjusts it over the loop.
Frequently Asked Questions
Self-reported keto-flu symptoms include headache, fatigue, nausea, dizziness, brain fog, digestive discomfort, decreased energy, faintness, and heartbeat changes. Reports peaked during the first week in a 2020 online-forum analysis, but the study did not establish how often keto flu occurs.1
In a small group of self-reports, eight users described symptom resolution between days 3 and 30. The median was 4.5 days, with an interquartile range of 3 to 15 days.1 The timeline came from eight users, so tell your physician if your symptoms persist or worsen.
Keto raises LDL cholesterol by about 8 mg/dL on average across randomized trials, although individual responses vary.2,3 Small studies in lean, young adults found much larger increases.6,7 Long-term cardiovascular outcome data are lacking, so use baseline and follow-up lipid panels to measure your response.
Kidney stones have been documented in people following ketogenic diets. The pooled estimate was 5.9%, but most participants were children using therapeutic ketogenic diets for epilepsy.4 The adult estimate had a wide confidence interval, so it does not define the risk for a healthy adult following keto for weight management. Tell your physician about any past stones so kidney monitoring can be built into your plan.
Keto is inappropriate for people with certain disorders of fatty acid oxidation, carnitine transport or use, pyruvate carboxylase activity, or porphyria.8 Acute pancreatitis, advanced liver or kidney disease, familial hypercholesterolemia, pregnancy, disordered eating, and glucose-lowering medication use require physician review before starting.5,8
The cited research does not establish a separate group of keto side effects for men. If you are considering keto, monitor your symptoms and lipid response. Your physician may also interpret testosterone and insulin markers as part of a wider assessment, as explained in keto diet guidance for men.
The first weeks of keto are easier to read with a baseline. An Opt Health membership pairs 55+ biomarkers with a one-on-one physician consultation and repeat testing every 3 to 4 months, so changes in your lipids are caught and acted on.
References
- Bostock ECS, et al. Consumer reports of “keto flu” associated with the ketogenic diet. Front Nutr. 2020;7:20. doi:10.3389/fnut.2020.00020. PubMed
- Chang C, et al. The impact of the ketogenic diet on the lipid profile in adults: a comprehensive review and meta-regression analysis of randomized controlled trials. Endocr Pract. 2026;32(5):819-828. doi:10.1016/j.eprac.2026.01.009. PubMed
- Zhao J, et al. Ketogenic diet-induced changes in adult lipid metabolism: a comprehensive systematic review and meta-regression of randomized controlled trials. BMC Cardiovasc Disord. 2026;26(1):520. doi:10.1186/s12872-026-05799-5. PubMed
- Acharya P, et al. Incidence and characteristics of kidney stones in patients on ketogenic diet: a systematic review and meta-analysis. Diseases. 2021;9(2):39. doi:10.3390/diseases9020039. PubMed
- Evert AB, et al. Nutrition therapy for adults with diabetes or prediabetes: a consensus report. Diabetes Care. 2019;42(5):731-754. doi:10.2337/dci19-0014. PubMed
- Burén J, et al. A ketogenic low-carbohydrate high-fat diet increases LDL cholesterol in healthy, young, normal-weight women: a randomized controlled feeding trial. Nutrients. 2021;13(3):814. doi:10.3390/nu13030814. PubMed
- Retterstøl K, et al. Effect of low carbohydrate high fat diet on LDL cholesterol and gene expression in normal-weight, young adults: a randomized controlled study. Atherosclerosis. 2018;279:52-61. doi:10.1016/j.atherosclerosis.2018.10.013. PubMed
- Dyńka D, et al. The ketogenic diet is not for everyone: contraindications, side effects, and drug interactions. Ann Med. 2026;58(1):2603016. doi:10.1080/07853890.2025.2603016. PubMed
This content is for informational purposes and does not replace evaluation, diagnosis, or treatment by a qualified medical professional.
This article is part of our guide to weight loss. See the doctor-guided weight loss program at Opt Health.
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