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Keto for Beginners: What Your First Week Is Really Like

Reviewed · 14 sources · By Anthony Falcone

The short answer: A ketogenic diet cuts carbohydrate to roughly 20–50 g a day or less, low enough that your liver starts turning fat into ketones for fuel. The first week is often rough, with headaches and tiredness the most common complaints, and that usually fades within a few weeks. Keto can make eating less feel easier, but when calories are held equal it does not burn extra fat.1,7,6,5

Free calculator: Protein Calculator: How Much Protein Per Day?

Before you start

  • If you take medicine for diabetes, especially insulin or an SGLT2 inhibitor (drug names ending in -gliflozin), talk to your doctor before cutting carbs. Doctors have reported ketoacidosis, a medical emergency, in people on SGLT2 inhibitors even when their blood sugar looked normal, and they advise extra care when carbohydrate intake drops.13
  • Also see your doctor first if you take blood-pressure medicine, have kidney or heart disease, have been told to limit salt, are pregnant or breastfeeding, or have a history of eating disorders.
  • Heart palpitations, fainting, or feeling unwell for more than a couple of weeks are reasons to stop and get checked, not to push through.
  • 20–50 g carbs a day or less on a typical ketogenic diet1
  • 1 in 3 forum users who described keto flu themselves (101 of 300)7
  • 0.91 kg more weight lost than on low-fat, across 13 trials of a year or more3
  • 3–4 weeks for keto-adaptation to be well under way8

What keto actually is

Normally your body runs largely on glucose from carbohydrate. When carbohydrate stays very low, insulin falls and glucagon rises. Once the liver's glycogen store is run down, the liver turns fatty acids into ketones (beta-hydroxybutyrate, acetoacetate and acetone), which almost every cell in the body, including the brain, can use for fuel.1

This state, nutritional ketosis, is not the same as diabetic ketoacidosis, the dangerous condition that develops when the body has no insulin. The names sound alike; the two are very different.1

In numbers: blood beta-hydroxybutyrate is normally below 0.3 mmol/L. Nutritional ketosis is usually defined as 0.5–3 mmol/L. Prolonged fasting pushes it to 5–10 mmol/L, and ketoacidosis is far higher still. A blood ketone meter measures exactly this.2

A common description of a well-formulated ketogenic diet: about 5–10% of energy from carbohydrate, which works out to under 20–50 g a day, enough protein, and fat to fill the rest until you're satisfied.1

How many carbs you can eat

Long-term keto trials usually draw the line at 50 g of carbohydrate a day. That was the cut-off for every trial in a meta-analysis of 13 randomised trials lasting at least a year.3

Shorter studies often go lower. A three-week randomised trial in healthy young adults, for example, used under 20 g a day.11

How long until you're in ketosis

None of the studies we cite gives a reliable day count, so be wary of anyone who promises one. What the research does say: ketone levels climb once liver glycogen is depleted, and fasting, very low carbohydrate intake and intense exercise all run glycogen down.1

Being in ketosis is not the same as being adapted to it. A 2004 review of keto and exercise concludes that adaptation clearly takes longer than one week and is likely well advanced within 3–4 weeks. It also notes that people who keep dipping in and out with carb-heavy meals report a harder time exercising.8

What the first week feels like

The most detailed picture of 'keto flu' comes from an unusual study: researchers read 448 posts by 300 people across 43 online forums. About a third described symptoms of their own. The most common were flu-like feelings, headache, fatigue, nausea, dizziness, brain fog, stomach upset, low energy, feeling faint and changes in heartbeat.7

Reports peaked in the first week and tailed off after four weeks. Of the 60 people who said how bad it was, 15 called it mild, 23 moderate and 22 severe. This is self-reported forum data, not a trial, so read it as a description of what people go through rather than a measure of how often it happens.7

Hunger often moves the other way. A meta-analysis found people in ketosis were slightly less hungry. The more useful effect was that hunger didn't climb during weight loss, as it normally does when you eat less.6

Salt and fluids in the first weeks

Very low-carb and fasting diets make the kidneys flush out more sodium, and keto researchers plan for it. In the studies described in Phinney's 2004 review, participants were given extra sodium as bouillon and extra potassium every day, and the review counts sodium and potassium among the reasons their endurance held up in ketosis.8

On the keto-flu forums, the remedy suggested most often was more sodium (58 mentions), followed by electrolyte supplements (38), broth (27), magnesium (25) and potassium (24). These are tips from users, not tested remedies.7

For scale: the EU's safe and adequate sodium intake for adults is 2 g a day. One gram of salt contains 0.4 g of sodium, so that's 2 ÷ 0.4 = 5 g of salt. Extra salt is a short-term adjustment, and not for everyone: if you have high blood pressure, heart or kidney disease, or take blood-pressure medicine, ask your doctor first.14

Will you lose more fat than on other diets?

A little more, at best. Across 13 randomised trials that lasted at least a year (1,415 people), keto lost 0.91 kg more than a low-fat diet.3

In the DIETFITS trial, 609 adults were randomly assigned a healthy low-fat or a healthy low-carb diet for 12 months. They lost 5.3 kg and 6.0 kg, a difference that was not statistically significant. The low-carb group averaged 30% of energy from carbohydrate by the end, so it was low-carb rather than strict keto.4

Why so close? In a tightly controlled metabolic-ward study, 17 men switched to keto with their calories held the same, and their fat loss did not speed up. Keto's advantage, where it has one, is that eating less feels easier.5,6

Training in the first weeks

Expect training to feel flat at first. In one study described in Phinney's review, trained cyclists noticed a modest drop in energy on training rides in the first week, after which it largely came back, except for sprinting.8

Hard, fast efforts are where keto costs most. Elite race walkers who trained hard for three weeks on a ketogenic diet burned far more fat but needed more oxygen at race speed, and their 10 km times didn't improve. The groups on higher-carbohydrate diets improved by 6.6% and 5.3%.9

If you lift, keep protein up. The International Society of Sports Nutrition puts 1.4–2.0 g of protein per kg of body weight a day as enough for most people who exercise.10

The blood test worth booking

On average, keto nudges blood fats in mixed directions. In the long-term trials, LDL cholesterol was 0.12 mmol/L higher on keto than on low-fat, while triglycerides were 0.18 mmol/L lower and HDL 0.09 mmol/L higher.3

Averages hide big individual swings. In a three-week trial in healthy, normal-weight young adults eating under 20 g of carbohydrate a day, LDL rose from 2.2 to 3.1 mmol/L on average, 44% more than in the control group, and individual responses varied enormously.11

A survey of 548 people on carbohydrate-restricted diets found that the leaner people were, the bigger their LDL rise tended to be. It's self-reported survey data, but it means being lean or fit is no reason to skip the test.12

What to do with this

  • Keep carbs low and steady. 20–50 g a day or less is the range keto studies use, and dipping in and out makes adaptation harder.1,8
  • Plan for a rough first week: lighter training, plus enough salt and fluid unless your doctor has told you to limit salt.7,8
  • If you train, keep protein at 1.4–2.0 g per kg of body weight a day.10
  • Judge keto by whether it helps you eat less without fighting hunger. It won't burn fat on its own.5,6
  • Get your cholesterol checked before you start and again once you've settled in, and take a big LDL rise to your doctor.11,12

How sure is this?

  • The weight-loss trials here lasted a year or more, but none of them tells you what many years of keto does to heart health.3,4
  • What we know about keto flu comes from self-reported forum posts, not a controlled trial.7
  • The LDL survey relied on people who chose to respond online, and the journal published letters disputing the authors' interpretation.12
  • The metabolic-ward study was small (17 men) and ran keto for 4 weeks.5

Common questions

How long does it take to get into ketosis?

No study we cite gives a reliable day count. Ketones rise once your liver glycogen runs low, and fasting or hard exercise run it down too. Adapting to keto takes longer: more than a week, and usually well under way by 3–4 weeks.1,8

How many carbs can you eat on keto?

Studies typically use roughly 20–50 g a day or less. The long-term trials in one meta-analysis all capped carbohydrate at 50 g a day.1,3

How long does keto flu last?

In the most detailed description we have, from online forum posts, symptoms peaked in the first week and faded after about four weeks. The few people who said when it ended reported anywhere from 3 to 30 days.7

Will I lose muscle on keto?

Protein and training matter most. The International Society of Sports Nutrition puts 1.4–2.0 g per kg a day as enough for most people who exercise, and says 2.3–3.1 g per kg may be needed to hold on to lean mass while dieting if you lift.10

Is keto safe?

It depends on who you are. The clearest risks in the research are ketoacidosis in people taking SGLT2-inhibitor diabetes drugs, and large rises in LDL cholesterol in some healthy people. If either could apply to you, talk to your doctor first and get tested.13,11

Sources

  1. Gershuni VM, Yan SL, Medici V. Nutritional Ketosis for Weight Management and Reversal of Metabolic Syndrome. Current Nutrition Reports, 2018. Review. doi.org/10.1007/s13668-018-0235-0
  2. Miller VJ, Villamena FA, Volek JS. Nutritional Ketosis and Mitohormesis: Potential Implications for Mitochondrial Function and Human Health. Journal of Nutrition and Metabolism, 2018. Review. doi.org/10.1155/2018/5157645
  3. Bueno NB, de Melo ISV, de Oliveira SL, da Rocha Ataide T. Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. British Journal of Nutrition, 2013. Meta-analysis. doi.org/10.1017/S0007114513000548
  4. Gardner CD, Trepanowski JF, Del Gobbo LC, et al. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial. JAMA, 2018. Randomised trial. doi.org/10.1001/jama.2018.0245
  5. Hall KD, Chen KY, Guo J, et al. Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men. American Journal of Clinical Nutrition, 2016. Randomised trial. doi.org/10.3945/ajcn.116.133561
  6. Gibson AA, Seimon RV, Lee CMY, et al. Do ketogenic diets really suppress appetite? A systematic review and meta-analysis. Obesity Reviews, 2015. Meta-analysis. doi.org/10.1111/obr.12230
  7. Bostock ECS, Kirkby KC, Taylor BV, Hawrelak JA. Consumer Reports of “Keto Flu” Associated With the Ketogenic Diet. Frontiers in Nutrition, 2020. Survey / forum study. doi.org/10.3389/fnut.2020.00020
  8. Phinney SD. Ketogenic diets and physical performance. Nutrition & Metabolism, 2004. Review. doi.org/10.1186/1743-7075-1-2
  9. Burke LM, Ross ML, Garvican-Lewis LA, et al. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. Journal of Physiology, 2017. Randomised trial. doi.org/10.1113/JP273230
  10. Jäger R, Kerksick CM, Campbell BI, et al. International Society of Sports Nutrition Position Stand: protein and exercise. Journal of the International Society of Sports Nutrition, 2017. Position stand. doi.org/10.1186/s12970-017-0177-8
  11. Retterstøl K, Svendsen M, Narverud I, Holven KB. Effect of low carbohydrate high fat diet on LDL cholesterol and gene expression in normal-weight, young adults: A randomized controlled study. Atherosclerosis, 2018. Randomised trial. doi.org/10.1016/j.atherosclerosis.2018.10.013
  12. Norwitz NG, Feldman D, Soto-Mota A, Kalayjian T, Ludwig DS. Elevated LDL Cholesterol with a Carbohydrate-Restricted Diet: Evidence for a “Lean Mass Hyper-Responder” Phenotype. Current Developments in Nutrition, 2022. Survey / forum study. doi.org/10.1093/cdn/nzab144
  13. Peters AL, Buschur EO, Buse JB, Cohan P, Diner JC, Hirsch IB. Euglycemic Diabetic Ketoacidosis: A Potential Complication of Treatment With Sodium–Glucose Cotransporter 2 Inhibition. Diabetes Care, 2015. Case series. doi.org/10.2337/dc15-0843
  14. EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA). Dietary reference values for sodium. EFSA Journal, 2019. Official reference values. doi.org/10.2903/j.efsa.2019.5778

The Keto Blueprint is the step-by-step version of this guide: a 14-day plan to start keto safely, with your macros, what to eat, how to handle the keto flu, and how to train, measure and decide what comes next. 33 sources cited. The Keto Blueprint · Hydration & Electrolyte Protocol · Fat Loss Protocol

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