KS1 continuous ketone monitor for tracking ketone levels on a keto diet in NZ

Keto Diet in NZ: Ketosis, Ketone Levels and Keto Flu Explained

By the Rose Optometry optometrist team, Hamilton. General information only — talk to your GP before a major change of diet, especially if you take medication. Numbered references are listed at the end.

A ketogenic (keto) diet is a very-low-carbohydrate way of eating that aims to put the body into ketosis: with little carbohydrate available, the liver turns fat into ketones and the body uses them for fuel [1].

How a keto diet works

A ketogenic eating pattern usually means 20 to 50 grams of carbohydrate a day [2,3]. Ketosis usually begins within about two to four days, though this varies and has been measured in only a few small studies [4].

Ketone levels: what the numbers mean

Blood ketones (mmol/L) What it generally means
Below 0.5 Normal, not in ketosis [1]
About 0.5 to 3 Nutritional ketosis, with normal blood glucose and pH [5]
3.0 or above with acidic blood, in someone with diabetes Diabetic ketoacidosis — an emergency [6]

Higher is not better. A meta-analysis found no difference in appetite between higher and lower levels of ketosis, and concluded there seemed no need to push ketones much above 0.5 mmol/L [7].

Ketosis is not ketoacidosis

Diabetic ketoacidosis (DKA) is a medical emergency in which ketones rise high and the blood becomes acidic, in people who lack insulin [6]. In people who make enough insulin, insulin restrains ketone production [1,5]. That protection does not apply to people with type 1 diabetes or to people taking SGLT2 inhibitor medicines: very-low-carbohydrate diets are a recognised risk factor for DKA in people on those medicines [6], and guidelines advise against a ketogenic eating pattern for them [3,8]. More in ketogenesis vs ketosis vs ketoacidosis.

Keto flu

Some people feel unwell in the first days: headache, tiredness, light-headedness, nausea and sometimes muscle cramps [9,10]. It typically starts within two to four days and lasts a few days to a week [10], occasionally longer; in one analysis of online reports, symptoms peaked in the first week and had dwindled by four weeks [9]. The likely cause is extra salt and water loss as insulin levels fall [11]. Extra fluid and salt are commonly recommended and make physiological sense, but they have not been tested in trials [9]. Symptoms that are severe or persistent need a GP visit.

How to tell if you are in ketosis

  • Urine strips detect a different ketone (acetoacetate) and give only a rough colour reading [12]. In one small study of people six weeks into a ketogenic diet, strips missed ketosis in up to half of people in the middle of the day and were most reliable early in the morning or late in the evening [13].
  • Finger-prick blood ketone meter — a blood reading at the moment you test.
  • Continuous ketone monitor — a wearable sensor that tracks ketones around the clock, with accuracy still being established. See our guide to continuous ketone monitors. We stock the KS1 Continuous Ketone Monitor in our wearables range. ✔ Stock on hand — ships NZ-wide from Rose Optometry, Hamilton.

Breath odour and reduced appetite can occur, but the appetite effect is small on average [7] and neither has been validated as a way of telling whether you are in ketosis.

What the evidence says about benefits and risks

  • Type 2 diabetes. In a meta-analysis of 23 trials, low-carbohydrate diets lowered HbA1c by about 0.5 percentage points more than comparison diets at six months; by 12 months the difference had roughly halved [14].
  • Long-term effects. Most trials last weeks to months. An umbrella review found LDL cholesterol can rise [15]. Kidney stones occurred in about 6% of people on long-term medical ketogenic diets, mostly children treated for epilepsy [16].
  • Endurance sport. A systematic review found ketogenic diets have not been shown to improve endurance performance; most studies showed no difference and a few showed worse results [17].

Who should talk to a GP first

  • People with diabetes on insulin or other glucose-lowering medicines, which may need adjusting to prevent low blood glucose [3]
  • People taking SGLT2 inhibitors, for whom a ketogenic pattern is discouraged [3,8]
  • People who are pregnant or breastfeeding [3] — severe ketoacidosis has been reported in breastfeeding mothers on ketogenic diets [18]
  • People with kidney disease [3]
  • People with liver failure or a history of pancreatitis [10]
  • People with, or at risk of, disordered eating [3]
  • Children and teenagers [3]

Keto and your eyes

There are no clinical trials of ketogenic diets for any eye condition. What exists:

  • Animal research. In mice that spontaneously develop glaucoma, eight weeks of a ketogenic diet preserved more retinal ganglion cells and optic nerve fibres than a control diet, without changing eye pressure [19]. Mouse results cannot be assumed to apply to people.
  • Human observational data. In 185,638 US adults, lower-carbohydrate eating was not associated with glaucoma risk; this was not a test of ketosis, since even the lowest-carbohydrate group ate 37 to 43% of their energy as carbohydrate [20].
  • A caution from medical ketogenic diets. In 1979, two patients on ketogenic diets for seizure control developed optic nerve damage linked to thiamine (vitamin B1) deficiency; both recovered normal vision with thiamine treatment [21].

Where diet does matter to the eye is through glucose. If a change of diet brings long-standing high glucose down quickly, your glasses prescription can shift for weeks to months, and existing diabetic retinopathy should be checked — see diabetic retinopathy and diabetes eye checks and blood sugar and HbA1c levels in NZ.

Common questions

How long does it take to get into ketosis?

Usually about two to four days, with variation between people [4].

What ketone level is ketosis?

Blood ketones of about 0.5 mmol/L or above [5].

How long does keto flu last?

Typically a few days to a week [10].

Is the keto diet safe long term?

Data beyond two years are minimal [10]. Regular check-ins with your GP, including blood tests, are sensible.

References

  1. Laffel L. Ketone bodies: a review of physiology, pathophysiology and application of monitoring to diabetes. Diabetes Metab Res Rev. 1999;15(6):412-426. PMID 10634967
  2. Evert AB, Dennison M, Gardner CD, 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
  3. American Diabetes Association Professional Practice Committee. 5. Facilitating positive health behaviors and well-being to improve health outcomes: Standards of Care in Diabetes—2026. Diabetes Care. 2026;49(Suppl 1). doi:10.2337/dc26-S005
  4. Harvey CJDC, Schofield GM, Williden M, McQuillan JA. The effect of medium chain triglycerides on time to nutritional ketosis and symptoms of keto-induction in healthy adults: a randomised controlled clinical trial. J Nutr Metab. 2018;2018:2630565. doi:10.1155/2018/2630565
  5. Gershuni VM, Yan SL, Medici V. Nutritional ketosis for weight management and reversal of metabolic syndrome. Curr Nutr Rep. 2018;7(3):97-106 (corrected 2025). doi:10.1007/s13668-018-0235-0
  6. Umpierrez GE, Davis GM, ElSayed NA, et al. Hyperglycemic crises in adults with diabetes: a consensus report. Diabetes Care. 2024;47(8):1257-1275. doi:10.2337/dci24-0032
  7. Gibson AA, Seimon RV, Lee CM, et al. Do ketogenic diets really suppress appetite? A systematic review and meta-analysis. Obes Rev. 2015;16(1):64-76. doi:10.1111/obr.12230
  8. Danne T, Garg S, Peters AL, et al. International consensus on risk management of diabetic ketoacidosis in patients with type 1 diabetes treated with sodium-glucose cotransporter (SGLT) inhibitors. Diabetes Care. 2019;42(6):1147-1154. doi:10.2337/dc18-2316
  9. Bostock ECS, Kirkby KC, Taylor BV, Hawrelak JA. Consumer reports of "keto flu" associated with the ketogenic diet. Front Nutr. 2020;7:20. doi:10.3389/fnut.2020.00020
  10. Kirkpatrick CF, Bolick JP, Kris-Etherton PM, et al. Review of current evidence and clinical recommendations on the effects of low-carbohydrate and very-low-carbohydrate (including ketogenic) diets for the management of body weight and other cardiometabolic risk factors: a scientific statement from the National Lipid Association Nutrition and Lifestyle Task Force. J Clin Lipidol. 2019;13(5):689-711.e1. doi:10.1016/j.jacl.2019.08.003
  11. Harvey CJDC, Schofield GM, Williden M. The use of nutritional supplements to induce ketosis and reduce symptoms associated with keto-induction: a narrative review. PeerJ. 2018;6:e4488. doi:10.7717/peerj.4488
  12. Nguyen KT, Xu NY, Zhang JY, et al. Continuous ketone monitoring consensus report 2021. J Diabetes Sci Technol. 2022;16(3):689-715. doi:10.1177/19322968211042656
  13. Urbain P, Bertz H. Monitoring for compliance with a ketogenic diet: what is the best time of day to test for urinary ketosis? Nutr Metab (Lond). 2016;13:77. doi:10.1186/s12986-016-0136-4
  14. Goldenberg JZ, Day A, Brinkworth GD, et al. Efficacy and safety of low and very low carbohydrate diets for type 2 diabetes remission: systematic review and meta-analysis of published and unpublished randomized trial data. BMJ. 2021;372:m4743. doi:10.1136/bmj.m4743
  15. Patikorn C, Saidoung P, Pham T, et al. Effects of ketogenic diet on health outcomes: an umbrella review of meta-analyses of randomized clinical trials. BMC Med. 2023;21(1):196. doi:10.1186/s12916-023-02874-y
  16. Acharya P, Acharya C, Thongprayoon C, 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
  17. Murphy NE, Carrigan CT, Margolis LM. High-fat ketogenic diets and physical performance: a systematic review. Adv Nutr. 2021;12(1):223-233. doi:10.1093/advances/nmaa101
  18. Borhan MK, Vethakkan SR, Sarvanandan T, Paramasivam SS. A case of severe lactation ketoacidosis in a nondiabetic mother on a ketogenic diet. JCEM Case Rep. 2023;1(6):luad134. doi:10.1210/jcemcr/luad134
  19. Harun-Or-Rashid M, Pappenhagen N, Palmer PG, et al. Structural and functional rescue of chronic metabolically stressed optic nerves through respiration. J Neurosci. 2018;38(22):5122-5139. doi:10.1523/JNEUROSCI.3652-17.2018
  20. Hanyuda A, Rosner BA, Wiggs JL, et al. Low-carbohydrate-diet scores and the risk of primary open-angle glaucoma: data from three US cohorts. Eye (Lond). 2020;34(8):1465-1475. doi:10.1038/s41433-020-0820-5
  21. Hoyt CS, Billson FA. Optic neuropathy in ketogenic diet. Br J Ophthalmol. 1979;63(3):191-194. doi:10.1136/bjo.63.3.191
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