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Ketosis is a metabolic reaction which occurs during the Optifast VLCD Intensive phase. Your body will begin to burn fat cells for the energy it needs, as this is no longer being provided by the food you are eating, producing Ketones.

A decrease in appetite is experienced during Ketosis, making it easier you to follow this phase.


Before your body transitions into ketosis, you will most likely feel hungry and you may experience some side-effects such as fatigue, lack of concentration, nausea & headaches. These side-effects should only last about 3 days and most symptoms usually pass in 4 - 6 days. This is what we refer to as the 3 Day Challenge.

Energy Change

OPTIFAST low carbohydrate diet (< 43g/d) decreases hunger compared to higher carbohydrate during weight loss diets through inducing ketosis.1 It is important to know that these are typical symptoms and you should try to stick to the program as prescribed otherwise you will simply pro-long these first few days of the OPTIFAST VLCD Program. Of course, if you are concerned about your symptoms or if they persist, speak to your doctor, dietician or pharmacist as they can advise you on how to overcome or correct these before coming off the program.

Excess Ketones

Once your body is in ketosis, any excess ketones that your body does not use for energy are excreted via your urine and breath (thus the incidence of bad breath in some cases). This therefore allows you to measure your ketone levels in your urine with Ketone Reagent Strips or Ketostix. Ketostix can be purchased over the counter from your local pharmacy. Alternatively, your healthcare professional may test for ketones on your regular visits. Do not worry if you have a little bad breath whilst using OPTIFAST VLCD. This will only occur and last if you are following the Intensive phase. Chewing on a low-calorie mint, sugar free chewing gum or even parsley can be very effective in managing this.

Carbohydrate Requirement

The amount of carbohydrate required to induce ketosis depends on a number of factors and can actually vary from day to day depending on activity levels and fluid intake. A diet containing 50-70g of carbohydrate is generally considered low enough to produce ketones but there is no absolute level that guarantees ketone production. The OPTIFAST VLCD Program generally only produces a mild ketosis, therefore for some reason, some people find that Ketostix do not show ketones or the levels fluctuate. If you are losing weight steadily, then there is no need to be concerned. You can rely on the other symptoms of ketosis such as a reduction in your appetite and change in your breath as a guide instead.

1. Sumithran P, Proietto J. Ketogenic diets for weight loss: A review of their principles, safety and efficacy. Obes Res Clin Pract. 2008 Mar;2(1):I-II. doi:10.1016/j.orcp.2007.11.003. PubMed PMID: 24351673.

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