Mechanics of a VLED

 Mechanics of a VLED

A VLED works by restricting total energy intake to approximately 800 calories (3300kJ) or less per day. Weight loss with a VLED is achieved by restriction of both carbohydrate and total energy intake. This enables the body to use its body fat stores for energy via a metabolic process called ketosis.

Ketosis is a metabolic reaction that occurs when following the OPTIFAST VLCD Program Intensive level.  When glucose is in short supply, ketones are produced which provide an alternative fuel source derived from fat1.

A diet containing 50-70g carbohydrates is generally considered low enough in carbohydrates to produce ketones, but there is no absolute level that guarantees ketone production. The amount of carbohydrates required to induce ketosis depends on several factors. It can be individual and can vary from day to day depending on activity levels, fluid intake and other factors.

The term "ketones" refers to three compounds:

  • Acetoacetate (AcAc)
  • 3-β-hydroxybutyrate (BHB)
  • Acetone


The circulating level of ketones in the body at any one time is dependent on their rate of production and rate of utilisation. Acetoacetate and 3-β-hydroxybutyrate are the two primary ketones generated and used for fuel under low carbohydrate conditions. Acetone gives the characteristic odour to the breath of patients in ketosis, similar to that of nail polish.

Dietary ketosis used in the management of weight loss should not be mistaken for ketoacidosis. Dietary ketosis is a controlled, insulin-regulated process which results in a mild release of fatty acids and ketones into the circulation in response to a low carbohydrate intake. Ketoacidosis, on the other hand, is a condition in which abnormal quantities of ketones are produced due to no insulin regulation of ketone levels. To reach a state of ketoacidosis, the body has to be in a stage of not producing any insulin. This is why ketoacidosis is generally only seen in uncontrolled type 1 diabetes.

Level of circulating blood ketones for different conditions2

  Blood Ketone Level

Weight loss using a VLED and inducing mild ketosis

Refractory paediatric epilepsy 2.0-7.0mmol/L
Diabetic ketoacidosis >3.0mmol/L, can be as high as 25mmol/L


Once the body is in ketosis, any excess ketones that the body does not use for energy, are excreted via the urine and breath (thus the incidence of halitosis in some cases). Measuring ketone levels on the breath or in the urine can be used as a guide to assessing compliance. Confirm your patient suitability to start the program.

1.    Gibson, A.A., et al., Do ketogenic diets really suppress appetite? A systematic review and meta-analysis. Obes Rev, 2015. 16(1): p. 64-76.
2.    National Health and Medical Research Council., Clinical Practice Guidelines for the Management of Overweight and Obesity in Adults, Adolescents and Children in Australia. National Health ad Medical Research Council, 2013.