Checking your ketone level is one way to know if you’re in ketosis. This metabolic state usually kicks in after three or four days of restricting your carbohydrate intake or going through periods of intermittent fasting. You don’t have to visit a doctor to measure your level. Pick up a ketone urine test from a nearby drug store, or use a blood sugar meter that’s capable of measuring ketones.
In 1921, Rollin Turner Woodyatt reviewed the research on diet and diabetes. He reported that three water-soluble compounds, β-hydroxybutyrate, acetoacetate, and acetone (known collectively as ketone bodies), were produced by the liver in otherwise healthy people when they were starved or if they consumed a very low-carbohydrate, high-fat diet.[10] Dr. Russell Morse Wilder, at the Mayo Clinic, built on this research and coined the term "ketogenic diet" to describe a diet that produced a high level of ketone bodies in the blood (ketonemia) through an excess of fat and lack of carbohydrate. Wilder hoped to obtain the benefits of fasting in a dietary therapy that could be maintained indefinitely. His trial on a few epilepsy patients in 1921 was the first use of the ketogenic diet as a treatment for epilepsy.[10]
The fatty acids then flow into the bloodstream and are taken up by body tissues.  Once in the cells, the fatty acids are transported into the mitochondria of the cell to be metabolized carbon by carbon in a process called beta-oxidation. As glucose levels fall and fatty acid levels in the blood rise, the liver cells ramp up beta-oxidation which increases the amounts of a molecule called Acetyl-CoA. As the level of Acetyl-CoA rises, it is shunted to a process called ketogenesis. Ketogenesis generates a ketone body called acetoacetate first, and this ketone is then converted into the two other types of ketones: beta-hydroxybutyrate, and acetone.  Meanwhile, the glycerol part of the fat molecule gets converted into glucose in a process called gluconeogenesis, which means "make new sugar".
Watermelon is a staple summer fruit and another low-carb way to help satisfy your sweet tooth on keto. Each ½ cup of diced watermelon has 5.4 g of net carbs. It’s also an acceptable choice when dieting because of its high water content. The ½ cup serving size of watermelon has about 23 calories and 4.7 g of sugar. This juicy fruit also offers 432 IU of vitamin A, which is 8.6 percent of the DV.

If you would like to read more on ketogenic diets and ketosis, Jeff Volek and Steve Phinney discuss the new method of checking blood ketones in their book "The Art and Science of Low Carbohydrate Performance", and they also offer another good book "The Art and Science of Low Carbohydrate Living", which is a good book for those who need an introduction to the science of ketogenic diets. 
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