Am confused on ketosis and fat loss

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  • I read that if on a water fast the fat loss begins in 3-4 days. That’s when body shifts to using fat. But Dr. Mosley says fat loss begins after 12 hours of fasting. ????
    I am a senior , and based on experience, won’t fast longer than 18 hours. That should give me 6 hours of fat burning. (I want to reduce my waist from 38 to 33 inches) Grateful for clarification.

    Hi Russ and welcome:

    Please don’t get all concerned about ketosis and fat loss – they are not the same.

    Virtually all standard weight loss diets, like weight watchers, slim fast, slimming world, and god knows how many others, never allow the body to get into ketosis because they constantly have you eating carbs. Nevertheless, everyone that follows those diets loses weight – fat weight. The body burns fat whenever it is suffering from a calorie deficit – that is why the body stores fat – that is what fat is for.

    So you don’t have to get your body into ketosis to burn fat. The key to burning fat is to make the body suffer a caloric deficit. 5:2 does that by seriously restricting caloric intake two days a week.

    Your question, and many others, are addressed in the threads linked in this topic:

    Good Luck!

    Hi Russ,

    The primary source of energy for our bodies is glucose. When we fast your body will use up the glycogen stores in our liver first and then it will use fat stores.
    Ketosis occurs when the body converts to using fat as a primary source of energy rather than glucose.

    If the body needed to be in ketosis to lose weight the majority of diets based on caloric restriction wouldn’t work, including 5:2.

    Having said that, timings are not precise ie it is not possible to say that you’ll be burning fat because you’ve not eaten for 12 hours as it will depends on how much glycogen your body is able to store and how much energy the body needs depending on what you are doing.

    EDIT, simco has nipped in before me!

    Basically correct above. Your body is burning a combination of glucose, glycogen and fat ALL the time. The ratio of the three does however vary depending on your activity and current state. Being in ketosis just means that the majority of your energy is provided from the conversion of stored fat. It is highly unlikely that you will enter ketosis after 12 hours. You could adopt a keto diet and then yes you would be in ketosis. That means eating less than about 20-50g of carbs per day. Don’t worry too much about ketosis just start doing 5:2. Good luck.

    Just to support what @bigbooty said, almost all processes in the body are happing all the time, it is just a matter of degree. People like to say when you have high insulin you can’t have any fat burning going on, but that simply isn’t true as there is always fat floating around in your blood. Over time the body regulates how much fat can be used and how much is stored, but there isn’t one valves some happens all the time, it is just a matter of how much.

    When you start fasting your body is working on your last meal, even up to 8 hours later but as that is digested your liver supplies glucose form its supply of glycogen. At the same time there is more lipolysis that starts happening as fats are needed in the blood stream. The body doesn’t have food to work on so it turns to the stored fats. The amount of stored fats broken down is likely small at first but it slowly increases as the fasting continues.

    At some point the liver will run out of stored glycogen and then some more glucose is needed from other sources. Typically there will be a period where proteins are also being converted to glucose ( Neoglucogenesis ) but at the same time the breaking down of fat ramps up. Eventually the production of glucose from non-carb sources results in ketone production which many parts of the body can use for fuel too. When the ketone concentrations in the blood become high enough the kidneys start leaking ketones into the urine and that is what most people measure to see if they are in ketosis.

    How long it takes to be in the ketosis state depends on many factors but a rule of thumb is about 2 days for women and 3 days for men. It can happen much faster or slower. However just because you can’t measure ketone levels doesn’t mean you aren’t using fat as fuel. Ketosis is a byproduct of breaking down fat for glucose, not the other way around.

    The amount of fat being burned can actually be measured by analyzing one’s breath. When we burn fat we create waste that is released in our breath. This is mostly carbon-dioxide and water. The amounts change as the amount of fat being consumed changes.

    While we are almost always using some fat, most of us simply store more fat than we use and that is why we gain fat. However not being about to use fat is pretty much incompatible with life.

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