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I've been on a ketogenic diet for the past two months, while also doing weightlifting three times a week. I need 2900 kcal a day to maintain my weight, but I've been eating at a 20% deficit to lose weight - 25g carbs, 160g protein and 175g fat daily.

But the fact that I'm in ketosis means I'm already burning mostly fat - why not burn my body's fat instead of sticks of butter?

As I understand it, all that really matters in preventing muscle wasting is that you get enough protein and exercise. So can I spend a couple days eating only 160g protein, and expect at the end to have around 250g less fat in me (i.e., the fat I didn't eat, that thus had to come from my body's stores) for every day mostly-fasting? And would that happen without losing significant muscle mass?

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If you do this, don't do it for very long! Protein only = rabbit starvation. You really need dietary fat (for "health" not weight loss, per se) –  Greg Jul 31 '13 at 14:23

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Ketogenic diets are of questionable benefit unless done for medical reasons (Such as an attempt to control epilepsy). Studies of ketogenic diets have had very mixed results.

If you only eat 160g of protein, that is a daily intake of 640 calories, which is borderline starvation diet. That isn't really a desirable outcome either. Fat is essential, but if you keep eating ~ 1600 calories a day of fat, you're going to have other health problems. Consuming that much fat will start to result in other conditions, especially if done for a prolonged period of time.

Metabolism is not a 1:1 relationship, i.e. if I don't eat 200g of fat, my body will take 200g of fat to make it up from stores. The body basically decides what and where it will take from to get the nutrients and energy that it needs. If it opts to take it from muscle, it will. All that you will do if you eat nothing but protein is force your body to take what it needs from where ever it can find it.

Studies have shown many times that the type of diet is almost irrelevant, whether it's Atkins, South Beach, grapefruit, ketogenic, whatever. Simple calorie restriction is the single most important factor.

Eat a healthy diet of varied elements (Shop the edges of the store), ensure you get enough protein and keep the 10-20% calorie reduction, and you will see desirable results. Given that you are weighlifting, that may be higher than BMI charts (Which are notoriously unreliable for athletic/muscular bodies) say you should be, but let the mirror and your health be the best guide.

Given the question regarding the Inuit diet which is routinely 50-75% fat based depending on season; the Inuit diet is high in fats that are monounsaturated and very high in Omega-3. Unless someone is eating wild caught seal/fish/walrus, it's going to be much higher in saturated fats, which is very common in Western diets and unhealthy.

The Inuit diet also varies from 50-75% fat depending on season, the diet described in the question is a straight 65-70% from fat daily. And, while the Inuit may not have the health problems most commonly related to high fat intake, they do suffer from obesity, which negates the weight loss that is desired.

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Your assertion that a diet high in fat will cause health problems needs a reference. Inuit have survived for eons eating diets composed of ~95% protein and fat: en.wikipedia.org/wiki/Inuit_diet –  Doc Jul 31 '13 at 17:25
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You need to read your own references. The Inuit diet is high in fats that are monounsaturated and very high in Omega-3. Unless the OP is eating wild caught seal/fish/walrus, it's going to be much higher in saturated fats, which is very common in Western diets. The Inuit diet also varies from 50-75% fat depending on season, the OP is eating a straight 65-70% from fat daily. And, while they may not have the health problems related to fat intake, they do suffer from obesity, which the OP doesn't want either. –  JohnP Jul 31 '13 at 19:41
    
@JohnP Thats all good information. Would you be able to edit it into your answer? –  Lego Stormtroopr Jul 31 '13 at 23:30
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@LegoStormtroopr - Done. It is a good comment, because people don't differentiate, and it's important to specify the type of fat. Most people will hear "Hey, the Inuit do it and they aren't unhealthy!" and start loading up on unhealthy manufactured fats. –  JohnP Aug 1 '13 at 14:34
    
Also you're forgetting the Inuit generally have/had a high parasite load that reduce their cholesterol levels. You need to do some more background research before sprouting advise. –  Chad May 27 at 6:46

you need to eat a lot of dietary fat, in the absence of carbohydrates. No need to cut calories, you will lose weight. you will need to eat 70-80% fats,20-25% protein, and 5% carbs, (if you feel you need to eat vegetables). I f you believe saturated fats are harmful, then make most of your fat intake mono-unsaturated, olive oil, avocado, but remember the lipid theory has been proven as based on lies and deceit, and will be proven wrong in the near future. Link Dr James Carlson, for some good, expert insight, into your low carb journey. Cheers.

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So, if I get you right, you suggest that he'll lose weight with no caloric deficit? How is that possible, thermodynamics-wise? –  LarissaGodzilla May 21 at 7:20

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