There is no simple answer to this question – in fact, the amount of carbs you need to lose fat is likely going to be different than the amount of carbs your coworker, training partner, best friend or whomever, needs to lose fat.
Two people can have identical body compositions and activity levels, yet due to individual metabolic variations, one may be able to eat twice as many carbs as the other and still lose fat. This, naturally, can be very frustrating for some people. The only way to know for sure the proper amount of carbs you should eat to lose fat is to experiment and find what works best for you. Still, there are some general rules and guidelines that can at least give you a starting point from which to work.
The answer to this question, on a strictly physiologic level, is zero. The body can manufacture glucose from other substances, and although the brain’s preferred fuel is glucose, it can use ketones(the by-products of fatty acid breakdown produced in very low insulin states) when no glucose is available. However, from a practical and performance standpoint, zero carbohydrate nutrition plans are not ideal for the vast majority of people. If you are interested in this kind of diet, there are countless books and articles about them, though I do not recommend following a ketogenic diet.
Some people believe that it is not calories, but carbohydrates, and your hormonal response to their ingestion (i.e. insulin release), that are responsible for weight gain. They argue that if insulin levels are kept low then fat storage is impossible regardless of the amount of calories consumed. Others believe that a calorie is a calorie and the laws of thermodynamics dictate that if you eat more calories than you burn, you will gain weight. If you eat fewer calories than you burn, you will lose weight.
I think that there is some truth to both arguments and they aren’t necessarily mutually exclusive. Fat loss can be different from “weight loss.” It is theoretically possible to eat fewer calories than you burn but lose muscle tissue and gain body fat, as it is possible (though very difficult) to eat more calories than you burn, gain muscle but lose fat.
As a practical matter, however, it may not matter. What works for you, works for you. If you can get away with stuffing your face with protein and fat without regard for total calories, maintain a low body fat percentage, feel well and have optimal biochemistry (low CRP, high HDL, low triglycerides, etc.), then more power to you. Most people, however, will find that calories must be controlled to lose fat. (Note: oftentimes, people who go on low carb diets attribute their weight loss to the lack of carbs when in fact it is the total calorie reduction that occurs with carb restriction that is responsible for the weight loss. These people would likely lose the same amount of fat with a modest reduction in carbs and fat, feel better while they are doing it, and have a better chance of keeping it off in the long run.)
Additionally, people generally feel better with at least some carbohydrate in their diets. The body uses stored sugar to fuel high intensity exercise, which should form the foundation of any fat loss exercise plan, so carbohydrates = improved exercise performance.
Certain sources of carbohydrates are better at blunting hunger than others. Vegetables and many fruits, with high water and fiber content, contribute to feeling full without providing significant amounts of calories. Other foods, specifically processed carbs like white bread, pasta and sugary candy and cereals pack a strong calorie punch and will likely leave you feeling hungry soon after eating them. Again, though, there is a huge amount of variability from person to person and you must experiment to see which carbs you react well to and which you’d do well avoiding.
If you have diabetes, insulin resistance or metabolic syndrome, you will likely feel better eating a more moderate carbohydrate, higher (healthy) fat and protein diet. Because you feel better you will be more likely to stick with it in the long run. If you feel better eating higher amounts of carbs and are still able to lose body fat (and control blood sugar), then by all means do so. Below are some very general guidelines for fat loss, but remember that everyone is different and certain “carb-sensitive” people may need to severely limit carbs in order to lose fat. In addition, the amount of protein and fat in your diet will also influence the amount of carbs you should be eating per day. Use these are starting points and adjust based on your response:
I recommend starting at the upper end and carefully tracking your intake. If your weight is not coming down appropriately (1-2 pounds per week), then begin reducing your carb intake until it does. Again though, if you feel sluggish or tired eating that much (or that little) carbohydrate, then adjust accordingly.
Most importantly, use common sense, work with someone who knows what they are talking about, and listen to your body.