Some people are so afraid of dietary fat, and the accompanying pitifully small protein recommendations (due to the last 40 years of low fat/low cholesterol food pyramid/My Plate propaganda) that they really are eating far too little, even when trying to do induction. Yes, Atkins tells you to be unafraid of fat, and eat enough protein to feel full, but sometimes the old teachings are difficult to shake, especially if you're used to low fat diets, with their strict limits on protein and fat.



So in that situation, let's say for instance that instead of eating a bowl of cereal with fat free milk and a big glass of orange juice for breakfast, when they start induction, they're eating a spinach omelet with 2 egg whites cooked in 1/4 tsp butter for breakfast. For lunch they still have a salad, but instead of just greens and fat free dressing with a big crusty fat free roll on the side, they'll have 2 oz albacore tuna on salad greens with low fat dressing; For dinner, they have 3 oz boneless skinless chicken breast cooked in 1/2 tsp olive oil, with plain steamed broccoli on the side, skipping the big baked potato that they used to eat with it. Yes, they're definitely eating much lower carb than before, but it's also still extremely low in fat, too low in protein, and even lower in overall calories - they haven't done anything other than allow the tiniest bit of fat, and cut back on carbs. The result is way too low in calories to lose weight effectively, because it's so low in calories, the body can go into a protective starvation mode. (That being said, if you ate that way for a week, you'd still very likely lose any excess fluid, due to the carb restriction, but after that, weight loss would slow because of the protective mechanism that conserves energy to prevent starvation)



A decade or so ago, I was doing LC, but unintentionally eating just a couple hundred calories below my BMR



[BMR, basal metabolic rate, along with the variables involved in estimated caloric needs is explained here You can enter gender, weight, height, and age to determine approximate BMR, plus the additional calories expended at various activity levels. You want to avoid caloric intakes below your BMR if at all possible.]


In addition, I was exercising at least an hour daily, which increased my maintenance calorie needs (the amount I would have needed to maintain my weight while exercising). I lost weight for a while, but then it stopped - my body rebelled after a time, apparently sensing a near-famine situation, since it wasn't getting enough to supply it's basic caloric needs. It wasn't a caloric deficit situation nearly as bad as described above with the person trying to do LC while not getting nearly enough protein, fat, or calories, but it was low enough that at some point, my body decided it was too much of a sustained deficit, even if it was only by a couple hundred calories daily, so my body went into protective mode, and I stopped losing.



With IF, there's no calorie limit - the real limit is just to stay within the window of time they've allowed for eating each day. If you do IF while low carbing, it can limit your calorie intake to a certain extent, since you're limiting the number of hours you allow yourself to eat. If you were to intentionally seriously restrict your calories in addition to limiting your carbs on IF, depending on your basal metabolic rate, that could potentially also prevent weight loss. But since you're free on IF to eat as much as you want during your eating window, that's not as likely to happen.