Obesity is defined generally as having a body mass index of 30. By definition, a BMI of 18.5 is considered undernourished. What this means is that many of us are carrying a food supply with us.
Let's look at an average American male weighing 170 lbs. If obese, they are carrying fat reserves of 51 lbs, but they need to maintain 18.5 percent to stay healthy, so their ideal weight is around 135 lbs.
Fat can be catabolized into energy, but it's not a straight forward process. While a healthy person can go upwards of three weeks with a severely reduced intake under stringent medical supervision, a lack of adequate blood glucose will result in losing consciousness.
Still, for the first few weeks of a SHTF scenario, most of us can manage quite tolerably on a reduced diet of 1500 calories. A normal diet for men is 2300 calories for maintenance. Most people do not eat a maintenance amount of calories especially when stressed. They eat far more and this is a major consideration for SHTF scenarios. You do not want to cut calories much lower than this. Food spoils easily, and it is more efficient to eat it, than carry it, within a rational level of consumption.
A problem is that while people are generally sedentary, under a stressed situation, they likely are increasing their caloric needs from exertion. Chopping wood is a labor intensive activity.
Very careful measurements of food servings will have to be done based upon the caloric needs of you family, and taking into account exertion and stress. We will all be a lot thinner.
In a perfect world, we would lose fat first. That is not the case. The first weight loss is from water and muscle tissue. Dehydration is a concern in the absence of clean water. Most people are out of touch with their normal thirst mechanism. Dehydration in hot climates, dry climates, even during winter time (low humidity) can significantly affect the diet and overall health.
Children will of course be our primary concern. Their bodies are growing, and any nutritional deficiency will become evident in their behaviour, skin, teeth, and hair in a very short amount of time. You must watch this carefully.
Pine needle tea can be consumed to prevent scurvy or a lack of Vitamin C. There is not much excuse for a lack of Vitamin C. Just about everyone can identify it and prepare it. Teeth can become loose, gums bleed, and wounds don't heal.
Hair loss is indicative of low protein consumption generally. The hair will become brittle and break easily.
Low blood sugar will result in stuporous behaviour and poor judgment. You have to watch for this as local sweeteners, fruits, and normal sugar in the diet will be severely curtailed.
A rapid reduction in normal caffeine consumption will cause withdrawal due to a chemical mechanism. Severe headaches can result and irritability. Many children consume a lot of caffeinated beverages, and these new symptoms could be this, or something far worse, but should be easy enough to identify based upon the time period they occur. Knowing this, if coffee is in short supply, you will want to ration it to avoid this issue.
Spicy foods are more satisfying, so adding more may stave off hunger at least at first. Make one pot meals. Think camping meals. You need to minimize the amount of dishes to wash anyway. Ramen noodles are cheap, add fat calories, filling, and are pre-flavored with bouillon. Hamburger Helper or mac and cheese can be also used to make a simple dish. The Italians say, "Good food praises God." I know it adds an enormous morale factor, so take the time to eat together and talk, and do your best to make the meal, such as it is, enjoyable.