Calories: how much food you really need

Why calories decide how long you last

A food stockpile is really a stockpile of calories. Shelves full of cans look reassuring, but what matters is how many days of energy they hold for the people who have to live on them. A pantry that looks like three months of food can turn out to be five weeks once you do the math.

The goal in a long emergency isn't to eat as little as possible. It's to eat enough to stay even: to hold your weight, your strength and your health while you do the hard physical work that survival takes. This guide explains how much that is, and the calculator below works it out for every person in your home.

What a calorie is, and where they go

A calorie is a unit of energy. Food supplies it, and the body spends it on three things.

The biggest is simply staying alive: breathing, pumping blood, keeping warm and running the brain. This resting burn takes roughly 60 to 70 percent of a typical person's daily calories even if they lie in bed all day. Digesting food takes about another 10 percent. Everything else, from walking to chopping wood, comes on top.

• Bigger, taller and younger bodies burn more at rest. Men usually burn more than women of the same size because they carry more muscle.
• Physical work raises needs sharply. Hauling water, splitting firewood, gardening by hand and walking everywhere can add 1,000 calories a day or more.
• Cold raises needs too. A body shivering in an unheated house burns extra fuel just to stay warm.
• Children need fewer calories than adults overall, but far more for their size, because they are growing.

How many calories people need

The US Dietary Guidelines estimate that most adult women need about 1,600 to 2,400 calories a day and most adult men about 2,000 to 3,000, depending on age, size and activity. Those figures assume ordinary modern life. A grid-down life of physical work pushes most adults toward the top of those ranges or beyond.

• Resting, mostly sitting: the low end. An office worker on a normal day.
• Light activity, such as housework and short walks: a little more.
• Moderate work, on your feet most of the day, carrying water, gardening: where most people land in a long emergency.
• Hard work, such as chopping wood, digging or hiking with a load: often 3,000 to 4,000 calories or more for a large man.
• Pregnancy adds about 340 calories a day in the second trimester and about 450 in the third. Breastfeeding adds about 330.
• Children from 2 to 17 range from about 1,000 calories a day for a toddler to over 3,000 for an active teenage boy.

Protein: holding on to your muscle

Protein is what the body builds and repairs itself with: muscle, skin, blood, the immune system. When protein runs short, the body takes it from its own muscle, and you get weaker every week.

The standard recommendation for adults is 0.8 grams of protein per kilogram of body weight a day, about 0.36 grams per pound. That's around 55 grams for a 150-pound person. People doing hard physical work, adults over 65, and anyone pregnant or nursing do better on more, about 1.0 to 1.2 grams per kilogram. The calculator below adjusts for that.

• Storage sources: dry beans, lentils and split peas, canned meat and fish, peanut butter, powdered milk, wheat and oats.
• Rice and beans together give a far better balance of protein building blocks than either one alone.
• Spread protein through the day. The body uses 20 to 30 grams at a meal better than one huge serving.

Carbohydrates: the everyday fuel

Carbohydrates are the body's main fuel and the brain's preferred one. They supply 4 calories per gram, and the guidelines suggest they make up about 45 to 65 percent of daily calories, with at least 130 grams a day for adults to keep the brain well fueled.

In a survival pantry, carbohydrates are the cheap, bulk calories: rice, wheat, oats, pasta, corn, potatoes, sugar and honey. They are why rice and wheat sit at the center of almost every long-term storage plan.

Fat: the densest fuel

Fat carries 9 calories per gram, more than twice as much as protein or carbohydrate. That makes it the lightest, most compact way to store energy, and the best fuel for cold weather and hard work. The guidelines suggest about 20 to 35 percent of calories come from fat.

The weak point of a rice-and-beans pantry is that it has almost no fat. Without added fat, people struggle to eat enough bulk to meet their calories and feel hungry all the time. Store cooking oil, shortening, peanut butter and canned meats with fat, and rotate them: fats go rancid in a year or two.

What happens when you don't eat enough

When people eat fewer calories than they burn, the body makes up the difference from its own stores, first stored sugar, then fat, and increasingly muscle. A deficit of about 500 calories a day takes off roughly a pound a week.

A little weight loss may not hurt a heavy adult. But a long shortfall wears people down in ways that matter in a crisis.

• Weakness and fatigue: work gets harder just when there's more of it to do.
• Feeling cold all the time, as the body turns its own heating down.
• Irritability, poor concentration and bad judgment, which lead to accidents and arguments.
• Slower healing and weaker immunity, so small cuts and colds become serious.
• In children, stunted growth and development that may never fully catch up.

Vitamins and minerals: the hidden shortfalls

Calories keep people going, but a diet of only stored staples can quietly run short of vitamins. Historically, long sieges, voyages and famines caused deficiency diseases even when people had enough to eat.

• Vitamin C: with none in the diet, scurvy symptoms such as bleeding gums, bruising and exhaustion appear within about one to three months. Store vitamin C tablets, canned fruit and juice. Sprouted beans and grains also make some vitamin C.
• Thiamine (vitamin B1): a diet mostly of white rice can cause beriberi. Mix in beans, oats, wheat and enriched foods.
• Salt and minerals: hard work and sweating use up salt. Store plenty of iodized salt.
• A daily multivitamin is a cheap insurance policy. Store a year's supply per person, and rotate it.

Your food calculator

Add every person in your household with their age, sex, weight, height and how active they'll be. Be honest about activity: in a long emergency, most adults are moderate or hard. Then choose how many days you want to plan for.

For each person, the calculator shows the calories, protein, carbohydrate and fat they need each day to hold their weight, and what that looks like on storage food. At the bottom it adds up the whole household for the number of days you chose. Your entries are saved only on this device.

How it works: for adults it uses the Mifflin-St Jeor equation, a standard estimate of resting energy use, multiplied by an activity factor. For children from 2 to 17 it uses the calorie estimates published in the US Dietary Guidelines. Fat is set at 30 percent of calories, protein by body weight, and carbohydrates make up the rest. These are estimates, not medical advice. The calculator works on this page at afterlightnews.com.

Turning numbers into a shopping list

The calculator turns calories into a simple storage-food mix: about 55 percent of calories from rice or other grain, 20 percent from dry beans, 10 percent from cooking oil, and 15 percent from canned fruit, vegetables, meat and milk for variety, vitamins and morale.

Example: a family of four doing moderate work, a 40-year-old man, a 38-year-old woman, a 12-year-old boy and an 8-year-old girl, needs about 9,050 calories a day. For 30 days that comes to roughly 90 pounds of rice or other grain, 35 pounds of beans, just under a gallon of cooking oil, about 135 cans of fruit, vegetables, meat or milk, and 120 gallons of water.

• Use the mix as a starting point, then adjust it to what your family actually eats.
• Swap in equal calories freely: oats, pasta and wheat for rice; lentils and split peas for beans; peanut butter for some of the oil.
• Recheck the numbers once a year. Children grow, and their needs climb fast.