The scale can fluctuate at least 3 to 5 pounds on any given day. To gain a full pound of fat, you must consume 3,500 calories more than you burn. You'd be challenged to eat this much, or more, above and beyond what you normally need to maintain your weight in two days. If the scale has gone up significantly in two days, it's likely because of natural fluctuations caused by water retention, hormones, constipation or dehydration.
When you have the intention to gain weight in two days, you can pack on a pound or two with a serious eating effort. But such fast weight gain is likely to occur by eating unhealthy foods and will increase your body fat, not muscle mass, which is a healthier tissue.
The Reason for Scale Fluctuations
Your scale measures more than your actual body fat. It measures bone, muscle, connective tissue, organs and any fluid being retained in your body. If you just ate and haven't yet digested and processed a meal, it'll show up as extra weight. The same is true if you're holding onto urine, feces or inflammation due to a salty meal, hormones or a tough workout. Dehydration can also make you a bit heavier as your body compensates by retaining the water you do have in case of emergency.
Weighing yourself daily can make you crazy worrying about these fluctuations. Judge changes in your size by how your clothes fit and a weekly weigh-in to minimize anxiety.
Aim for Slow Weight Gain
Two days isn't long enough to promote substantial, healthy weight gain. You want to add primarily muscle, whether you're underweight due to illness, trauma or a high metabolism. The most muscle you can realistically expect to gain in one week is 1/2 pound. This requires an addition of 250 to 500 calories per day and strength training at the gym. In two days, you might add a few ounces, but probably not enough to be noticeable. Gaining muscle is a slow process that requires diligence and large portions of nutrient-dense foods.
Fast Weight Gain Pitfalls
Make smart choices if you absolutely have to gain weight in two days to make a team or pass a health evaluation. It's easy to look at weight gain as a free-for-all and opt for foods that are high in calories but contain large amounts of sugar, saturated fats and refined grains and fail to supply nutrients to support a healthy body.
For example, you could pad your diet with a sausage and egg biscuit from your local drive-through; a vanilla shake with two slices of pan pizza; a cup of ice cream and two chocolate-chip cookies for a snack; and a fried fish and chips dinner washed down with a 20-ounce cola. These choices certainly supply calories but give you little fiber, no phytonutrients, more than 146 grams of fat, about 50 grams of which is saturated fat, and 130 grams of sugar.
The American Heart Association recommends that no more than 5 to 6 percent of your daily calories come from saturated fats -- even if you're eating 3,000 to 4,000 calories per day, that's only 20 to 27 grams daily. The association also recommends women consume no more than 24 grams of added sugar per day, with a 36-gram limit for men. These choices far exceed recommendations for sodium, too.
Even if you're underweight, you're still at risk of health problems associated with eating too much of these low-quality options.
Healthy Ways to Gain Weight
If you must gain weight in two days, do so with healthy food choices. Unsaturated fats, starchy vegetables, whole grains, lean proteins and dairy offer nutrition and calories.
You're better off scrambling a few eggs with a whole-wheat bagel alongside peanut butter and a banana in the morning; adding avocado and nuts to a salad at lunch; and enjoying a large serving of whole-wheat pasta tossed in olive oil and topped with ground turkey, broccoli, Parmesan cheese and marinara sauce for dinner.
Snack on yogurt mixed with granola, cottage cheese with raisins and almonds, trail mix or liquid milk mixed with dried milk instead of a milkshake. These foods are calorically dense, contain higher levels of protein and other nutrients and support healthy weight gain. You'll still gain mostly fat if you pack on a few pounds, but at least supply yourself with quality nutrition in the process.
Use the two days to kick off a strength-training routine to help you gain more muscle in the long term. You can't gain any measurable weight from strength training in two days, though.