There are two upsides to eating just one food every single day at every single meal. The first is that you could contend for the title of Worst Dinner Party Host of All Time. The second is that you could play a game to distract yourself from your dull diet: See how long you last before you wind up in hospital.
The conventional wisdom says you're supposed to fill your plate with a variety of foods, including fruits, vegetables, grains, lean protein and dairy. There's good reason for that dietary mandate: No single food provides everything you need. Instead, the macronutrients (carbohydrates, fat, protein), micronutrients (vitamins and minerals) and nonessential nutrients (like antioxidants) work synergistically to keep you healthy.
"All foods provide specific nutrients your body needs, says Dr. Joy Dubost, a registered dietitian and spokesperson for the Academy of Nutrition and Dietetics. "If you start eliminating foods, you need to get the nutrients they provide elsewhere."
If you only eat one food without any supplementation, you're setting yourself up for one or many nutrient deficiencies. Depending on which food you're eating, the symptoms of those deficiencies can manifest in different ways, like lack of energy, low blood sugar, anemia, thinning hair, dry skin, weak fingernails, and weight loss or weight gain. If you miss out on foods containing vitamin C, within several weeks you could be afflicted with scurvy, a disease that causes weakness, gum disease and skin hemorrhages.
You probably wouldn't expect good results from a diet of just donuts, burgers or candy bars, but it's not just drive-through junk: Even a food that plays a role in a nutritious diet can cause health problems if it comprises all of your calories. Grilled chicken, for example, contains no carbohydrates, the body's preferred source of energy. If you're not getting energy from your diet, your body begins cannibalizing muscle and fat for fuel, which can result in weakness and impaired brain function in the short term.
By the same token, eating just bread, fruit or another carbohydrate means you aren't taking in fat and protein, which results in amino acid deficiencies that cause organ failure if left untreated. Eating just one food also results in overconsumption of certain nutrients that can also have health impacts: Too much saturated fat and sodium from pizza could drive high blood pressure and heart disease, while an excess of potassium from too many bananas could cause electrolyte imbalances, resulting in heart palpitations.
While you could probably last for several days eating the same food without any problems at all, those ill effects could begin to appear within a matter of weeks. Eventually, you'd need medical interventions like tube feeding to stave off fatal results. "[Eating only one food] can impact your quality of life, your long-term health, and ultimately, it could lead you to your deathbed," Dubost says. "That's not pleasant to hear, but it's the truth."