The Nutrients People Most Commonly Undereat in the American Diet

Dietary fiber is perhaps the most widespread nutritional shortfall in the American diet. Most adults consume roughly half the amount recommended by federal dietary guidelines, which suggest around 25–38 grams per day depending on age and sex — though individual needs vary and these figures are general population benchmarks, not personal prescriptions.

Fiber supports digestive health, helps regulate blood sugar response after meals, and contributes to a feeling of fullness. It also feeds beneficial gut bacteria, which research increasingly links to broader aspects of health. Whole grains, legumes (beans, lentils, chickpeas), vegetables, and fruits are the primary dietary sources. Processed and refined foods, which dominate much of the American food supply, are largely stripped of fiber during manufacturing.

Vitamin D stands out because the body can synthesize it through sun exposure — yet deficiency and insufficiency remain widespread. NHANES data has consistently found that a substantial share of the U.S. population has low circulating levels of vitamin D. Dietary sources are limited: fatty fish (salmon, mackerel, sardines), egg yolks, and fortified foods like milk and some cereals are the main contributors.

Vitamin D plays well-established roles in calcium absorption and bone health, and research continues to investigate its involvement in immune function and other systems. Geographic latitude, skin tone, time spent outdoors, and sunscreen use all influence how much the body can produce from sunlight — which is why food and, where clinically indicated, supplementation become important considerations for many people.

Calcium is essential for bone and tooth structure, muscle contraction, and nerve signaling. While it is strongly associated with dairy, it's also found in fortified plant milks, tofu made with calcium sulfate, canned fish with soft bones, and leafy greens such as kale and bok choy. Spinach contains calcium but also high levels of oxalates, which significantly reduce how much the body can absorb.

Inadequate calcium intake over time is associated with reduced bone density, making it particularly important across the lifespan — including during childhood and adolescence when bone mass is being built, and in older adulthood when bone loss accelerates. Women and older adults are disproportionately affected by low calcium intake. Vitamin D and calcium work in tandem: sufficient vitamin D is needed for the gut to absorb calcium effectively.

Potassium is a mineral and electrolyte that helps regulate fluid balance, supports normal muscle function, and plays a role in maintaining healthy blood pressure. Despite its importance, most Americans consume well below the adequate intake level. The foods richest in potassium — beans and lentils, sweet potatoes, avocados, bananas, leafy greens, and yogurt — tend to be underrepresented in typical American eating patterns.

The relationship between potassium and sodium is also worth understanding: diets high in sodium and low in potassium are associated with less favorable blood pressure outcomes in population research. Our article on sodium and your diet explores that balance in more depth. Potassium shortfalls are largely a reflection of low fruit and vegetable consumption across the population.

Magnesium is involved in hundreds of enzymatic processes throughout the body, including energy production, protein synthesis, blood glucose regulation, and nerve and muscle function. It is found in nuts and seeds, whole grains, legumes, and dark leafy greens. Like fiber, magnesium is substantially reduced when grains are refined — so diets built around white bread, white rice, and processed snacks tend to be low in this mineral.

Survey data consistently shows that a meaningful portion of Americans, particularly older adults, consume less magnesium than recommended. Low magnesium intake has been associated in research with a range of health concerns, though establishing clear cause-and-effect relationships in nutrition science is complex. The nutrient's broad functional role makes it one of the more consequential gaps to address through food choices.

Iron deficiency is the most common micronutrient deficiency worldwide, and certain groups in the U.S. — particularly women of childbearing age, pregnant women, and young children — face notably higher risk. Iron is necessary for producing hemoglobin, the protein in red blood cells that carries oxygen. Symptoms of inadequate iron status can include fatigue, difficulty concentrating, and reduced physical endurance, though these overlap with many conditions.

There are two forms of dietary iron: heme iron (from meat, poultry, and fish), which the body absorbs more efficiently, and non-heme iron (from legumes, tofu, fortified grains, and dark leafy greens), whose absorption varies more by context. Consuming vitamin C-rich foods alongside non-heme iron sources can meaningfully improve absorption. Plant-based eaters should be aware that their iron needs and absorption dynamics differ from those who eat meat regularly — a topic explored further in our article on plant-based versus animal protein trade-offs. Special-population concerns around iron — including during pregnancy — warrant consultation with a healthcare provider.