Food and energy: what to check before you change your diet

Fatigue is a lack of energy and motivation, different from drowsiness, and it is a normal response to hard work, stress or short sleep. When it is not relieved by enough sleep, good nutrition and a low-stress stretch, MedlinePlus advises having it evaluated MedlinePlus. The usual first tests are blood tests for anemia, diabetes and infection, plus kidney, liver and thyroid function testsMedlinePlus. Three nutrient and hormone causes come up often enough to name.

• Iron. Low iron at first causes no obvious symptoms because the body draws on its stores. Once iron deficiency anemia sets in, symptoms include weakness, tiredness, lack of energy and trouble concentrating. Teen girls and women with heavy periods, pregnant women, frequent blood donors and people with gut disorders are more likely to run short NIH ODS. Do not take iron supplements just in case: the adult upper limit is 45 mg a day, and a doctor decides on higher doses NIH ODS.
• Vitamin B12. A shortage can make you feel tired or weak. Many older adults absorb B12 poorly from food, people who eat little or no animal food may not get enough, and the diabetes medicine metformin can lower B12 levels NIH ODS.
• Thyroid. An underactive thyroid slows many body functions. Symptoms include fatigue, weight gain, trouble tolerating cold, constipation and dry skin, and because it develops slowly, many people do not notice for months or years. Women and people over 60 are at higher risk, and a TSH blood test is the usual check MedlinePlus.

A review of caffeine and performance found that low to moderate doses (roughly 40 to 300 mg) improve alertness, vigilance, attention and reaction time, with less consistent effects on memory and judgmentMclellan 2016. The lift is short-term, and it does not touch the cause of lasting fatigue.

For most adults the FDA cites 400 mg a day, about two to three 12-ounce cups of coffee, as an amount not generally linked to negative effects. Too much can cause a fast heart rate, palpitations, insomnia, anxiety, jitters and an upset stomach FDA. Late caffeine is a common hidden cause of next-day tiredness: see the sleep page for the six-hour rule. The coffee and green tea pages grade their other claims.

There is little trial evidence that any meal pattern raises energy in healthy adults, so the advice here is practical rather than graded. Long gaps without food, very low-calorie diets and meals built on refined starch alone leave many people flat by mid-afternoon; a meal with some protein, fiber and fat tends to carry you further. Malnutrition and eating disorders are on the MedlinePlus list of causes of fatigueMedlinePlus.

Fluid matters too. In a meta-analysis of 33 studies, dehydration impaired attention, executive function and coordination, and the effect was larger once people had lost more than 2% of body weight in waterWittbrodt 2018. Thirst and urine color are the simple guides; the hydration page has the amounts.

Ginseng, ashwagandha and similar herbs are widely sold for tiredness. Their grades are in the table on this page and on the Panax ginseng and ashwagandha pages. Energy drinks and "energy" supplements mostly deliver caffeine, sometimes in large or unlabeled amounts, so count them toward the 400 mg.