The RDAs for thiamin in the USA were revised in 1998. The minimum suggested amounts for men are 1.2mg per day and 1.1mg for women and slightly more when pregnant or lactating. Thiamin supplements may be useful during times of stress, fever, diarrhea and during and after surgery. Many experts recommend as much as 100mg of thiamin per day for those who drink alcohol.

Raw freshwater fish and shellfish contain an enzyme which breaks down thiamin. This can happen during food storage and preparation or as food passes through the gut. Thus large intakes of raw fish and shellfish can increase the risk of thiamin deficiency. Drinking large quantities of tea and coffee may reduce thiamin absorption.

Deficiency symptoms are still seen in parts of the world where white rice makes up a major part of the daily diet. Those at greatest risk of deficiency include some young children and teenagers, stressed adults, those who exercise very heavily, alcoholics, pregnant women, those on fad diets and people suffering from malabsorption diseases, who are not supplemented with any B1. Marginal deficiencies, without clinical symptoms, may be common among these groups.

Elderly people are also at risk of thiamin deficiency and this may lead to reduced mental functioning, depression, weakness, suppressed immunity and gastrointestinal problems. Early thiamin deficiency may be easily overlooked as the symptoms are generalized and can include fatigue, depression and stress-induced headaches.

Severe thiamin deficiency causes beriberi. Beriberi can affect the cardiovascular system (wet beriberi) and the nervous system (dry beriberi). One of the earliest signs of thiamin deficiency is reduced stamina. Depression, irritability and reduced ability to concentrate are later followed by fatigue, muscle cramps and various pains. Dry beriberi symptoms include numbness and tingling in the toes and feet, stiffness of the ankles, cramping pains in the legs, difficulty walking, and finally, paralysis of the legs with wasting of the muscles. Permanent damage to the nervous system can occur if the deficiency is not corrected in time. Thiamin deficiency may also be associated with reduced tolerance to pain.

Thiamin deficiency can also lead to nausea, lack of appetite, weight loss and constipation. Carbohydrate digestion and the metabolism of glucose are diminished. In the advanced stages of thiamin deficiency, the symptoms of wet beriberi include heart enlargement. Symptoms of cardiac failure such as breathlessness, ankle swelling and fatigue may follow. Marginal thiamin deficiency may contribute to heart disease.