Recommendations on the adequacy of nutrient intake indicate that lactating mothers have higher nutritional needs than do pregnant mothers. High nutrient intake is necessary to help mothers recover after childbirth, produce milk, and maintain the quantity and quality of breast milk. It also prevents maternal malnutrition. Research has shown, however, that the dietary energy consumption of mothers during lactation was significantly lower than that during pregnancy. The current study explored the factors associated with decreased nutritional intake during maternal lactation. The study was conducted in March–April 2013, and the subjects were mothers with infants aged >6 months. Results revealed that the factors causing low dietary energy consumption among breastfeeding mothers were poor nutritional knowledge and attitude toward high energy intake requirements during lactation, lack of time to cook and eat because of infant care, reduced consumption of milk and supplements, dietary restrictions and prohibitions, and suboptimal advice from midwives/health personnel. Beginning from the antenatal care visit, health personnel should conduct effective counseling on the importance of nutrient intake during lactation. Advice should be provided not only to mothers, but also to their families to enable them to thoroughly support the mothers as they breastfeed their infants.