VITAMIN FORTIFICATION AND OBESITY PREVALENCE

Although there are few studies linking the increased prevalence of obesity to vitamin fortification, existing evidence suggests that high-risk populations are those who are most likely to have an increased intake of synthetic vitamins and decreased vitamin elimination, e.g., populations in fortified countries[6], individuals with low SES in developed countries[6-10] or with high SES in developing countries[11,12,55], formula-fed infants[15-17], and those who live in fortified countries with less rigorous physical activity[56-59].

The prevalence of obesity varies from country to country. It seems that this variation may be related to different food fortification policies and standards among countries. As shown in Table ​Table4,4, the ranking of countries according to their prevalence of child obesity is similar to the ranking by the fortification standards of B vitamins. Evidently, flour fortification prohibited countries have a low prevalence of obesity, while countries with high flour fortification standard have high rates of obesity. Over the past few decades, food fortification has spread from developed countries to developing countries[19]. Therefore, it is possible that the spread of obesity from developed countries to developing countries may reflect the time sequence of implementing food fortification with vitamins.

Implementation of a vitamin fortification policy in a country will surely cause a sudden nationwide increase in vitamin intake in a short period. The initiation of food fortification with B vitamins in the late 1930s-1940s and the update of fortification standards in the 1970s in developed countries led to three phases in the consumption of vitamin B1, B2 and niacin: a rapid increase in the 1940s, followed by a plateau period between the 1950s and the 1960s and a steep increase thereafter, as shown in Figure ​Figure2.2. Available evidence has suggested an association between these food events and the prevalence of obesity. Two birth cohort studies conducted in Switzerland[60] and Denmark[61] showed that there was a significant increase in the prevalence of being overweight and obesity which occurred mainly in the cohorts born in the 1930s and the 1940s and in the cohorts born in the late 1960s to the 1970s. A Fels longitudinal study also showed that the child obesity epidemic in the United States is a sudden event that started in the 1970s and the 1980s[62]. A similar phenomenon is also seen in Saudi Arabia. Saudi Arabia started wheat flour fortification in the 1970s[63]. Following its food system change, Saudi Arabia experienced a rapid increase in obesity rates in the 1980s and the 1990s, and its obesity rate in schoolboys sharply increased from 3.4% in 1988 to 24.5% in 2005[64]. Our ecological studies clearly showed that there are strong lagged correlations between United States per capita consumption of B vitamins (B1, B2 and niacin) and the prevalence of obesity and diabetes[25,26]. Figure ​Figure33 clearly shows that both the initiation of food fortification in the 1940s and the update of fortification standards in 1974 are followed by a sharp increase in diabetes prevalence. The update of fortification standards followed a sharp rise in obesity prevalence.

As mentioned above, low SES groups in developed countries but high SES groups in developing counties may have a high synthetic vitamin intake from fortified foods. This may explain the findings that obesity is more prevalent in low SES groups in developed countries[6-10] but in high SES groups in developing countries[10-12,55]. Formula-fed infants have a high vitamin intake. Studies have demonstrated that formula-fed infants have a higher plasma level of vitamins compared with human milk-fed infants[51-53]. It is known that formula feeding[65-67] and micronutrient-fortified human milk feeding[68,69] can lead to rapid infant weight gain, a known major risk factor for children developing obesity[70-72]. Therefore, excess vitamin intake may mediate the link between formula feeding and childhood obesity.

In most developed countries, the energy expenditure needed for daily life has decreased since the beginning of the 20th century because of increasing mechanization, urbanization, motorization and computerization[4]. However, it is only since the 1970s, when food fortification standards were dramatically increased, that obesity prevalence has risen substantially. Moreover, although formula feeding is associated with an increased risk for obesity[15-17], there is no evidence indicating that there is a decrease in energy expenditure in formula-fed infants compared with breast-fed infants[73,74]. Instead, evidence shows that formula-fed infants may have higher total daily energy expenditure[13,14]. These data suggest that increased B vitamin intake rather than decreased energy expenditure may play a major role in the development of obesity. On the other hand, many studies, especially those conducted in highly B vitamin fortified countries, such as the United States[56], Canada[57], Saudi Arabia[58] and Kuwait[59], found that moderate to vigorous physical activity is associated with a reduced risk of obesity. It is proposed that this association may involve increased elimination of vitamins through sweat because moderate to vigorous physical activity can increase the sweat rate[28]. We have demonstrated that excess nicotinamide can be rapidly removed through sweating[75]. Sweat-mediated elimination of nicotinamide may be a crucial factor in preventing nicotinamide toxicity because human kidneys hardly excrete nicotinamide due to the reabsorption of renal tubules[76]. Therefore, it is conceivable that under the same conditions of high vitamin intake, those individuals who live a life that inhibits the activity of sweat glands (e.g., physical inactivity) may be at greater risk of obesity. From this point of view, black people should be more sensitive to excess vitamins than whites, because the activity of sweat glands of blacks is lower than that of whites in the same temperature environment[77]. There is evidence showing that black women may have lower levels of physical activity than black men[78]. This may explain why obesity prevalence is greater in blacks, especially black women, than in whites in the United States[79,80]. Taken together, it may be concluded that food fortification-induced high intake of vitamins, especially B vitamins, may be responsible for the increased global prevalence of obesity.