Can vitamin K reduce body fat accumulation in postmenopausal women?

Obesity rates in the United States are now in excess of 36% and obesity remains one of the leading risk factors for heart disease^[1], stroke^[1], and diabetes. While weight gain and the rise in obesity affects all age groups, the increase in weight amongst middle-aged and postmenopausal women is of particular note. In 2002, women aged 50 to 59 years old were, on average, approximately 23 pounds^[2] heavier than they were in 1960. This compares to roughly 17 pounds for women aged 40-49 years old and 9 pounds for women over 60. Therefore, interventions that address weight gain in middle-aged, postmenopausal women are of specific importance for tempering the rise in obesity rates.

Hormonal changes that occur during menopause (e.g.,decreases in estrogen^[3]) have been shown to alter bone metabolism, resulting in lower bone mineral content in postmenopausal women. It has been hypothesized that changes in the metabolism of the bone-related protein osteocalcin may be responsible for at least some of the weight gain observed in postmenopausal women due to its potential role in energy metabolism^[4][5].

Osteocalcin is a protein found in bone tissue and is secreted by osteoblasts. Osteocalcin’s function is regulated by a process known as carboxylation, the chemical process of adding a carboxyl group to an amino acid. The metabolic (i.e. fat and glucose) roles of osteocalcin are largely dependent on its carboxylation status^[6], which is shown in Figure 1 and largely regulated by vitamin K. This is why it has been hypothesized that vitamin K, which acts as a carboxylating molecule, might be largely responsible for maintaining adequate levels of carboxylated osteocalcin and therefore maintaining normal fat metabolism in postmenopausal women. In small intervention studies, supplementation of vitamin K has been shown to increase osteocalcin carboxylation^[7] and decrease levels of decarboxylated osteocalcin^[8]. Therefore, the purpose of this study was to test the hypothesis that long term supplementation of vitamin K would result in lower body fat, or influence fat distribution.

Postmenopausal women are a high-risk group for both weight gain and decreases in bone mass. A bone-related protein, osteocalcin, may be a key regulator of fat and glucose metabolism and its function is regulated by its carboxylation status. Vitamin K supplementation has previously been shown to improve osteocalcin carboxylation. The study under review therefore sought to test whether vitamin K supplementation would improve fat and glucose metabolism and reduce body fat accumulation in postmenopausal women.