In an analysis of the phytosterol composition of nuts and seeds commonly consumed in the United States, it was found that sesame seeds had the highest total phytosterol content (400−413 mg/100 g). Phytosterols are plant components that have a chemical structure similar to cholesterol except for the addition of an extra methyl or ethyl group. Phytosterols reduce cholesterol absorption, which assists in the reduction of circulating levels of cholesterol.

A systematic review investigated the efficacy of phytosterols in lowering lipid concentration in individuals with non-familial high blood cholesterol. Randomized controlled intervention trials were identified through selected international journal databases and reference lists of relevant publications. The results of the systematic review indicated that phytosterols could significantly decrease LDL cholesterol, total cholesterol and triacylglycerol in treatment groups compared with control groups. Foods enriched with 2.0 g of phytosterols per day had a significant cholesterol lowering effect.

Sesamin is a major lignan constituent of sesame and possesses multiple functions such as antihypertensive, cholesterol lowering, lipid lowering and anti cancer activities.

The effects of sesamin on various aspects of cholesterol metabolism were examined in rats maintained on various dietary regimens. When given at a dietary level of 0.5% for 4 weeks, sesamin reduced the concentration of serum and liver cholesterol significantly irrespective of the presence or absence of cholesterol in the diet.

In a randomized, placebo-controlled, crossover study of 26 postmenopausal women, half of them consumed 50 g sesame seed powder daily for 5 wk, followed by a 3-wk washout period, then a 5-wk 50-g rice powder placebo period. The other half received the 2 supplements in reverse order. After sesame treatment, plasma total cholesterol, LDL cholesterol and the ratio of LDL cholesterol to HDL cholesterol decreased significantly by 5, 10 and 6%, respectively. The ratio of alpha- and gamma-tocopherol to total cholesterol increased significantly by 18 and 73%, respectively. 

Some of the nutrients found in sesame seeds are manganese, copper, calcium, magnesium iron, phosphorus, vitamin B1 (thiamin), vitamin E, zinc, dietary fiber and monosaturated fats. The vitamin E found in sesame seeds is mostly gamma-tocopherol.  Gamma-tocopherol is the major form of vitamin E in many plant seeds and in the US diet and alpha-tocopherol is the primary form of vitamin E in supplements. Better absorption of all the nutrients is attained if the seeds are ground or pulverized before consuming. Tahini is a paste used in food made from finely ground sesame seeds.
References:
1. Katherine M. Phillips, et al. Phytosterol Composition of Nuts and Seeds Commonly Consumed in the United States. J. Agric. Food Chem., 2005, 53 (24), pp 9436–9445. DOI: 10.1021/jf051505h.
2. Wu T, Fu J, Yang Y, Zhang L, Han J. The effects of phytosterols/stanols on blood lipid profiles: a systematic review with meta-analysis. Asia Pac J Clin Nutr. 2009;18(2):179-86. PMID: 19713176.
3. Hirose N, et al. Inhibition of cholesterol absorption and synthesis in rats by sesamin. J Lipid Res. 1991 Apr;32(4):629-38. PMID: 1856608.
4. Yokota T, et al. Sesamin, a lignan of sesame, down-regulates cyclin D1 protein expression in human tumor cells. Cancer Sci. 2007 Sep;98(9):1447-53. Epub 2007 Jul 19. PMID: 17640297.
5. Yamashita K, Ikeda S, Obayashi M. Comparative effects of flaxseed and sesame seed on vitamin E and cholesterol levels in rats. Lipids. 2003 Dec;38(12):1249-55. PMID: 14870927.
6. Jiang Q, Christen S, Shigenaga MK, Ames BN. gamma-tocopherol, the major form of vitamin E in the US diet, deserves more attention. Am J Clin Nutr. 2001 Dec;74(6):714-22. PMID: 11722951.
7. Wu WH, Kang YP, Wang NH, Jou HJ, Wang TA. Sesame ingestion affects sex hormones, antioxidant status, and blood lipids in postmenopausal women. J Nutr. 2006 May;136(5):1270-5. PMID: 16614415.