THAT is a good question! To answer that we need to look at a few different things. First is the digestive process itself. When we eat food and the food passes from the stomach to the small intestine bile is added to the mix. Bile emulsifies fatty acids in the food, which sets up the whole process of fatty acid digestion.
NOW! The point of the Oatmeal is that it binds to cholesterol (both dietary and cholesterol that is part of the bile salts) and pulls this cholesterol out of the body along with the feces (sorry for the graphic nature...just how it works). So total and LDL cholesterol are reduced because some of it is being removed from the body.
Oatmeal is not alone in this effect however! Fruit, veggies, nuts and seeds also have this effect. Oatmeal just has quite a bit more fiber than most grains and thus has a greater effect than say pearled barley or brown rice. Regarding the insulin, Oats are fairly low glycemic index so if quantities are tiny they do not pose a terrible problem with regards to insulin spiking and deranged cholesterol....but here is where it gets fun!
The first study below is a meta analysis (a combination of many studies which tends to solidify the findings statistically) which shows one receives only a slight decrease in cholesterol levels and this effect is not enhanced by increased oat consumption.
The second study shows that if the oats are processed and or have sugar added to them they pose significant insulin load as evidenced by the lack of fat loss as compared to un-processed oats EVEN WHILE CALORIE RESTRICTED! The last study really zeros in on the insulin question however in that no change is seen in plasma triglyceride levels. Triglycerides are an excellent indicator of insulin resistance and any nutritional intervention that does not lower triglycerides is highly suspect, as it will not reduce inflammatory cytokines associated with elevated insulin levels.
So...the whole "Oats lowers cholesterol" thing is true...kind of, but this was some grasping at straws by folks back in the cholesterol=heart disease days. As Ufe Ravanskove points out in the cholesterol myths: http://www.ravnskov.nu/cholesterol.htm
Dietary cholesterol levels are not correlative OR Causative with regards to heart disease. Now if we start talking C-reactive protein, apolo-protien-a.... dental organism infestation...now we are talking about some correlative and causative aspects of heart disease.
The real question here is: Does eating Oats favorably effect your health? That is a very simple "NO".
Cholesterol-lowering effects of dietary fiber: a meta-analysis. Brown L, Rosner B, Willett WW, Sacks FM. Department of Nutrition, Harvard School of Public Health, Boston, MA 02115, USA.
BACKGROUND: The effects of dietary soluble fibers on blood cholesterol are uncertain.
OBJECTIVE: This meta-analysis of 67 controlled trials was performed to quantify the cholesterol-lowering effect of major dietary fibers.
DESIGN: Least-squares regression analyses were used to test the effect on blood lipids of pectin, oat bran, guar gum, and psyllium. Independent variables were type and amount of soluble fiber, initial cholesterol concentration, and other important study characteristics.
RESULTS: Soluble fiber, 2-10 g/d, was associated with small but significant decreases in total cholesterol [-0.045 mmol L(-1).g soluble fiber(-1) (95% CI: -0.054, -0.035)] and LDL cholesterol [-0.057 mmol.L(-1).g(-1) (95% CI: -0.070, -0.044)]. The effects on plasma lipids of soluble fiber from oat, psyllium, or pectin were not significantly different. We were unable to compare effects of guar because of the limited number of studies using 2-10 g/d. Triacylglycerols and HDL cholesterol were not significantly influenced by soluble fiber. Lipid changes were independent of study design, treatment length, and background dietary fat content.
CONCLUSIONS: Various soluble fibers reduce total and LDL cholesterol by similar amounts. The effect is small within the practical range of intake. For example, 3 g soluble fiber from oats (3 servings of oatmeal, 28 g each) can decrease total and LDL cholesterol by approximately 0.13 mmol/L. Increasing soluble fiber can make only a small contribution to dietary therapy to lower cholesterol.
Effect of dietary carbohydrate source on the development of obesity in agouti transgenic mice. Morris KL, Zemel MB. Department of Nutrition, University of Tennessee, Room 229 Jessie Harris Building, 1215 West Cumberland Avenue, Knoxville, TN 37996, USA.
OBJECTIVES: Our objective was to evaluate the effects of a qualitative change in dietary carbohydrate source on body weight and adiposity in a rodent model of diet-induced obesity.
RESEARCH METHODS AND PROCEDURES: We evaluated the effects of high-fat diets (basal) varying in carbohydrate source in aP2-agouti transgenic mice. In the ad libitum study, animals were given free access to the basal diet or one of four test diets for 6 weeks. In two of the diets, dietary carbohydrate was derived from a single source: mung bean noodles (MUNG) or rolled oats (ROLL). The remaining diets were designed to mimic commercially available instant oatmeal with added sugar (IO-S) or flavored instant oatmeal (IO-F). In the energy-restricted study, animals were given ad libitum access to the basal diet for 6 weeks. Subsequently, animals were assigned to one of six treatment groups for 6 weeks. One group was continued on the basal diet ad libitum. The remaining groups were maintained with energy restriction (70% ad libitum) on either the basal, MUNG, ROLL, IO-S, or IO-F diet.
RESULTS: Subcutaneous fat pad mass was significantly higher (p<0.05) in the energy-restricted basal and IO-S groups compared with the energy-restricted ROLL diet. Similarly, visceral fat pad mass was significantly lower with ROLL and MUNG diets (p<0.05 for both) compared with basal and IO-S diets, and the insulin:glucose ratio was reduced (by 23% to 34%, p<0.05) in these two diets compared with all others. In ad libitum-fed animals, liver fatty acid synthase expression was 43% to 62% lower (p<0.05) with ROLL and MUNG diets compared with all others. DISCUSSION: These data suggest that a qualitative change in dietary carbohydrate source modulates body weight and adiposity. 1: Am J Clin Nutr. 1981 Oct;34(10):2061-7. Related Articles, Links The effect of rolled oats on blood lipids and fecal steroid excretion in man. Judd PA, Truswell AS. Rolled oats (125 g daily) were substituted for breakfast cereals and wheat flour in the metabolically controlled diets of 10 subjects for 3 wk. Fat and energy intakes in the 2-wk control periods before and after the oat period were adjusted by addition of an oil with a similar fatty acid composition to the lipid in the oats. Plasma total cholesterol concentrations were reduced in seven of 10 subjects, but over the whole group the mean reduction of 8% was not significant (0.05 less than p less than 0.01). High-density lipoprotein cholesterol concentrations and plasma triglyceride levels were unchanged. Fecal fat excretion was increased by 47% (p less 0.005) and fecal bile acid excretion by 35% (p less than 0.01) but neutral steroid excretion was unchanged on the oatmeal diet. PMID: 6270999 [PubMed - indexed for MEDLINE]