What is dietary fiber and how is it defined?

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In this "Ask Me Anything" (AMA) episode, Peter breaks down the science of dietary fiber, moving beyond the blanket advice to "eat more fiber" to uncover what it actually does in the body and where its benefits are truly supported by evidence. He explains how different types of fiber—soluble, insoluble, viscous, and fermentable—affect digestion, satiety, weight management, and glycemic control, and compares their impact to other, more potent metabolic tools. Peter also examines how certain fibers influence lipid metabolism and cardiovascular risk, evaluates the strength of evidence for fiber's role in colorectal cancer prevention, and highlights why some individuals may not tolerate specific fibers well. The discussion concludes with practical guidance on moving past generic fiber targets toward a more strategic and personalized approach that maximizes the true benefits of fiber.

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We discuss:

• Why it's time to re-examine the evidence behind dietary fiber recommendations [2:00];
• Why it's hard to isolate fiber's true effects on health: the limits of nutritional epidemiology [5:45];
• Defining dietary fiber: what it is, how it's digested, and why different types have different effects [8:15];
• Understanding fiber properties: how solubility, viscosity, and fermentability shape its effects in the body [11:15];
• Resistant starches explained: types, food sources, and how cooking and cooling influence their benefits [16:30];
• A framework for evaluating each of the major health claims linked to fiber [19:15];
• How fiber can support weight loss: mechanisms, realistic expectations, and its complementary role to broader dietary strategies [20:30];
• How fiber modestly improves glycemic control by reducing glucose spikes and insulin demand [26:15];
• How fiber modestly lowers LDL cholesterol and supports cardiovascular health [34:30];
• How fiber compares to other available tools and strategies for managing lipids, blood sugar, and weight [42:00];
• Fiber's role in colon cancer prevention: mechanisms, evidence, and limitations [45:30];
• Is fiber necessary for colon cancer prevention in otherwise healthy individuals? [53:30];
• Why some people have adverse reactions to certain types of fiber, and how to manage them [56:00];
• A general strategy for dietary fiber: combine multiple fiber types through whole foods and supplements [58:45];
• Why total fiber intake is more important than the ratio of soluble-to-insoluble fiber [1:02:45];
• The optimal timing and context for consuming fiber to maximize blood sugar control and metabolic benefits [1:05:00];
• How food processing affects the functional properties of fiber, the differences between supplement forms and natural sources, and why whole foods generally remain the best option [1:06:45];
• Fiber's potential to interfere with medication absorption [1:09:30];
• How to safely increase fiber intake: ramp up gradually and stay hydrated [1:12:00];
• Final takeaway on fiber: modest benefits, strong rationale, low downside [1:13:00];
• Peter's carve-out: lessons and inspiration from the Acquired podcast [1:14:30]; and
• More.

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