Gut health has become a major focus in nutrition, medicine, and consumer wellness, but the term is often used loosely. Claims about microbiome testing, probiotics, fermented foods, fibre, and "boosting" the gut microbiome are now common, yet the evidence behind these claims varies substantially.

In this episode, Dr. Emily Leeming examines what gut health actually refers to, why it cannot be reduced to the microbiome alone, and where current microbiome science is being applied before it is ready. The discussion covers the limits of commercial stool testing, the difficulty of defining a healthy microbiome, and the practical strategies most strongly supported by current evidence.

• [02:48] Interview start
• [04:17] Defining gut health
• [09:03] What is a "healthy microbiome"?
• [15:25] Microbiome testing - any clinical utility?
• [24:08] Interpreting microbiome studies
• [34:39] "30 plants a week" is not evidence-based
• [39:53] Serotonin and gut brain
• [45:34] Fiber research frontier

Links/Resources:

• Go to episode page (w/ links to mentioned studies)
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• Enroll in the next cohort of our Applied Nutrition Literacy course
• Dr. Leeming's newsletter: Second Brain

In this episode, we examine what nutrition can realistically do in the condition historically known as PCOS, now renamed polyendocrine metabolic ovarian syndrome, or PMOS.

We begin by explaining why the name change matters: the condition is not defined by ovarian cysts, but is better understood as a broader endocrine-metabolic and ovarian syndrome involving insulin resistance, androgen excess, ovulatory dysfunction, metabolic risk, and psychological burden.

We then assess the nutrition evidence, including energy restriction, weight loss, carbohydrate quality, glycaemic index and load, protein intake, fat quality, appetite regulation, fertility outcomes, and phenotype differences. Rather than seeking a single "PCOS diet", the episode asks which dietary features may plausibly help, how strong the evidence is, and where uncertainty remains.

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Body composition goals, particularly bodyfat loss, are among the most common reasons people seek support from a nutritionist or health and fitness professional. While the principles are well established, the challenge is helping individuals apply them consistently in real-world conditions.

Many people struggle due to hunger, unrealistic expectations, emotional eating, inconsistent routines, or overly restrictive dieting approaches. These challenges can make fat loss difficult to sustain, even when someone understands what they "should" be doing.

In this episode, Luke Hanna discusses practical strategies for improving body composition, including food diaries, energy-density manipulation, preloads, mindful eating, and realistic goal-setting. The discussion emphasizes identifying individual barriers, collaborating with clients, and building repeatable behaviours that support both fat loss and long-term maintenance.

Luke Hanna holds a Master's degree in Obesity and Clinical Nutrition from University College London and a degree in Sport and Exercise Science from the University of Portsmouth. He currently works as a nutrition coach and personal trainer.

Timestamps:

• [03:15] Interview
• [05:39] Client assessment basics
• [11:59] Alternatives to tracking
• [13:57] Volume eating
• [18:56] Preloads before meals
• [22:25] Snacking and hunger types
• [26:44] Habits and food environment
• [30:40] Managing expectations
• [33:51] Transition to maintenance
• [39:09] Key ideas (premium-only)

Links:

• Go to episode page (with resources)
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• Subscribe to Sigma Nutrition Premium
• Instagram: @lukehannanutrition

Fasting, nutrient timing, chrono-nutrition, and continuous glucose monitoring are all topics that have generated substantial interest, but they are also areas where exaggerated claims can easily outpace the underlying evidence.

In many cases, tentative hypotheses are presented as if they were already well-established conclusions, despite the fact that the research base is often more mixed and context-dependent than popular narratives imply. It is one thing for an idea to appear biologically coherent. It is another for that idea to translate into meaningful, reliable effects in real-world interventions.

In this episode, Professor James Betts discusses how to think clearly about these topics, why common errors in interpretation can lead to overstated conclusions, and what is required to properly evaluate whether an observed effect reflects a true intervention effect rather than baseline differences, inappropriate comparisons, within-group changes, or mechanistic signals being mistaken for meaningful health outcomes.

Timestamps:

• [04:24] Background into Prof. Betts' research
• [07:28] Evidence in fasting research over past 5-6 years
• [10:15] Hype vs evidence in intermittent fasting
• [16:44] Spotting spin in study conclusions
• [17:31] Common statistical red flags
• [24:45] Methods matter in fasting trials
• [31:10] Exercise nutrient timing
• [38:32] CGMs what they measure, misuse and patterns
• [53:59] Key ideas (premium-only)

Links:

• Go to episode page & resources (study links, bio, etc.)
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• Enroll in the next cohort of our Applied Nutrition Literacy course

How should we decide what counts as trustworthy evidence? Scientific rigor is not a single characteristic of a study, but a chain of decisions made from the moment a question is conceived to the point at which findings are communicated to the public.

Errors can occur at every stage: the question may be ill-posed, the design may be incapable of answering it, the measurements may be weak, the analysis may be inappropriate, the interpretation may overreach, and the public-facing communication may become distorted.

In this episode, Dr. David Allison, PhD discusses the deeper methodological issues that shape the field's conclusions. The discussion moves from the philosophy of scientific inquiry to the practical realities of study design, statistical analysis, interpretation, and dissemination.

• [03:30] Interview start
• [06:17] What is true scientific rigor?
• [10:06] Study design and analysis problems in nutrition
• [12:56] The DINS error
• [14:14] Conflation of heterogeneity in response vs. in outcomes
• [17:31] Misunderstanding of p-values and hypothesis testing
• [27:01] Incorrect labelling of "responders" and "non-responders"
• [34:49] Errors related to analysis of secondary outcomes
• [45:01] How can nutrition science improve as a field?
• [51:30] Key ideas segment (Premium-only)

Links:

• Go to episode page (with list of episode resources)
• Subscribe to Sigma Nutrition Premium
• Enroll in the next cohort of our Applied Nutrition Literacy course

Dietary fiber is widely recognized as an important component of a healthy diet, yet it is not typically classified as an essential nutrient. In this episode, Dr. Andrew Reynolds explores whether that distinction still holds, arguing that the traditional criteria used to define essentiality may be outdated when applied to modern nutrition science.

The discussion moves beyond simply acknowledging the benefits of fiber and instead examines whether it meets the foundational requirements of an essential nutrient. This includes considering its physiological roles, the body's inability to synthesize it in sufficient quantities, and whether low intake leads to a meaningful and reversible dysfunction.

Drawing on evidence from prospective cohort studies, randomized controlled trials, and mechanistic research, Reynolds outlines the strength of the evidence linking higher fiber intakes to reduced risk of cardiovascular disease, type 2 diabetes, colorectal cancer, and premature mortality.

Reynolds presents a compelling case that fiber may play a fundamental role in maintaining normal physiological function and therefore warrants reconsideration within the framework of essential nutrients.

• [03:50] Interview starts
• [05:53] Understanding essentiality
• [09:26] Could there be a deficiency-state for fiber?
• [15:38] What are fiber guidelines based on?
• [23:52] Fiber and chronic disease risk: dose-response
• [28:59] Different types of fiber
• [37:21] Fermentation and SCFAs
• [42:55] Research priorities ahead
• [50:04] Low fiber health risks
• [58:02] Key Ideas segment (Premium-only)

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• Enroll in the next cohort of our Applied Nutrition Literacy course
• Reynolds et al., 2026 – Dietary fibre as an essential nutrient:
• Reynolds et al., 2019 – Carbohydrate quality and human health: a series of systematic reviews and meta-analyses
• Episode 482: Carbohydrate Quality & Health – Andrew Reynolds, PhD

Avoidant/Restrictive Food Intake Disorder (ARFID) is an eating disorder diagnosis characterized by a persistent restriction or avoidance of food intake that results in clinically significant consequences (medical, nutritional, and/or psychosocial), but without the weight- and shape-driven psychopathology typical of anorexia nervosa and bulimia nervosa.

In this episode, Megan Hellner and Katherine Hill outline how ARFID presents across the lifespan, why it is frequently missed in routine healthcare, and what an evidence-informed assessment and treatment pathway can look like in practice.

A central theme is that ARFID is not synonymous with "picky eating" and not confined to any one body size. Patients may present at any point on the weight chart, including those who are weight-stable or in larger bodies, and the condition can begin in early childhood and persist into adulthood.

The episode also highlights ARFID in athletes and physically active people, where restricted dietary variety and/or low intake can contribute to low energy availability and RED-S-like presentations, sometimes without an obvious intent to lose weight.

• [03:48] Interview start
• [06:23] What is ARFID? DSM-5 definition vs "picky eating"
• [09:36] Clinical red flags: when restriction becomes a disorder
• [11:37] ARFID isn't always underweight: missed cases & diagnostic pitfalls
• [16:46] ARFID presentation profiles: low interest, sensory sensitivity, fear
• [18:59] Comorbidities & nutrition consequences
• [25:16] Evidence-based ARFID treatment
• [29:16] How to expand foods without pressure
• [32:28] Weight restoration, stabilization, and long-term maintenance
• [35:44] What research still needs
• [38:16] Differential diagnosis & referral

• Go to episode page (with links to papers and ARFID resources)
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This episode examines what we actually know (and importantly, what we do not know) about diet in relation to gallstones and gallbladder conditions. Much of the public-facing guidance around gallstones focuses on "avoiding fatty foods", yet Dr. Angela Madden explains that this long-standing practice sits on surprisingly weak direct evidence, particularly when judged against the standards typically expected for clinical dietary recommendations.

A central theme is the need to separate two distinct questions: dietary factors that influence the risk of developing gallstones (prevention), versus dietary strategies intended to reduce symptoms or complications once gallstones exist (management). While the prevention literature suggests plausible, consistent associations with overall diet quality and lifestyle factors, the specific question of prescribing a low-fat diet to manage symptomatic gallstones lacks robust randomized trial evidence.

Dr Angela Madden is a clinical researcher in nutrition and dietetics at the University of Hertfordshire, where she established and led the nutrition and dietetics subject group and now focuses her research on improving nutritional assessment, dietary interventions, and patient-centred outcomes in clinical and public health settings.

• [02:09] Discussion with Dr. Angela Madden begins
• [06:53] Understanding the gallbladder
• [08:08] Gallbladder disorders and their prevalence
• [13:42] Risk factors and pathophysiology
• [22:15] Dietary factors and gallstone formation
• [27:20] Exploring dietary fat and gallstones
• [34:09] Broader dietary considerations
• [45:44] Practical dietary recommendations

• Go to episode page
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• Enroll in the next cohort of our Applied Nutrition Literacy course
• Dr. Madden's univeristy page
• Cochrane Review: Madden et al., 2024 – Modified dietary fat intake for treatment of gallstone disease in people of any age
• Related episode: 513: Kidney Stones & Diet – Deepa Kariyawasam, RD