Why We Eat: The Science of Appetite, Nutrition and Weight - Prof Alexandra Johnstone
Professor Alexandra Johnstone joins Dr James McIlroy to explore what makes us hungry, what helps us feel full, and why healthier choices are rarely a matter of willpower alone.
A conversation on chrononutrition, protein, food inequality, GLP-1 medicines and the future of public health.
The big idea: Our bodies, habits and surroundings all influence how we eat. Better health, therefore, needs more than individual resolve: it needs practical advice, supportive environments and policies that make nourishing food easier to choose.
00:00 Meet Professor Alex Johnson
Professor Alexandra Johnstone, Chair of Nutrition at the University of Aberdeen, joins Dr James McIlroy in this episode of Inside Matters to discuss appetite, obesity, food inequality and the future of nutrition science.
01:53 Breakfast and Chrononutrition
Is breakfast really the most important meal of the day? Professor Johnstone’s answer is an emphatic yes—but not because breakfast has any magical property. Her research focuses on chrononutrition: the relationship between what we eat, when we eat it and the body’s roughly 24-hour circadian rhythms.
The timing of the largest meal may not dramatically alter how much energy the body burns, she explains, but it can change how eating feels. A larger breakfast appears to help people feel fuller and may make a calorie-reduced diet easier to follow. The body is also more sensitive to insulin earlier in the day. Insulin helps regulate blood glucose, so shifting more of the day’s food towards the morning may suit our physiology better than saving the biggest meal for late evening.
05:13 Fasting and Eating Windows
There is no universal definition of breakfast time. A useful way to think about it is the first meal within two or three hours of waking—an approach that also makes sense for shift workers. Context matters: rotating shifts can disrupt circadian rhythms, and someone who eats late at night may simply not feel hungry first thing the next morning.
Fasting is a tool, not a loophole. Intermittent fasting and time-restricted eating can be useful for some people, but Professor Johnstone stresses that they are behavioural strategies rather than shortcuts around energy balance. Counting hours, calories or points can all reduce overall intake; the best approach is one a person can realistically sustain.
Long fasts can make some people think about food more, not less. If someone chooses a restricted eating window, she would favour including the morning rather than routinely skipping breakfast. Her practical summary borrows an old phrase: eat breakfast like a king and dine like a pauper. It is not a rigid rule, but a reminder that front-loading food may support appetite control.
07:59 What Makes a Healthy Breakfast
The UK has broad healthy-eating guidance, but little specific public-health advice on breakfast or on eating well while working shifts. That leaves plenty of room for confusion. A traditional cooked breakfast can include nutritious ingredients such as eggs, tomatoes and mushrooms, but bacon, sausages, black pudding and fried accompaniments can also bring substantial salt, saturated fat and processed meat.
Breakfast cereal can play a useful role, especially for children, because many products are fortified with vitamins and minerals. The important distinction is between everyday choices and treats: high-sugar cereals need not be banned, but they are better treated as occasional foods rather than the default every morning.
10:42 Hunger vs Appetite Explained
One of the conversation’s most helpful distinctions is between hunger and appetite. Hunger is the body’s cue to start looking for food. We learn to recognise it through sensations such as a rumbling stomach, mild nausea or a dip in energy. Appetite describes what happens next: what we want, how much we choose and whether we are drawn to something sweet, salty or comforting.
Biology is part of the picture. Gut hormones, including ghrelin and GLP-1, help signal hunger and fullness, and rare genetic conditions can produce extreme, persistent hunger. Yet food choices are also shaped by stress, emotion, menstrual phase, routines, culture and opportunity. The same stressful event may lead one person to eat more and another to eat less. Sometimes we eat early because a busy schedule tells us there will be no chance later.
This is why reducing appetite to ‘willpower’ is so misleading. People differ in their biological signals, learned responses and daily circumstances. Fullness—the counterweight to hunger—matters just as much, and the composition of a meal can influence how long that fullness lasts.
15:49 Protein and Satiety Science
Why is protein filling? Protein is generally the most satiating of the energy-providing nutrients. In Professor Johnstone’s weight-loss research, higher-protein diets often draw on varied sources, including poultry, red meat, peas, beans and pulses. The aim is not simply to add protein on top of everything else, but to use it within an overall eating pattern that supports fullness and nutrition.
There is no single explanation for the protein’s effect. Proteins are broken down into amino acids; some of these influence the brain, while digestion affects stomach emptying and gut hormones such as GLP-1. These mechanisms overlap, like circles in a Venn diagram, to tell the body that it has eaten enough.
18:20 Obesity Paradox and Poverty
The obesity paradox: when calories are cheap but nourishment is not. Obesity is patterned by inequality. People on low incomes may have access to plenty of calories while struggling to afford foods rich in fibre, protein, vitamins and minerals. This is sometimes called the obesity paradox: food insecurity can coexist with overweight and obesity because the cheapest, most heavily promoted options are often energy-dense and nutritionally poor.
Professor Johnstone is clear that stigma is not the answer. Telling people to make better choices ignores the price of food, disposable income, transport, time, retail promotions, and the pressures families face.
The first 1,000 days of a child’s life are especially important, making support during pregnancy, maternity leave, and infancy a public-health issue as well as a household one.
19:53 Fixing the Food Environment
Changing the environment changes behaviour. We live in what researchers call an obesogenic environment: food is available around the clock, inexpensive high-fat, high-sugar and high-salt products are heavily promoted, and tempting items are positioned where people must pass them.
Restrictions on prominent placement and promotions do not remove choice. A person can still buy chocolate, but it need not be placed directly in front of them at the till. Professor Johnstone argues that these population-level changes can reduce purchases at scale while leaving room for pleasure and occasional treats.
23:11 Gut Brain Axis and Weight Defence
Professor Johnstone’s work on food insecurity and obesity brings researchers together with retailers and policymakers to ask a more constructive question: how can the food system make healthier choices affordable, accessible and environmentally sustainable?
27:06 Obesogenic Retail and Policy Changes
The discussion continues on how retail environments and policy can support healthier defaults without eliminating pleasure.
31:16 Alcohol Mediterranean Diet and Blue Zones
Alcohol, Mediterranean patterns and the value of connection. Alcohol provides energy and can stimulate appetite, but it is not required for a healthy diet. Red wine often appears in descriptions of the Mediterranean diet, alongside vegetables, fruit, whole grains, olive oil, physical activity and shared meals. It should not be treated as the ingredient responsible for longevity.
The discussion of so-called Blue Zones makes a broader point: health is social. Genetics, physical activity and diet all matter, but so do belonging, purpose and eating with other people.
38:05 Why Protein Fills You Up
(Continued from earlier) Whole-food form matters too. In one study recalled during the conversation, meat was more satiating than the equivalent free amino acids. Solid protein also tends to be more filling than liquid protein because it remains in the stomach longer.
38:52 Protein Satiety Mechanisms
Proteins influence the brain and gut hormones, working together for satiety.
39:32 Amino Acids vs Whole Protein
Whole foods outperform isolated amino acids for satiety.
40:48 Liquid Meal Replacements
A shake can be convenient, especially on a hectic day, but convenience is not the same as a complete replacement for the enjoyment, texture, culture and social meaning of food.
43:49 Ultra Processed Food Debate
Ultra-processed food: a debate that needs more evidence. Ultra-processed food is an emotive subject, and the scientific picture is more nuanced than many headlines suggest. There is no universally accepted definition, although the NOVA system is the most widely used. Such foods make up a large share of UK energy intake and are associated with weight gain.
The unresolved question is why. Controlled human trials remain limited, so Professor Johnstone resists claiming that the mechanism is settled. Additives and preservatives need context too—they can improve safety and reduce waste.
50:41 Scotland Diet Reality Check
The conversation addresses real-world dietary patterns in Scotland and beyond.
52:37 Red Meat and Cancer Risk
Discussion of evidence around red and processed meats.
53:47 GLP-1 Weight Loss Challenges
GLP-1 medicines: powerful treatment, unanswered questions. GLP-1-based medicines have transformed the treatment of obesity by reducing appetite and supporting substantial weight loss. Yet they are tools, not a cure. Weight regain is common after stopping.
The idea of long-term low-dose or ‘microdosed’ treatment is attracting attention, but human evidence is not yet sufficient. Access is another concern: medicines could widen health inequalities.
There is also a nutritional challenge. People taking appetite-suppressing medicines eat much less, making the quality of what they do eat more important. Adequate protein and resistance exercise can help protect muscle.
55:07 Microdosing and Inequality
Access and equity issues around long-term GLP-1 use.
01:04:19 Muscle Loss and Nutrition
Protecting muscle mass during weight loss, especially with medications.
01:05:50 Fibre Deep Dive
Fibre does a different—but equally important—job. If protein is the headline nutrient for fullness, fibre deserves equal attention for gut health. Most people in the UK do not eat enough of it. Professor Johnstone’s recent comparison showed protein performed better for satiety, while fibre produced stronger indicators of gut health. A healthy pattern should include both.
01:08:21 Sweeteners and Microbiome
Her work also explores how fibre and different non-nutritive sweeteners may affect the gut microbiome.
01:10:44 Future Vision and Wrap Up
A practical takeaway: build patterns, not perfection. The science of nutrition rarely delivers one universal menu. A sensible starting pattern is to eat earlier when it suits your routine; make the first meal substantial enough to prevent constant grazing; include solid sources of protein and fibre; keep sugary cereals, processed meats and alcohol as occasional rather than automatic choices; and pay attention to the quality of food when appetite is reduced.
Just as importantly, avoid turning food into a moral test. Hunger is biological and learned. Appetite is personal and contextual. None of this removes individual agency, but it explains why kinder, more realistic support works better than blame.
What comes next. Professor Johnstone would like to see stronger dietary guidance for shift workers, updated protein recommendations that support healthy ageing, and clearer evidence on ultra-processed foods. The future of nutrition will combine better medicines with better food systems.
