Understanding carbohydrates, protein and fibre: how they affect hunger, energy and body weight
One of the most important studies examining this question is the DIETFITS trial, a large, well-designed clinical trial published in JAMA in 2018.
What the science says about healthy weight loss
The study included 609 adults aged 18–50 years with overweight or obesity (BMI 28–40 kg/m²), of whom 57% were women. Participants were randomly assigned to follow either a healthy low-fat diet or a healthy low-carbohydrate diet and took part in a 12-month programme that included health education and behavioural change support to help them adopt healthier eating habits.
The main aim of the study was to compare weight loss after 12 months and to investigate whether certain factors, such as genetic makeup or baseline insulin secretion, could predict which diet would work best for different individuals.
The low-fat group consumed substantially more carbohydrate, with carbohydrates providing around 50% of their daily calorie intake. If carbohydrates were the primary driver of overeating and weight gain, we would expect this group to gain weight or lose less weight than the low-carbohydrate group. However, this was not the case.
Participants in both groups naturally reduced their calorie intake without being instructed to count calories or deliberately restrict food. Both diets focused on improving food quality and reducing highly processed foods. As a result, participants in both groups lost a similar amount of weight, averaging around 5–6 kg over the 12 months. Although there was a small numerical difference favouring the low-carbohydrate group, this was not statistically significant.
Importantly, the researchers found that neither genetic differences nor baseline insulin secretion predicted which diet would be more effective for weight loss. In other words, these factors did not help identify who would do better on a low-fat or a low-carbohydrate diet.
The findings suggest that there is no single diet that is universally superior for weight loss. Rather than focusing solely on the amount of carbohydrate or fat in the diet, success appears to depend more on improving food quality, reducing calorie intake naturally through more filling foods, and finding a healthy eating pattern that can be sustained long term.
For most people, the best diet is not necessarily the lowest in carbohydrate or fat, but the one that helps them eat well, control hunger, maintain a healthy calorie intake, and achieve lasting weight loss.
Insulin sensitivity is influenced far more by body fat than by the exact amount of carbohydrate or fat in the diet. When people lose weight, particularly harmful fat stored around the organs (visceral fat) and in the liver and muscles (ectopic fat), insulin sensitivity usually improves. Research shows that if body weight remains stable, changing the proportion of carbohydrates and fats in the diet has little effect on insulin sensitivity. Conversely, gaining weight on any type of diet can increase insulin resistance.
The most effective eating pattern is therefore one that helps you maintain a healthy weight or lose excess body fat in a sustainable way. This may be lower in carbohydrates, higher in carbohydrates, or somewhere in between, depending on personal preference and what you can stick to long term.
Food quality also plays an important role because it influences how much we eat. We are much more likely to overconsume ultra-processed foods, sugary drinks, alcohol, highly palatable foods rich in fat, sugar and salt, energy-dense foods, and foods low in protein and fibre. In contrast, minimally processed foods rich in protein, fibre, and whole-food carbohydrates tend to be more filling and can help reduce overall calorie intake.
Not all carbohydrate-rich foods are the same. Whole grains, beans, lentils, vegetables and fruit can be very satisfying and support weight management, whereas sugary cereals, cakes, biscuits, crisps and sugary drinks can make it easier to consume excess calories.
The key message is to focus on food quality, satiety, and achieving a healthy body composition rather than simply counting carbohydrates.
Dr Kratz’s poster below shows the amounts of different protein-rich foods one needs to eat to get 10 g of protein. Just an ounce or two of most meat or fish. A couple of eggs or three to four ounces of cottage cheese or Greek yoghurt. Legumes such as lentils and beans, as well as soybean products like tofu, are other great sources.
Why protein helps with weight management
Research involving thousands of people across 44 feeding studies found that our bodies seem to have a natural appetite for protein. Regardless of whether a diet was low or high in protein, people tended to eat a similar amount of protein each day.
When meals contained less protein, people often ate more food and consumed more calories overall to meet their protein needs. In contrast, higher-protein diets helped people feel satisfied while eating fewer calories.
This may explain why meals that are low in protein and high in refined carbohydrates and fats can sometimes leave us feeling hungry soon after eating. Including a source of protein at each meal, such as beans, lentils, tofu, eggs, dairy products, fish, poultry, or lean meat, can help reduce hunger, increase fullness, and make weight management easier.
In simple terms, when we don’t eat enough protein, we may keep eating to try to get it. When we eat enough protein, we often feel satisfied with less food.
Why protein is especially important in people living with obesity
People living with obesity often need proportionally more protein than people of a lower body weight. This is partly because insulin resistance can make the body less efficient at using and preserving protein, particularly in muscles.
As a result, the body may break down muscle protein more readily and use some of its building blocks for energy. This means that more dietary protein may be needed to maintain muscle health, support metabolism, and help the body function well.
Including protein-rich foods regularly throughout the day helps support muscle mass, improve fullness, and can make weight management easier.
Fibre and weight management
Research suggests that certain types of fibre, particularly soluble and viscous fibre, may be especially helpful for reducing hunger, lowering calorie intake, and supporting weight loss. These fibres absorb water in the gut, helping you feel fuller for longer after eating.
Good sources include vegetables, fruits, berries, beans and whole grains.
Dr Kratz’s poster above shows that some foods contain particularly high amounts of fibre. These include fruits (especially berries), non-starchy vegetables, beans, lentils, and whole grains. Some starchy vegetables, such as carrots, squash and beetroot, can also make a valuable contribution to fibre intake.
Nuts and seeds are also good sources of fibre and protein and can be part of a healthy diet. However, they are also very calorie-dense because they contain relatively little water and more fat. For example, almonds are nutritious and filling, but they contain considerably more calories per ounce/gram than fruits and vegetables.
For weight management, the best approach is usually to increase fibre intake from a variety of sources, particularly vegetables, fruits, legumes and whole grains, while enjoying nuts and seeds in sensible portions as part of a balanced diet.
Choosing foods that keep you fuller for longer
Some foods are much more filling than others, even when they contain the same number of calories. The Nourished by Science satiety score is a way of estimating how well a food satisfies hunger relative to the calories it provides.
Foods tend to be more filling when they are high in protein, rich in fibre, and lower in calorie density (meaning they contain fewer calories for their weight or volume). These foods help people feel satisfied for longer and naturally reduce the urge to eat between meals.
Research shows that meals rich in protein and fibre, such as vegetables, fruit, beans, lentils, whole grains, yoghurt, eggs, fish, and lean meats, can help people eat fewer calories overall without consciously restricting food intake.
By choosing more filling foods and reducing foods that are less satisfying, it is often possible to lower calorie intake naturally, making weight management easier without constantly feeling hungry or needing to count calories.
For more dietary advice, get in touch to arrange a consultation with our Weight Loss Clinic.
References
Gardner CD, Trepanowski JF, Del Gobbo LC, et al. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion. JAMA. 2018;319(7):667–679. doi:10.1001/jama.2018.0245
Raubenheimer D, Simpson SJ. Protein appetite as an integrator in the obesity system: the protein leverage hypothesis. Philos Trans R Soc Lond B Biol Sci. 2023 Oct 23;378(1888):20220212. doi: 10.1098/rstb.2022.0212. Epub 2023 Sep 4. PMID: 37661737; PMCID: PMC10475875.
Dr Mario Kratz, PhD, Founder, Nourished by Science
About the author
Dr Ulrike Naumann
Dr Ulrike Naumann runs our Weight Loss Clinic and is committed to providing holistic, individualised care to help you develop healthier habits, including an improved relationship with food and exercise. She has over three decades of experience in General Practice with a special interest in obesity.
This post has been written or personally reviewed by Dr Naumann.
