Carbohydrates, Fasting and Health: What the Science Really Says
Few areas of nutrition generate as much confusion as carbohydrates and fasting.
Carbohydrates come under a lot of flack and are often blamed for weight gain, inflammation and poor metabolic health. Fasting, meanwhile, is frequently promoted as a shortcut to fat loss, insulin control and even longevity. As usual, the reality is more nuanced.
At Orion, we do not initially recommend fasting as a primary weight-loss strategy. This is not because fasting is useless. In the right context, it can be a helpful tool. But for most people beginning a health or body-composition journey, the priority should be building consistent foundations:
sufficient protein
fibre-rich carbohydrates
healthy fats
resistance training
sleep
hydration
sustainable eating habits.
Fasting can work, but it is not magic. And carbohydrates are not the enemy.
What Are Carbohydrates?
Carbohydrates are one of the three main macronutrients, alongside protein and fat. They provide approximately 4 calories per gram, the same as protein and less than half that of fat, which provides around 9 calories per gram.
When digested, most carbohydrates are broken down into glucose, which is used by the body as the major source of energy. Glucose is particularly important for the brain, red blood cells and exercising muscle. Muscles themselves hold up to 80% of your body’s glycogen while your liver can hold the remaining 20%. The glycogen in your muscles becomes especially important during exercise as it is the fuel for muscle contraction, whilst the glycogen in your liver is used to stabilise the glucose level in your blood.
It is important to note that not all carbohydrates behave the same way. A bowl of oats, a portion of lentils, a banana and a fizzy drink all contain carbohydrate, but their effects on appetite, blood sugar, gut health and long-term disease risk are very different.
Prioritise Quality Over Quantity
The glycemic index (GI) indicates how quickly a carbohydrate-rich food will raise your blood sugar compared to pure glucose. Generally speaking, a low GI food is preferable but there are instances when a higher GI food is more appropriate. The better question is: what type of carbohydrate, in what amount, for which person, and in what overall dietary pattern?
High-quality carbohydrate foods include:
whole grains
oats
potatoes and sweet potatoes
beans, lentils and chickpeas
fruit
vegetables
minimally processed starchy foods
These foods tend to contain fibre, micronutrients, polyphenols and water. They are generally more filling and usually produce a slower rise in blood glucose than refined carbohydrates.
By contrast, highly processed carbohydrates, for example, sweets, biscuits, cakes, sugary drinks and many refined snack foods are often energy-dense, low in fibre and easy to overconsume.
The World Health Organization recommends that carbohydrate intake should come mainly from whole grains, vegetables, fruits and pulses, and advises adults to consume at least 400g of vegetables and fruit and 25g of naturally occurring fibre per day. In the UK, adults are advised to consume 30g of fibre per day, yet most people consume around 20g or less, and only about 4 in 100 adults meet the recommendation.
This matters because fibre is not just “roughage”. It slows digestion, improves satiety, supports the gut microbiome, helps regulate bowel function and can blunt sharp rises in blood glucose after meals.
Sugar Is Different From Whole-Food Carbohydrate
One of the most important distinctions is between naturally occurring carbohydrates in whole foods and free sugars.
Free sugars include sugars added to food, as well as sugars naturally present in honey, syrups and fruit juices. They do not include the sugars naturally contained within whole fruit, vegetables or milk.
The UK Scientific Advisory Committee on Nutrition recommends that free sugars should make up no more than 5% of total daily energy intake. In practical terms, UK guidance recommends that adults consume no more than 30g of free sugars per day, roughly equivalent to seven teaspoons.
This is where carbohydrate advice often becomes confused. Reducing sugary drinks, sweets and ultra-processed snacks is sensible for most people. But that is not the same as avoiding oats, berries, potatoes, beans or whole grains.
Do Low-Carbohydrate Diets Work Better for Weight Loss?
Low-carbohydrate diets can lead to weight loss, particularly in the short term. But the evidence does not show that they are consistently superior to other well-structured diets when calories and food quality are controlled.
One of the most useful trials is the DIETFITS study. It randomised 609 overweight adults to either a healthy low-fat diet or a healthy low-carbohydrate diet for 12 months. At one year, the low-fat group had lost 5.3kg on average, while the low-carbohydrate group had lost 6.0kg—a difference of only 0.7kg, which was not statistically significant.
The important word in that study is healthy. Both groups were encouraged to reduce added sugars, refined grains and highly processed foods, and to focus on whole foods. In other words, the biggest win was not low-fat or low-carb. Both of these diets would have led to consuming reduced calories and this is the important figure for weight loss.
Large observational research also suggests a U-shaped relationship between carbohydrate intake and mortality, with the lowest risk seen around 50–55% of energy from carbohydrate. Both very low and very high carbohydrate intakes were associated with higher mortality, and outcomes appeared to depend strongly on what replaced the carbohydrate—plant-based fats and proteins looked more favourable than animal-derived replacements.
This does not mean every person needs to eat 50–55% carbohydrate. It does mean that extreme carbohydrate restriction is not automatically healthier.
What Happens When We Fast?
Fasting simply means going for a period of time without energy intake. Overnight fasting is normal; most people fast for 10–12 hours between dinner and breakfast without thinking about it.
During the fed state, insulin rises and the body stores energy. As the hours pass without food, insulin falls and the body begins drawing on stored fuel. In the early stages of fasting, blood glucose is maintained largely by liver glycogen. The production of glucose from non-carbohydrate sources begins around 4–6 hours after fasting starts and becomes more important as fasting continues.
After roughly 24 hours, liver glycogen becomes substantially depleted, and the body relies more heavily on fat stores, free fatty acids and ketone bodies. Longer fasting increases this shift, although the degree varies depending on activity, previous diet, body size and metabolic health.
This “metabolic switching” is one reason fasting is so interesting scientifically. A major New England Journal of Medicine review described evidence that eating within a 6-hour window and fasting for 18 hours can trigger a shift from glucose-based to ketone-based energy metabolism.
However, this does not mean that a routine fast automatically creates dramatic ketosis or superior fat loss.
Does Fasting Improve Weight Loss?
Fasting can help some people lose weight, but mainly because it can reduce total energy intake.
A common pattern is time-restricted eating, where someone eats within a 6–8 hour window and fasts for the rest of the day. This can naturally remove snacks, late-night eating and grazing. In some studies, people reduce intake by around 500 calories per day simply because the eating window is shorter.
But when fasting is compared with ordinary calorie restriction, the advantage often disappears.
A 2022 New England Journal of Medicine trial randomised 139 adults with obesity to either calorie restriction alone or calorie restriction plus time-restricted eating between 8am and 4pm. After 12 months, time-restricted eating was not more effective for reducing body weight, body fat or metabolic risk factors than calorie restriction alone.
Similarly, a one-year JAMA Internal Medicine trial compared alternate-day fasting with daily calorie restriction in 100 adults with obesity. At 12 months, weight loss was similar: 6.0% in the alternate-day fasting group versus 5.3% in the daily calorie restriction group. The fasting group also had the highest dropout rate: 38%, compared with 29% in the calorie-restriction group and 26% in the control group.
Most recently, a 2026 Cochrane review of intermittent fasting in adults with overweight or obesity found that, compared with traditional dietary advice, intermittent fasting may result in little to no difference in weight loss or quality of life. The review included 21 studies and 1,430 people for the comparison with traditional dietary advice, and the authors concluded that these approaches did not produce clinically meaningful differences in most outcomes.
This is the main reason Orion does not initially recommend fasting as a weight-loss strategy. The evidence suggests it is an option, not a superior solution.
Why Fasting Is Not The Initial Solution
For people beginning a weight-loss or health programme, fasting can sometimes distract from the fundamentals.
The most common problems include:
under-eating protein
eating too little fibre
compensatory overeating later in the day
increased cravings
poor training performance
disrupted social eating
a restrict-and-rebound cycle
excessive focus on rules rather than food quality
Weight loss is not just about creating a calorie deficit. It is about creating a calorie deficit that preserves muscle, supports training, manages hunger and can be maintained long-term.
A person who skips breakfast, drinks coffee until midday, under-eats protein, trains poorly, then overeats in the evening has not improved their metabolism. They have simply compressed a chaotic diet into a shorter window.
For many people, the better initial approach is more boring but more effective: regular meals, protein at each meal, fibre-rich carbohydrates, appropriate portions, reduced ultra-processed foods and consistent movement.
When Might Fasting Be Beneficial?
Fasting can be useful when it solves a specific problem.
For some people, time-restricted eating reduces evening snacking. For others, it simplifies decision-making because they prefer fewer meals. Some people find they feel more mentally clear with a longer overnight fast, although responses vary.
Fasting may also have a role in patients with specific diseases but this should only be done under medical advice and supervision.
At Orion Medica, fasting may be considered when:
a person already has good dietary quality
protein intake is adequate
training performance is not compromised
there is no history of disordered eating
the person finds fasting easy rather than stressful
it reduces late-night snacking or grazing
it is medically appropriate
In this context, fasting is best viewed as a behavioural structure, not a metabolic hack.
Who Should Avoid Fasting?
As mentioned above, fasting is not appropriate for everyone.
Caution is particularly important for:
pregnant women
people with a current or past eating disorder
underweight individuals
adolescents
frail or older adults at risk of muscle loss
people taking glucose-lowering medication
people with a history of fainting, hypoglycaemia or significant medical conditions
The Bottom Line
Carbohydrates are not inherently fattening, inflammatory or unhealthy. The quality, quantity and context matter. Whole-food carbohydrates such as oats, potatoes, fruit, vegetables, pulses and whole grains can form part of a very healthy diet.
Fasting is also not inherently good or bad. It can help some people reduce energy intake, improve structure and potentially support metabolic health. But the best current evidence does not show that fasting is consistently superior to conventional calorie restriction for weight loss.
Before considering fasting, most people should first be able to answer yes to the following:
Am I eating enough protein?
Am I getting 25–30g of fibre per day?
Am I eating fruit, vegetables, pulses or whole grains daily?
Am I strength training?
Am I sleeping well?
Am I avoiding frequent ultra-processed snacks and sugary drinks?
Am I able to maintain my approach socially and psychologically?
At Orion, the initial focus should be on nutritional foundations: protein, fibre, whole foods, intelligent carbohydrate choices, strength training and sustainable routines. Once those are in place, fasting may be considered for the right person, at the right time, for the right reason.
The goal is not to eat as little or as rarely as possible.
The goal is to eat in a way that supports health, performance, body composition and long-term consistency.
