Natalie Strachan

Registered Dietitian & Nutritionist


Do carbs cause weight gain?

This isn’t usually a question that I hear - it’s more of a statement! The belief that all carbohydrates spike our blood sugar, raise our insulin levels and therefore automatically promote fat storage is ubiquitous. However, as with most things nutrition, it’s not quite as simple as that.   

Evidence-based nutrition guidelines recommend that around 50% of our energy should come from carbs. For most people this amounts to at least a cup’s worth of cooked grains with each meal, plus some extra carbs with snacks.  With the wealth of misinformation available you’d be forgiven for thinking that this is a high carbohydrate diet, but it’s moderate - it's the amount that research shows is best for our health. When researchers fed a group of people an actual high-carb diet (77%-86% of total energy), they found that there was still no appreciable fat storage until several consecutive days on the diet. This was partly because in response to the additional carbohydrate intake, the subjects’ bodies just stored more carbs in the liver and muscles as glycogen, and increased the rate at which they turned carbohydrates into usable energy.

A review article in the Journal of Clinical Nutrition and Metabolism summarises this research by saying  “Thus, the human body can easily accommodate the daily ingestion of relatively large amounts of carbohydrates without having a need to convert carbohydrate to fat.” 

What about the role of insulin in fat storage? After we eat a meal or snack containing carbohydrates the pancreas releases the hormone insulin which pushes the digested carbohydrate (glucose) from the blood into our cells where it can be used to power all the important jobs that the cells needs to do. Insulin also inhibits our fat stores from being used for energy and can promote the storage of excess blood glucose as fat. But remember that on the extremely high-carbohydrate diet, it still took several days for this to take effect. With a moderate intake of carbohydrates the insuin response is also moderate and does not promote excess fat storage.

Additionally,  a carbohydrate-rich meal will stimulate the release of other hormones which inhibit fat storage - such as leptin, which also helps us feel full and satisfied after a meal. It's only when carbohydrates are excessive that the balance is tipped in favour of fat storage. Plus, if we eat balanced carbohydrate meals along with sources of protein, fibre and fat, we slow the release of the carbohydrate into the blood which helps keep our insulin levels regulated. 

So why do low carbohydrate diets result in rapid weight loss? Low carbohydrate diets initially deplete us of water and glycogen (stored glucose). This happens quickly, resulting in rapid weight loss which perpetuates the belief that carbohydrates are “fattening”. These diets are usually low in calories, which is why people continue to lose weight, but unfortunately low carbohydrate diets can result in more muscle, bone and organ tissue depletion than a similar-calorie diet that contains adequate carbohydrates. So yes there is weight loss, but much of this is because the body is consuming it’s own essential non-free tissues.

Do carbs promote visceral fat? No, not in moderation, but we know that a LONG-TERM  diet containing REGULAR and VERY EXCESSIVE amounts of refined carbohydrates can encourage the storage of fat around the internal organs, especially if it’s in combination with other lifestyle factors.  This is the fat we don’t see and can be present in ALL body sizes.  It’s the fat that is linked to increased risk of heart disease and diabetes, as supposed to the more visible fat under our skin which appears to protect our health.

But this definitely doesn’t mean that fun foods like chocolate, ice cream and pastries should be banished from our lives! It’s just a reminder to nourish ourselves with these foods within the broader context of a balanced diet, because that way moderate amounts of these delicious foods each day can be handled just fine by the body. 

So meeting our carbohydrate needs (~50% of total energy intake) is not known to cause weight gain, but we do know that it supports thyroid function; preserves muscles, bones and organs; supports mood, memory and hormone balance; it’s the favourite food of our gut microbes - the guys who will help keep us healthy as long as we feed them; and of course carbs provide us with energy.  In fact they are the body’s favourite fuel, and our bodies will remind us of this if we restrict carbs over the day, only to find we are preoccupied with them by the evening. Our bodies are intelligent and will let us know if something’s wrong!  Having some carbohydrates with each meal and snack can help us feel satisfied throughout the day and keep the mind and body relaxed into the evening. 

Remember that how we respond to carbohydrates is highly individual and depends on factors such as genetics, hormones, growth, microbiome health, muscle mass, exercise levels and nutritional status. This is where individualised advice from a registered dietitian or registered nutritionist can help you find the right balance.

Sources:  10.1016/j.eclnm.2011.01.005 ; 10.1093/ajcn/48.2.240 ; 10.1093/ajcn/38.6.989