Intermittent fasting: restricting carbs twice a week may improve metabolic health

Intermittent fasting is more than a potentially effective weight-loss strategy: it has also been linked to a range of metabolic health advantages that occur regardless of weight loss. However, many people find it difficult to follow, particularly the 5:2 approach, which involves dramatically cutting calorie intake on two days each week.

My most recent study suggests that sharply reducing calories may not be necessary to achieve intermittent fasting’s metabolic advantages. Limiting carbohydrate intake on just two days per week could be sufficient to improve metabolic health.

Intermittent fasting seems to support health so effectively because it changes how the body manages metabolism.

Following a meal, the body moves into the postprandial state. During this period, metabolism directs cells to use carbohydrates as an immediate fuel source, while saving some carbohydrates and fat for later.

After several hours without eating, however, the body enters the postabsorptive "fasted" state. At this point, metabolism begins drawing on fat stores to provide energy.

Intermittent fasting can therefore create a healthier balance between the energy sources the body uses. This improves metabolic flexibility, which is associated with stronger cardiometabolic health.

Put simply, this is linked to a lower risk of cardiovascular disease, insulin resistance and type 2 diabetes.

Intermittent fasting and metabolic health

In an earlier study, my colleagues and I examined how fasting affects the body. We found that after one day of either complete fasting or substantial calorie restriction - consuming only around 25 percent of each person’s usual daily calorie needs - participants’ bodies were more able to clear and burn the fat from a full English breakfast the following day.

Fasting moved the body away from relying on carbohydrates and towards using fat. This effect continued throughout the fast and into the next day.

Our research has also compared intermittent fasting with diets matched for calories or based on calorie restriction. Both groups continued their respective diets until they had lost 5 percent of their body weight.

Although each group lost the same 5 percent of body weight at the same pace, metabolic processing improved more in the intermittent fasting group, echoing the results of the earlier trial.

Researchers who have similarly compared the 5:2 form of intermittent fasting against calorie-matched, calorie-restricted diets have also concluded that fasting benefits metabolic health.

Metabolic health benefits of carbohydrate restriction

But what makes intermittent fasting particularly helpful for metabolic health? My latest study aimed to explore this question.

On the 5:2 intermittent fasting diet, fasting days typically involve very low calorie intake, often amounting to only a few hundred calories daily. Since so little food is eaten on these days, carbohydrate consumption is also very low.

As carbohydrate availability controls the postprandial state, this raised the question of whether intermittent fasting’s metabolic effects result from calorie restriction or carbohydrate restriction.

We enrolled 12 participants who were overweight or obese. On one day, participants received a very low-carbohydrate diet. On another, they were given a severely calorie-restricted diet, containing around 75 percent fewer calories than they would ordinarily consume.

After each fasting day, participants ate a meal high in fat and sugar, similar to an English breakfast, allowing us to assess how readily their bodies burned fat.

The change towards fat burning, along with the improved processing of the high-calorie meal, was almost identical after both the conventional calorie-restricted "fast" day and the low-carbohydrate day. Put another way, reducing carbohydrates can produce the same positive metabolic effects as fasting.

Further research involving a larger group of participants will now be needed to verify these findings.

A low-carb 5:2 approach

These results may help tackle some of the practical difficulties associated with intermittent fasting and conventional low-carbohydrate diets.

With intermittent fasting diets, severely limiting calories on fasting days can raise the risk of nutritional deficiencies without careful planning. It may also trigger disordered eating.

Maintaining strict carbohydrate restriction over the long term can be difficult as well, and could encourage an unhealthy fear of carbohydrates.

Another drawback shared by intermittent fasting and continuous carbohydrate restriction is that weight loss is a probable consequence. These methods may therefore not suit everyone who wants to improve their health without losing weight, or those seeking to maintain their current weight.

We are now assessing whether an intermittent carbohydrate-restriction diet - or low-carb 5:2 - is feasible. Rather than reducing calories on two days a week, people would reduce the amount of carbohydrates they eat twice weekly.

If this approach is shown to work, it could provide the benefits of fasting without requiring calorie restriction on "fast" days.

Adam Collins, Associate Professor of Nutrition, University of Surrey

This article is republished from The Conversation under a Creative Commons license. Read the original article.