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

Intermittent fasting is widely used for weight loss, but evidence also links it to improvements in metabolic health that occur even when weight change is not the main driver. Even so, many people struggle with intermittent fasting in practice-particularly the 5:2 approach, where calories are tightly limited on two days each week.

My most recent study indicates that you may not need such severe calorie restriction to see the metabolic upsides of intermittent fasting. Simply cutting the amount of carbs you eat on two days a week may be sufficient to enhance metabolic health.

How intermittent fasting changes metabolism

Intermittent fasting seems to support health largely because it shifts how the body fuels itself.

After eating, we move into the postprandial state. During this period, metabolism encourages cells to use carbohydrates as an immediate energy source, while storing some carbohydrate and fat for later.

After several hours without food, the body enters the postabsorptive "fasted" state, and metabolism transitions towards using some stored fat to meet energy needs.

By prompting this switch more regularly, intermittent fasting can improve the balance between the fuels used for energy. This supports better metabolic flexibility, which is associated with stronger cardiometabolic health.

Put simply, this means a reduced risk of cardiovascular disease, insulin resistance and type 2 diabetes.

What earlier studies found about fasting and fat use

In earlier work, my colleagues and I ran a study to show what fasting does in the body. We found that after either a full day of total fasting or marked calorie restriction (eating around only 25 percent of each person's daily calorie requirements), the body was more effective at clearing and burning the fat from a full English breakfast eaten the next day.

During the fast, the body shifted away from using carbs and towards using fat. Importantly, this change persisted into the following day.

We have also compared intermittent fasting with a calorie-matched or calorie-restricted diet. Participants stayed on their assigned diet until they lost 5 percent of their body weight.

Although both groups lost the same 5 percent of body weight at the same rate, those following intermittent fasting showed larger improvements in how their metabolism handled nutrients-similar to what we observed in the earlier trial.

Other researchers comparing the 5:2 version of intermittent fasting with a calorie-matched, calorie-restricted diet have likewise reported benefits of fasting for metabolic health.

Metabolic health benefits

What, specifically, makes intermittent fasting so helpful for metabolic health? That was the central question behind my latest study.

On the 5:2 intermittent fasting diet, “fasting” days are typically very low in energy-often only a few hundred calories for the day. Because intake is so limited, carbohydrate intake tends to be very low as well.

Since the postprandial state is strongly influenced by carbohydrate availability, this raised a key question: is it the calorie restriction, or the carbohydrate restriction, that produces the metabolic effect seen with intermittent fasting?

What my latest study tested

We enrolled 12 participants who were overweight or living with obesity. On one occasion, participants followed a very low-carb diet for a day. On another occasion, they completed a severely calorie-restricted day (around 75 percent fewer calories than they would usually consume).

After each of these days, we gave participants a high-fat, high-sugar meal-similar to an English breakfast-to assess how readily their bodies could burn fat.

The results showed that the move towards fat burning and the improved handling of fat from the high-calorie meal were near identical after both the traditional calorie-restricted "fast" day and the low-carb day. In other words, restricting carbs can produce the same favourable metabolic effects as fasting.

It is now important that larger studies, using bigger groups of participants, are carried out to verify these findings.

Practical implications and limitations

These findings could help tackle some of the practical issues associated with both intermittent fasting and standard low-carb diets.

With intermittent fasting, very low calorie intake on fasting days can increase the risk of nutritional deficiencies if people are not careful. It may also act as a trigger for disordered eating.

Likewise, strict long-term carbohydrate restriction can be difficult to maintain, and may contribute to an unhealthy fear of carbs.

Another shared limitation of intermittent fasting and continuous carb restriction is that weight loss is a likely outcome. That means these methods are not universally suitable for people who need to improve health without losing weight, or for those aiming to maintain their current weight.

We are currently testing whether an intermittent carb restriction diet-effectively a low-carb 5:2-can work in practice. Rather than restricting calories on two days a week, the idea is to restrict the number of carbs consumed twice a week.

If this approach proves beneficial, it could deliver the advantages of fasting without the need to cut calories on "fast" days.

Adam Collins, Associate Professor of Nutrition, University of Surrey

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