Fasting may influence metabolism and immunity differently depending on body type, study finds

Key Takeaways:

• New research shows that people living with obesity respond differently to fasting compared to lean individuals, particularly in how their immune systems react.
• A 48-hour fasting study found that lean participants’ immune cells adapted by burning more fat, while those with obesity maintained higher levels of pro-inflammatory signals.
• Researchers say the findings highlight the complex interaction between nutrition, metabolism and immune function, and stress the need for further investigation.
  

Growing interest in fasting and ketogenic diets

Fasting has become a popular dietary trend, especially among those seeking to manage weight. Increasingly, fasting is paired with ketogenic or very low-carbohydrate diets, which aim to shift the body’s energy use from carbohydrates to stored fat, producing ketones as an alternative fuel source.

Dr Hashim Islam, Assistant Professor in the School of Health and Exercise Sciences at the University of British Columbia Okanagan (UBCO) and a member of the Centre for Chronic Disease Prevention and Management, explains that while fasting and low-carbohydrate diets can be beneficial, their effects are not uniform across all body types.

“These diet trends continue to grow in popularity. But our study found that people with obesity may respond to fasting differently than leaner individuals, especially in how their immune systems react,” says Dr Islam.

Fasting and immune health

Fasting has attracted significant attention in recent years, partly because of its coverage in mainstream media. Beyond public interest, scientists value fasting for its potential to alter metabolic processes in beneficial ways.

Lead author Dr Helena Neudorf notes that fasting forces the body to switch from burning glucose to burning fat, a process that produces ketones. “Fasting may improve health by changing metabolism to strengthen the immune system and reduce chronic inflammation, which is linked to many diseases,” she says.

However, the team was particularly interested in whether fasting has distinct effects on people living with obesity compared with lean individuals.

Study design and methods

To investigate, the researchers recruited participants with obesity and lean counterparts. Each group underwent a 48-hour fasting period, during which blood samples were collected before, during and after the fast.

The samples were analysed for hormones, metabolites, metabolic rate, inflammation markers and the activity of T cells – white blood cells that are crucial for fighting infections but can also drive chronic inflammation.

The study was a collaborative effort between Dr Islam’s group and Professor Jonathan Little’s research group at UBCO’s Centre for Chronic Disease Prevention and Management. The findings were published in iScience. 

Key findings

The results indicated that fasting did not affect all participants equally.

• In people living with obesity: T cells remained more pro-inflammatory, continuing to produce inflammatory signals even after fasting. These participants also showed a smaller increase in ketones and lower levels of important chemical reactions associated with immune regulation, such as ketones binding to amino acids or proteins.
  
• In lean participants: Immune cells adapted more readily to fasting by shifting to fat metabolism. This led to a stronger movement towards a balanced, anti-inflammatory state compared with their counterparts living with obesity.
  

“We also found the immune cells in lean participants adapted to fasting by burning more fat. This didn’t happen in those living with obesity,” explains Dr Neudorf. “Overall, their shift toward a more balanced, anti-inflammatory state was weaker in this particular group.”

Implications and future research

The findings suggest that while fasting may carry health benefits, these benefits may be blunted in people living with obesity. Dr Islam stresses that the implications of this are not yet fully understood.

“People living with obesity may respond differently to an isolated two-day fast compared to those who are leaner, but we don’t yet know if this is good or bad,” he says. “Our study shows the complex relationship between nutrition, metabolism and immune function, and that more research is needed to see how fasting can be used as a therapeutic tool for people with different body types.”

CCH Insight

This study is a good reminder that there is no one diet that suits everyone. We are all biochemically unique, due to a range of personal factors such as genetics, the makeup of our gut microbiome and our state of health, so it is always a good idea to be skeptical about ‘shiny new diets’ that appear to be the answer for weight loss or longevity. In this study, fasting appears to benefit people of a healthy weight, but not those living with obesity. Considering that one of the keys to good health and maintaining a stable, healthy body weight is to adopt healthy dietary habits that works for you and you can sustain, it is arguable that regular fasting could be unhelpful for people with obesity, who may be prone to binges or irregular eating patterns.