A Tale of Two Diets – a Primer on Intermittent Fasting and the Ketogenic Diet

Nuts, salmon, avocado, eggs and other high-fat and protein and low-carb food choices

On Jan. 25, RVNAhealth Registered Dietitian and nutrition educator, Ava Safir, JD, MS, RDN, presented A Tale of Two Diets at Fit Club in Ridgefield, offering education and information on Intermittent Fasting (IF) and the Ketogenic Diet. The presentation was fascinating, covering the science behind the diets as well as benefits and pitfalls of both.  Following is key information from the presentation. 

If you have additional questions regarding IF and Keto, or are considering a new diet approach for personal or medical reasons, contact Ava Safir, JD, MS, RDN for guidance, advice, or to arrange for a nutritional counseling session.

A Tale of Two Diets – a Primer on Intermittent Fasting and the Ketogenic Diet 

Intermittent Fasting (IF) and the Ketogenic Diet (Keto) are based on the hormone theory of obesity, which states that weight gain and obesity are driven by hormones rather than the old-school belief that they are based on calories in/calories out. While both IF and Keto are grounded in the hormone theory of obesity, each plan takes a very different approach to achieving lasting weight loss and improved health outcomes. Since these diets are currently very popular, and because we get a lot of questions on both, we offered a presentation to provide facts and information.

Intermittent Fasting

Intermittent Fasting is an eating pattern that cycles between periods of fasting and eating. The theory behind IF is that the metabolic switching between the use of glucose for energy in the “fed state” and the use of fat for energy in the “fasting state” positively impacts hormone levels and causes numerous downstream short-term and long-term adaptations that improve physical and mental health, increase disease resistance, and potentially lengthen the lifespan of animals and humans.

 The three most common forms of IF are:

  • 24 hours on/24 hours off (alternate days eating and fasting)
  • 5/2 fasting (five days eating, two days fasting)
  • 16/8 (eat all meals over eight hours each day, fast for remaining 16 hours each day)

When following an IF meal plan, there are no restrictions on the amount or types of foods consumed. There are only restrictions on the timing of each meal, and no snacking between meals is allowed.

According to the results of a meta-analysis of IF eating plans published in the New England Journal of Medicine in December 2019,1 an IF eating plan can: increase insulin sensitivity, improve lipid metabolism (use of stored fat for energy), improve the resistance of cells and organs to stress and disease, improve cell growth and plasticity, and possibly reduce abdominal fat, inflammation and blood pressure.

Some of the potential limitations of the IF diet include: viability as a lifestyle given cultural norms and activities surrounding meals and snacks; possible hunger and irritability in the early stages of IF; reality that many people eat for reasons other than hunger and IF does not allow for this. In addition, long-term studies have only been completed on animals, and clinical studies have focused on overweight young and middle-aged adults. The benefits of IF cannot be extrapolated to other age groups. 

Note: as part of her research, Ava Safir did the IF diet for two weeks. Here are Ava’s outtakes on the experience.

The Ketogenic Diet

The Ketogenic Diet focuses on macronutrient distribution, in particular, consuming a high-fat, moderate protein, low-carb food plan to keep insulin levels low and encourage weight loss. The standard Ketogenic Diet, which drastically reduces carbohydrate intake and replaces it with fat, recommends the following ratios: 75-80% fat; 15-20% protein and 5% carbohydrates. The theory behind the diet is that the reduction in carbohydrates significantly reduces insulin production (similar to a fasting state) and puts the body into a metabolic state called “ketosis.” Being in ketosis reduces insulin production and can lead to weight loss. There are no restrictions re: timing of meals/snacks or the amount of food consumed on the Ketogenic Diet.

Some of the short-term and long-term benefits that appear to result from the Ketogenic Diet are: Faster weight loss (initially); potential improvements in blood glucose control and insulin sensitivity in patients with Type 2 Diabetes (always check with your doctor before starting a new diet plan); a decrease in cholesterol levels over time. Additionally, there are ongoing studies regarding potential benefits for cancer treatment and tumor growth, as well as reducing the symptoms and progression of neuro-degenerative conditions such as Alzheimer’s Disease and Parkinson’s. 

Some of the potential limitations of the Keto Diet include: viability as a lifestyle over time given the strictness of the diet; concerns related to malnutrition; “Keto flu,” which includes constipation, nausea, fatigue, headache, and brain fog; the diet often involves the consumption of fatty, salty, processed foods; and the excessive intake of fat – or any macronutrient – will cause weight gain. 

1De Cabo, Rafael. Effects of Intermittent Fasting on Health, Aging and Disease. NEJM, 381;26 December 26, 2019.

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