With a new year comes renewed motivation to shed pounds and improve health, but navigating the sea of trendy diets can be overwhelming. From intermittent fasting to keto and smoothie-based plans, which approach is truly sustainable—and safe? According to accredited practicing dietitian and founder of NuSeasons Vivienne Wang and dietitian Trista Best, intermittent fasting stands out as the top contender.
“Intermittent fasting is the most sustainable option. Unlike keto or smoothie diets, intermittent fasting does not restrict the variety of foods you eat. It only changes the timing. With intermittent fasting, the body still gets all the nutrients it needs without strict restrictions that are hard to maintain in social situations,” Wang explains.
Struggling With Sleep? This Rechargeable Sound Machine Is A Total Game-Changer
Why Intermittent Fasting Wins for Sustainability
Wang emphasizes that intermittent fasting is not only effective but also socially adaptable. “Intermittent fasting is the safest and is the most socially adaptable. It’s achievable without putting strain on your body and psyche. Keto can be beneficial for epilepsy and provide rapid initial results for those with insulin resistance, but it’s unsafe for individuals with pancreatic disease, liver conditions, or gallbladder issues. It’s also hard to maintain socially. Dining out is difficult when you strictly eliminate carbohydrates,” she explains.
Best agrees, adding, "It is the most sustainable dietary approach, in my opinion, and can be modified to meet the individual's unique needs and preferences. There are no food groups being cut out or restricted and no strict food rules to stick to. These characteristics make it ideal for someone wanting to simply improve their health and potentially lose weight in a sustainable and ongoing manner that won't result in rebound weight gain."
For beginners, Wang recommends starting with the 16:8 fasting method, which involves eating during an 8-hour window and fasting for 16 hours, instead of jumping straight into the more aggressive 5:2 approach. “I advise clients to start with 16:8 fasting rather than the more aggressive 5:2 approach,” she notes. She also stresses the importance of structuring your meals properly: “Crucially, I recommend keeping breakfast and having an early dinner to fast through the night. Fasting during high-stress periods can raise cortisol levels, making weight loss harder and increasing cravings. Therefore, listening to your body's stress signals is essential.”
Meanwhile, the smoothie diet may help specific populations but comes with risks. “The smoothie diet is helpful for people with digestive issues or chewing problems (elderly). However, it’s not safe for diabetics or those with insulin resistance. When you blend fruit, you break down the fiber matrix, which can cause rapid spikes in blood glucose. It is essentially a 'crash diet' that is difficult to align with an active lifestyle.”
Wang also warns that intermittent fasting isn’t for everyone: “People with a history of eating disorders should avoid intermittent fasting. The ‘eating-window’ can trigger binge-restrict cycles. Individuals with high stress loads or adrenal fatigue should also be cautious, as prolonged fasting can elevate cortisol.”
Understanding the Risks and How to Fix Them
Even the most popular diets come with caveats, and Wang breaks them down:
- Keto Diet Risks: “The biggest nutritional risks here are a lack of fiber and micronutrient deficiencies (specifically electrolytes, Vitamin C, and B vitamins) due to the exclusion of fruits, whole grains, and starchy vegetables. This often leads to the 'Keto Flu' and chronic constipation," she reveals.
- Fix: Prioritize non-starchy vegetables like leafy greens, broccoli, and cauliflower, and supplement with electrolytes.
- Smoothie Diet Risks: Wang notes that the main risk is a lack of protein and healthy fats, "leading to muscle loss and metabolic slowdown. There is also a risk of consuming hidden sugars if the smoothies are fruit-juice-based."
- Fix: Wang warns: never drink a “fruit-only” smoothie. Add a protein source such as Greek yogurt or protein powder, and a healthy fat like chia seeds, nut butter, or avocado to slow sugar absorption and keep you full.
- Intermittent Fasting Risks: “The main risk isn't necessarily what you eat, but under-eating or 'backloading' calories," she says. "Some people starve all day and then binge on non-nutritious foods in their window, missing out on protein and fiber targets.”
- Fix: “Focus on the first meal of your window. Make it nutrient-dense with high protein and fiber to set the metabolic tone for the rest of the eating window. View the window as a time to nourish, not just a time you are allowed to eat," she advises.
The Bottom Line
If your goal is sustainable, socially adaptable, and nutritionally balanced weight loss, intermittent fasting remains the safest and most effective choice. While keto and smoothies may have short-term benefits for certain conditions, Wang’s advice underscores that the key to success is a diet you can maintain without stress, nutrient deficiencies, or lifestyle conflicts.
