Researchers in Denmark say “no.” French fries, maybe.
“The identified studies do not provide convincing evidence to suggest an association between intake of potatoes and risks of obesity, T2D, or CVD. French fries may be associated with increased risks of obesity and T2D although confounding may be present. In this systematic review, only observational studies were identified. These findings underline the need for long-term randomized controlled trials.”
Source: Potatoes and risk of obesity, type 2 diabetes, and cardiovascular disease in apparently healthy adults: a systematic review of clinical intervention and observational studies
I would like to see RCTs with potatoes. I’m currently testing a ketogenic diet, but before that, I would periodically eat potatoes, aiming to cook then cool then reheat the potato for maximum resistant starch. I’m not sure there was a benefit to eating potatoes, though I’m also not sure there was a detriment.
French fries are likely problematic because of the high omega 6 content of the oils. I assume if you used lard or tallow, they might not be problematic. Again, you’d need an RCT (preferably with control, fries fried in seed oils/high omega 6 oils, and fries fried in one of lard or tallow, preferably made from locally farmed and/or grass fed animals).